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Pacific Oyster

Knowledge repository

Scientific name: Crassostrea gigas (sometimes Magallana gigas) 
Family: Ostreidae, the oysterfamily 
Size: average 10-15 cm, but sometimes as big as 30 cm. 
Distribution: worldwide 
Status: not endangered 

Visit our expo about the pacific oyster!

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See also

  • Oesterbank

External characteristics

The Pacific oyster is an elongated bivalve shellfish. Bivalve means that the oyster has two shell valves, just like a mussel. The shells are a colour mix of different shades of white and grey. In an adult oyster, the lower shell is curved and deeper than the upper shell. The shells are irregularly shaped with edges and ridges. Sometimes you can find barnacles on the shells of the Pacific oyster; these are small crustacean-like animals in a sturdy white house. Have you ever come across a Pacific oyster? Or maybe you've eaten one.

The Pacific oyster grows from April to October. From November to March, the oyster does not grow and may even emaciate [8]. This is due to the temperature of the water. They prefer a temperature between 11 and 34 degrees celsius. The rate at which they filter food from the water is also higher in the warmer months than in the winter months [8].

Did you know...

...An oyster can live up to 20 years old. 

Oysters have growth lines, like trees do. These line are not easy to read, so we don't know how old an oyster actually is. [*]  

Groei van Japanse Oester

A Pacific oyster consists of the following parts [8]:  

  • Two shells 
  • Gills 
  • Mantle 
  • Mouth 
  • Mouthflaps 
  • Hinge 
  • Gonad 
  • Heart 
  • Contractor muscle 
  • Anus 

      Distribution and status

      The Pacific oyster has its origins in Japan. But the animal is also native to Korea. The Pacific oyster thrives in cold waters and is widely used in aquaculture. Aquaculture is the farming of aquatic plants and animals. In the last century, the species was imported by humans into several countries, including the United Kingdom, the United States, Ireland, Australia and the Netherlands, to name a few.  

      Although it was intended to be used only in aquaculture to replace the native flat oyster, the species 'escaped'. Soon after, the animal began to establish itself rapidly in those countries. In addition, the species also accidentally ended up in countries like Norway, Denmark and New Zealand. This also happened in the Netherlands. 

      The Pacific oyster can quickly take over new areas with relatively little effort. They grow quickly compared to other oyster species, and are often better able to withstand pollution and disease. Because of their ability to establish quickly and compete with native species, the Pacific oyster is considered an invasive species (and sometimes even a pest). 

      Status: not endangered

      There are concerns that Pacific oysters are displacing native species, which could lead to their local extinction. This would then lead to reduced biodiversity. The Pacific oyster itself is therefore not considered an endangered species. 

      The Pacific oyster in the Netherlands 

      The Pacific oyster is considered an exotic species, meaning it does not originally occur in the Netherlands. An exotic species can be transported to a new place either deliberately or accidentally. Accidental, for example, can be because they were lifted along with shipping to a new place where they could settle. This is not the case with the Pacific oyster in the Netherlands. 

      In fact, this oyster species was deliberately introduced to the Netherlands in 1964 to breed them in the Oosterschelde, Zeeland [1]. This was done because the other, native oyster species in the Netherlands - the flat oyster - was doing badly. Due to overfishing, diseases and cold winters, the number of flat oysters decreased enormously [2].  

      People thought the Pacific oyster could not reproduce in the Dutch sea because of the low water temperature. This turned out to be the case, and the Pacific oyster spread rapidly. The first outbreak of larvae of the oyster occurred in 1976, the second outbreak in 1982, and then the species was definitely established. At the end of 1976, imports of Pacific oysters were banned, but by then the Pacific oyster had spread [edepot]. The species started spreading in the Wadden Sea in 1983 near Texel [3].

      A place in the ecosystem

      The species is seen as a pest by some people. An exotic species introduced into a new area can have major consequences. Some species drive out native species, others take an extra or empty place in the ecosystem.  

      The Pacific oyster has taken the empty spot of the flat oyster. This oyster has a lot of impact on the environment in the Wadden Sea.. Meanwhile, the Pacific oyster has been established in the Netherlands for decades and will not disappear any time soon. What do you think of the Pacific oyster in the Wadden Sea? 

      As an exotic species, it has no protected status in the Netherlands. This means that small-scale manual harvesting is allowed. You can eat the Pacific oyster: restaurants serve mainly Pacific oysters.  

      Did you know...

      De Japanse oester in Zeeland op het menu staan onder de naam creuse?  

      Oysters in the Wadden Sea 

      In 2002, the first oyster bank in the Wadden Sea was mapped [12]. From then on, the number of oyster beds there increased. In the eastern Wadden Sea, the Pacific oysters mixed with existing mussel beds. But in the western part, the oysters found new places to expand.

      Family of oysters 

      The Pacific Oyster is very similar to the Portuguese oyster (Crassostrea angulata). So much so, that researchers first thought they were the same species. Research from 2010 on their DNA gives evidence that they are indeed two different species. About 2.7 million years ago, they were genetically separated from each other and so for that long they are a different species [6]. 

      The genome of their mitochondrial DNA differs 3%. For your imaging, in humans and chimpanzees this difference is 4% [16]. Because these oysters are so similar, there is a theory that they have a recent ancestor.  

      The Pacific oyster is native to Asia. The Portuguese oyster was described as originally occurring precisely in the Northeast Atlantic. There is a theory that the Portuguese oyster does originate from Asia and was introduced to Europe at different times in history [7]. It is not yet known where this species originally came from. 

      Living environment of the Pacific oyster: oyster reef

      The Pacific oyster needs a hard bottom to attach to, such as a rock bottom, stones or poles. They may also use shellfish such as other oysters and mussels to attach to.  

      Oysters are found in littoral areas (always underwater) and sheltered sublittoral areas (is dry with low tide). They can occur at a depth of 4 metres, but in a scan of the Oosterschelde in Zeeland, the shellfish were found at a depth of 42 metres [*]. 

      The Pacific oyster can form a reef with many other oysters. A reef is a shallow area in the sea made on a hard surface. In many places in the world, a reef consists of coral, but in the Netherlands, it is the shellfish that can form a reef. So you have oyster reefs and mussel reefs. 

      The oyster reef can consist entirely of oysters or a combination of oysters with mussels. Once an oyster has attached itself to a hard surface, this oyster can offer the same hard surface to a new Pacific oyster. This allows oyster reefs to grow continuously. Over time, the old oysters die, but their shells remain.  

      The environment, such as water flow velocity, affects the formation and maintenance of an oyster reef. If the current velocity is too hard, oyster larvae cannot establish. It can also cause oysters to break off from the reef. 


      The environment does not only affect the Pacific oyster: vice versa, the oyster bank also affects the environment of the Wadden Sea. This is why we also call oyster banks biobuilders: a biobuilder is a species that can greatly change its environment. A Pacific oyster does this by: 

      • Making a hard surface on a sandy area. 
      • Making the murky water clear so that more light shines through it: the water gets filtered. 

      Changing the environment so greatly affects other animals and plants living in the Wadden Sea. They provide a different habitat, so this attracts all kinds of living things.  

      On and between the oyster you will find mussels, but also overgrowth of weeds such as the sea oak. In the lee of the oyster bank and/or the tidal pool that remains after high tide you will find: barnacles, shrimps, crayfish, snails, periwinkles, crabs, anemones, sea squirts. As the top of the oyster bank dries, birds descend on it in search of food, such as spoonbills and turnstones. Fish seek the shelter of the oyster bank and descend on the food found there. A well-known example of this is the mullet.  

      The influence of the Pacific oyster on other species

      Research shows that mussels are more likely to survive on a mixed oyster bed than on a pure mussel bed. The Pacific oysters protect the mussels by making it harder for birds to pick out the mussels between the oysters. On the other hand, mussels on such a mixed bank do have less meat, probably because they have to compete for food with the Pacific oyster [10]. It was also seen in 2016 that the native flat oyster uses shells of Pacific oysters to attach to [2].

      Did you know...

      The Pacific oyster filters the seawater? This is how the animal gets oxygen and its food.

      Diet and foraging 

      The Pacific oyster filters seawater to get oxygen and eat plants and animals. They therefore depend on the flow and speed of the water, as well as the food in the water [8]. The gills filter the particles they eat. They eat mostly algae (phytoplankton), but also larvae and seeds of other animals such as mussels (zooplankton), bacteria and dead organic matter [8,*]. They also extract lime from the water to grow their shell.

      Enemies of the Pacific oyster 

      The Pacific oyster is itself a prey animal. It is eaten by humans, but also by a number of bird species such as gulls and oystercatchers [4]. Oystercatchers insert their beak into a Pacific oyster that is slightly open, allowing them to spread it open further and eat it.  

      Gulls drop oysters from the air onto the ground. This is why you often see broken (thrown) oyster shells around the Wadden Sea dykes [5]. In addition, crabs, lobsters, starfish and fish also eat the oyster species [8,*]. There is even a sea slug called the oyster borer that mainly bores open young oysters and eats them [13]. 

      A pearl on the mudflats

      Pacific oysters and also mussels can form pearls. This is a reaction of the animal to grains of sand they ingest while filtering the water. Those grains of sand are sharp and the animal protects itself by depositing pearls around the grains. It is quite rare to find a pearl from a Pacific oyster. 

      Lifecycle of a Pacific oyster 


      Pacific oysters are hermaphrodites, meaning they can change sex. Larvae of Pacific oysters usually start out male and can change sex during their lifetime.  

      According to a study in 2020, they can even change sex several times in their lifetime. Not all oysters changed gender. 58% were hermaphrodites, while 42% did not change sex. Of these, 34% were female, and 8% were male. After 6 years of observing every oyster, the distribution had tilted predominantly in favour of females. 

      At 8-10 months of age, they do not become sexually mature until the water temperature exceeds 12 degrees [9]. Then they can change sex after 3-4 years.  

      Fertilisation in Pacific oysters 

      In oysters, fertilisation of the egg takes place in the sea. Female oysters can release between 1,000,000 and 100,000,000 eggs per year [8]. They release the eggs from their bodies. The eggs then float in the water. At the same time, the males also release their sperm into the water. This process happens mainly in the months of July and August, when conditions are right [9]. That means: at least 15-16 degrees Celsius and enough food in the sea. But it can also take place in June and September [15].  

      Als een eicel een zaadcel vindt, dan wordt het eicel bevrucht en ontwikkelt het zich in 1 dag tot een larve [8]. De bevruchte larven drijven tussen de 15 en 30 dagen rond in zee door middel van de stroming [9]. Dat maakt de larve van een Japanse oester in die levensfase zoöplankton. Zoöplankton zijn dieren in zee die niet of nauwelijks tegen de stroom in kunnen bewegen.  

      The chances of surviving as a larva are incredibly low. All kinds of fish and other shellfish such as mussels and oysters filter their food out of the water. In the process, they also filter oyster larvae out of the water. 

      Adult phase 

      Over 15-30 days, oyster larvae develop a shell. When the shell becomes too heavy to float with, a larva sinks to the bottom to attach itself to a hard surface. They do this with a kind of 'cement' that comes from a gland at the base of their just-developed foot [11]. The oyster is permanently attached to this spot. 

      Young larvae can detect adult oysters by the substances the adult oysters secrete into the water. Since young oysters have a preference for a place where oysters are already present, they can seek out an oyster reef in this way and thus grow larger [11]. A Pacific oyster can thus reach an age of 20 years [14]. 


      6. Ren, J., Liu, X., Jiang, F., Guo, X., & Liu, B. (2010). Unusual conservation of mitochondrial gene order in Crassostrea oysters: evidence for recent speciation in Asia. BMC Evolutionary Biology, 10(1), 394. doi:10.1186/1471-2148-10-394  
      7. Deborah M. Power, Jonathan W. King, Frederico M. Batista, Ana Grade, Hicham Chairi, et al.. New insights about the introduction of the Portuguese oyster, Crassostrea angulata, into the North East Atlantic from Asia based on a highly polymorphic mitochondrial region. Aquatic Living Resources, 2016, 29 (4), pp.404. ff10.1051/alr/2016035ff. ffhal-01483208f 
      8. Ecologisch profiel van de Japanse oester.  
      15. Factsheet oester – Stichting geïntegreerde visserij (2017) 
      16. Varki A, Altheide TK. Comparing the human and chimpanzee genomes: searching for needles in a haystack. Genome Res. 2005 Dec;15(12):1746-58. doi: 10.1101/gr.3737405. Erratum in: Genome Res. 2009 Dec;19(12):2343. PMID: 16339373. 
        *Jaap Vegter, Stichting Geïntegreerde Visserij en coördinator van het veldstation Waddenloods in Lauwersoog.

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      Pox virus in seals

      Knowledge repository

      The pox virus occurs in seals. A virus in which seals get firm skin nodules on the head, neck and flippers, among other things. The pox usually goes away on its own. At the Sealcentre we do everything we can to prevent any contamination.

      See also

      • Zeehond - moeder en pup zogen

      Pox virus

      The pox virus in seals is a different species than the (chicken)pox virus that occurs in humans and belongs to the parapoxvirus family (1). But seals can transmit this virus to humans (2). To minimize the chance of this, our seal caretakers wear protective clothing, gloves and face masks.

      Symptoms of pox

      The name of the virus refers to one of its most obvious features: pox. Pox are small firm skin nodules. They are between 1 and 3 centimeters in size and they can be all over the body (3). In seals we often see them on the head, neck and flippers.

      How do seals get pox?

      Pox is a regular occurrence in seals in our sanctuary. This is because the disease is influenced by stress. It is unclear how seals contract the virus, but the seal already carries the virus before it is taken into care. We will only know this if the virus causes visible symptoms. If the seal is taken care of and gets stressed, the virus can 'express' itself, which means that after a while in the shelter, the seal will develop small firm skin nodules on its skin.


      Pox is not contagious until it opens and bleeds out. To prevent a seal with the virus from infecting another seal, seals with pox are staying at the Sealcentre. They should not be released until the pox has disappeared, so that wild seals do not become infected.

      Treating pox

      Usually no treatment is needed. The pox goes away on its own over time, usually around 4-6 weeks. If a seal suffers from the pox virus, we can only treat the sympotms. If the seal is in pain, we administer painkillers. If the pox is infected with a bacteria, we treat it with antibiotics. In addition, we keep the pool water as clean as possible and it contains salt to keep wounds clean. After the pox has cleared, the seals are sometimes left with bald spots or scar tissue.


      1. Becher P, Konig M, Muller G, Siebert U, Thiel HJ (2002) Characterization of sealpox virus, a separate member of the parapoxviruses. Arch Virol 147: 1133–1140 DOI 10.1007/s00705-002-0804-8
      2. Clark C, McIntyre PG, Evans A, McInnes CJ, Lewis-Jones S (April 2005). “Human sealpox resulting from a seal bite: confirmation that sealpox virus is zoonotic”.  J. Dermatol. 152 (4): 791–3. doi:10.1111/j.1365-2133.2005.06451.xPMID 15840117S2CID 38466772.
      3. Sealpox Virus in Marine Mammal Rehabilitation Facilities, North America, 2007–2009 Amira A. Roess,1 Rebecca S. Levine, Laura Barth, Benjamin P. Monr oe, Darin S. Carroll, Inger K. Damon, and Mary G. Reynolds. Emerging Infectious Diseases • • Vol. 17, No. 12, December 2011

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      Herpes in seals

      Knowledge repository

      The herpes virus that seals can carry is not the same herpes virus that we humans can get. It is a highly contagious virus, which is why we are doing everything we can to prevent any spread in the Sealcentre.

      See also

      • Zeehond onder water

      Types of herpes

      Herpes is a virus that seals carry for the rest of their lives after infection. This virus is not the same herpes virus that humans can get. There are seven variants of this phocine herpesvirus (PhHV).

      Did you know...

      Our vet Ana, together with other scientists, has discovered the seventh variant of the phocine herpes virus? (1)

      We think it is very special that our head veterinarian Ana Rubio García belongs to the group of scientists who discovered this seventh variant in 2014 (1). When seals in our rehabilitation centre carry herpes, it is almost always this seventh species.

      Symptoms of herpes

      Herpes is not clearly visible by one specific complaint or change in appearance. Seals can have several complaints at the same time. Possible symptoms of herpes viruses are:

      • Runny nose (often with blood)
      • Inflamed oral mucosa
      • Vomiting
      • Diarrhea
      • Fever
      • Cough
      • Pneumonia
      • Hair loss in grey seals (2)

      Herpes remains in the seal's body forever. As a result, the complaints can occasionally return. Something similar happens in humans as well. The cold sore, an irritated blister that some people get around the mouth, is a characteristic of the herpes virus (herpes labialis). If you become infected with the cold sore, you will keep this virus with you for the rest of your life (3). In case of stress or fever, you can regularly get a cold sore again, which always disappears over time.

      How do seals get herpes?

      Herpes is a highly contagious disease. Seals can transmit it to each other through virus particles in the air. That is why it is very important to immediately separate a seal with herpes from the other seals in the sanctuary, by placing them in separate enclosures.

      Treating herpes

      There is no cure for herpes. We do try to counteract the symptoms of the virus, for example by lowering the fever with medication. We can also treat the inflamed oral mucosa by giving mouthspray. As soon as the seal no longer has any symptoms, it is no longer contagious. The animal can then stay with other seals and eventually be released.


      1. Bodewes R, Contreras GJS, García AR, Hapsari R, van de Bildt MWG, Kuiken T, Osterhaus ADME. Identification of DNA sequences that imply a novel gammaherpesvirus in seals. J Gen Virol. 2015;96(Pt 5):1109–14.
      2. Field, C. L. (2022, 7 juli). Viral diseases of marine mammals. MSD Veterinary Manual. Geraadpleegd op 30 juni 2022, van
      3. Herpes labialis (koortslip) | RIVM

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      Influenza in seals

      Knowledge repository

      There are four types of influenza in total. Types A, B and C cause flu in humans. Influenza types A and B can occur in marine mammals, such as in seals. Influenza can cause a huge wave of disease in wild seals.

      See also

      • Zeehondenmoeders en pup

      Types of influenza

      There are four types of influenza in total. Types A, B and C cause flu in humans. Influenza types A and B can occur in marine mammals, such as in seals. Influenza can cause a huge wave of disease in wild seals.

      Symptoms of influenza in seals

      Influenza is not clearly visible by one specific complaint or change in appearance. Seals can have several complaints at the same time. The characteristics of influenza B are unknown. Possible features of influenza A are:

      • Weakness
      • Bad coordination
      • Short of breath
      • Swollen neck
      • White or bloody runny nose
      • Pneumonia

      How do seals get influenza?

      The virus droplets are spread by, among other things, coughing and sneezing, and are then inhaled again. Influenza is extremely contagious. Not only for seals, but also for humans. That is why it is very important to protect both the seals and ourselves if someone is carrying the virus.

      Common seals

      Common and grey seals can be sick with influenza. It is shown that mainly common seals suffer from the influenza virus. In several major outbreaks of the virus, mainly common seals died. The last outbreak of influenza was in 2014, when thousands of common seals washed up dead on the coasts of the Netherlands, Germany, Denmark and Sweden (1).

      Avian flu in seals

      Seals can also get influenza from birds. In October 2015, the avian flu virus (H10N7) was the cause of major deaths among harbor seals (2). Seals probably contracted the virus through direct or indirect contact with wild birds or their faeces.

      In June 2022, researchers found the avian flu virus in three seals in Germany, but it hasn't sparked a massive outbreak. The fact that the virus is transferred from a bird to a marine mammal means that there is a chance that humans can also be infected.

      Treating influenza

      It is not possible to cure influenza. There is no medicine for this virus. We can only try to reduce the symptoms and wait until the seal gets better.


      1. Bodewes R, Rubio García A, Brasseur SM, Sanchez Conteras GJ, van de Bildt MWG, Koopmans MPG, et al. (2015) Seroprevalence of Antibodies against Seal Influenza A(H10N7) Virus in Harbor Seals and Gray Seals from the Netherlands. PLoS ONE 10(12): e0144899. doi:10.1371/journal. pone.0144899
      1. Zohari S, Neimanis A, Härkönen T, Moraeus C, Valarcher JF. Avian influenza A(H10N7) virus involvement in mass mortality of harbour seals (Phoca vitulina) in Sweden, March through October 2014. Euro Surveill. 2014;19(46):pii=20967. Available online: 

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      Hooded seal

      Knowledge repository

      Scientific name: Cystophora cristata
      Family: Phocidae
      Size: male 3.50 meters; female 2.00 meters
      Weight: male: 400 kilo; female: 300 kilo
      Habitat: Northwest Atlantic Ocean and the Arctic
      Endangered status: vulnerable

      See also

      “A male hooded seal makes tries to attract females in an extraordinary way: they have an inflatable, stretchy, red cavity in their nose (also called hood).”

      External features of the hooded seal

      Hooded seals can reach a weight of about 145 to 352 kilograms and a length of 2 to 2.6 metres. Large animals can even reach more than 400 kilograms. Hooded seals have a large and broad but relatively short head with large nostrils. The head and flippers are usually coloured completely black. The rest of the body is white in both males and females with black spots of all sizes and shapes1,2. In contrast to their robust body, their flippers are relatively short.

      Gender differences

      Imagine this: you inflate a red balloon bigger than your head. You would stand out in an audience. So does a male hooded seal, but with a hood coming from their nose! This is where the species gets its name from. They do this to attract the attention of females, but also to threaten other males (and compete when mating with females)1,6.

      In fact, males have a rather large nose, the skin of which they can inflate into a black "balloon". This one can be bigger than their own head! But that's not all, as they can additionally inflate a sheet on the inside of their nose into a red "balloon". They inflate these different air sacs to show how big they are. The nose of females has no other notable modifications and they remain somewhat smaller than males.

      Distribution and status

      Hooded seals are found in the North Atlantic and the Arctic Ocean. They are native to Canada, Greenland, Iceland and Norway. It is estimated that there are about 600,000 hooded seals in the North Atlantic and another 100,000 or so in the Arctic Ocean. The hooded can swim in water from -1.9°C to 10.7°C9

      Did you know...

      The habitat of the hooded seal is decreasing due to climate change?

      Decreasing habitat

      This gives? the hooded seal the "vulnerable" status on the IUCN Red List, meaning that the species may be endangered in the future2,3The habitat of the hooded seal, like other Arctic or ice dependent species, is threatened by habitat loss - due to climate change. Therefore, it could be the case that the hooded seal will be listed as an "endangered" species2. Because of its vulnerable status, the hooded seal is protected by the Marine Mammal Protection Act1.

      Humans hunting hooded seals

      In the past, there was intensive and unsustainable commercial hunting of hooded seal mothers for oil and leather or the pups' fur. Usually, when pups were hunted for their fur, the mothers were also killed because they would try to defend their pup. In Greenland today, people still hunt hooded seals for meat or fur1,2,5,8.

      Further threats to the hooded seal population include entanglement or bycatch, competition for food with commercial fisheries or other predators, climate change and, as usual, typical diseases1,2.

      Diet and foraging

      The diet of hooded seals consists mainly of squid, starfish, mussels and some fish species such as Arctic and Atlantic cod and herring. Young hooded seals eat pelagic crustaceans, such as krill 

      Did you know...

      Hooded seals can dive as deep as 1 kilometre?

      While foraging, hooded seals dive as deep as 100 to 600 metres. They then do so for about 13 to 15 minutes. However, it is known that they can even reach a depth of 1,000 metres! Hooded seals can stay underwater for 1 hour and can travel as fast as 27 kilometres per hour 1,2,7.

      Behavior of the hooded seals

      Hooded seals are solitary animals. That means they prefer to go their own way. In general, hooded seals show aggressive and territorial behaviour. Males are known to patrol along the ice edge and often stay close to females. There is considerable fighting between males, with often bloody results, whilst showing off vocally their inflated red 'hood'1,5

      The only social contact with other hooded males occurs during the mating and moulting season 1,10. The males then appear together in small groups. But: even then they will avoid lying close together2.

      Did you know...

      Hooded seals can stay at sea for weeks without resting?

      The hooded seal is a migratory species: they migrate. Sometimes they spend weeks at sea without resting. When they do rest, they prefer to do so on floating pack ice 6,8. Their annual migration cycle begins when they are sexually mature1. Males are sexually mature at 6 years of age, females at 3 to 6 years of age7. The molting period is annual in July, after the young are born 1,2

      Voortplanting bij de klapmuts

      From April to June, the hooded seals have only 2.5 weeks to mate6. Hooded seals are polygamous: males mate with several females. Mating takes place under water2,4.

      Diapause and pregnancy

      As with all seal species, there is a diapause after fertilization. A diapause means that there is a time between fertilization and the actual pregnancy, so implantation is delayed. This delayed implantation of the fertilized egg takes about 3 to 4 months. After these months, gestation lasts about 8 to 11 months7,8

      Birth and nursing period

      A hooded seal gives birth to one single pup. The pup is born and suckled on the sea ice1,2. The mother will aggressively defend her pup. Usually, the mother will not forage while lactating. Usually, a male will assist the mother and her pup and stay nearby. When the mother will suckle her pup, the male can mate with her directly in the water5,8

      The birthing season usually takes place around March and April5. The pups are also known as "bluebacks" because their fur appears blue-grey. Unlike their backs, the pups have a whitish belly. The fur falls out during the molt when they are 14 months old1

      Did you know...

      Pups of hooded seals have the shortest lactation time of all mammals? Suckling lasts only 3-5 days after birth.

      Suckling lasts only three to five days after birth. This makes the pup of the hooded seal the fastest suckling mammal compared to other mammals. During the suckling period, the pup drinks up to 10 litres of the high-fat content milk per day1,8

      At birth, the pup weighs around 24 kilograms and is about 1 metre long1,2. After the suckling period, the pup will weigh about 48 kilograms. In other words, within five days, the pup doubles its weight (and gains about 7 kilograms a day)2,11. The pup first stays in the ‘nursing area’ for a while, but then starts learning to swim, dive and forage independently8. They do this based on their instincts.

      Natural enemies of the hooded seal

      Hooded seals are on the menu of polar bears. Killer whales could also be possible predators of the hooded seal, but this has not yet been observed. Greenland sharks can feed on young hooded seals2,11.

      1. ( Hooded Seal; NOAA Fisheries; 2022)

      2. (Hooded Seal; IUCN RedList; 2015)

      3. (Vulnerable species; dictionary; 2012)

      4. (Polygyn, dictionary; 2012)

      5. (Hooded Seal;Marine Species Identification Portal; n.d.)

      6. (Hooded Seal; Discovery of the Sound in the Sea; n.d.)

      7. (Hooded Seal; Oceanwide; n.d.)

      8. (Hooded Seal: Cystophora cristata; 2009)

      9. (Hooded seal Cystophora cristata foraging areas in the Northeast Atlantic Ocean-Investigated using three complementary methods; 2017)

      10. (Cystophora cristata hooded seal; Animal Diversity Web; 2010)

      11. (Hooded Seal; Norwegian Polar Institute; n.d.)

      12. Thomas A. Jefferson, Marc A. Webber and Robert L. Pitman, in Marine Mammals of the World  (Second Edition), 2015

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      Residents of the Wadden Sea

      Knowledge repository

      In het Waddengebied leven heel veel verschillende planten en dieren. Hoeveel van deze soorten zou je kunnen opnoemen? Misschien kom je wel tot een stuk of 5 soorten, of zelfs meer! Maar wist je dat er wel 10.000 soorten in zee en op het land leven? De helft hiervan leeft (deels) in zee¹. Op deze pagina maak je kennis met een aantal van deze zeebewoners. 

      See also

      • Kokmeeuw - Waddenzee

      • De wadpier leeft in de bodem van de zee

      • Brandganzen

      • De zeehond is de graadmeter van de zee

      Planten van de zee

      In zee leven allerlei soorten planten. Alleen zien ze er anders uit dan je misschien gewend bent van de landplanten 

      Vaste planten in de Waddenzee 

      Er bestaan planten die vastzitten aan de bodem van de zee. Zo heb je de bekende zeewieren, die je wel eens aangespoeld ziet op het strand. Een minder bekende plant is zeegras: dit is een plant die in de Waddenzee veel voorkwam, maar waar het sinds 1930 ontzettend slecht mee ging door onder andere de wierziekte en de bouw van de Afsluitdijk. Er wordt al jarenlang gewerkt aan een project om de zeegrasvelden te herstellen in de Waddenzee.²

      Zeewier - Waddenzee

      Zwevende platen in de Waddenzee

      Daarnaast heb je piepkleine planten die in het water zweven. Kleine, maar zeer fijne plantjes: Fytoplankton. Microscopisch kleine planten die in zee zweven en die dus niet vastzitten aan de zeebodem. In totaal zweven ze met miljarden in alle oceanen. Fytoplankton staan aan het begin van de voedselketen in zee en ze zijn daarom onmisbaar. Niet alleen voor het leven in de zee. Fytoplankton maken voor meer dan de helft van het zuurstof aan op aarde!³ Dus ook voor jou zijn ze onmisbaar.

      Plankton - zeevonk

      Bodembewoners van de Waddenzee 

      Op het eerste gezicht lijkt de bodem van de Waddenzee misschien levenloos. Maar niets is minder waar! Zowel in als op de bodem kun je vele bewoners vinden. 

      In de bodem

      Een van de belangrijkste dieren die in de bodem van de Waddenzee leven is de wadpier. Deze worm leeft in een u-vormige buis in de zeebodem. The wadpier eet fytoplankton op dat in de bodem zit and poept daarna schoon zand uit. Dat zijn de vele hoopjes die je op de zeebodem kan zien als het eb is.

      Schone zandhoopjes van een wadpier in de zeebodem.

      De wadpier heeft veel buren in de zeebodem. Zo zijn er verschillende schelpdieren die bescherming vinden in de grond. Denk aan kokkels, nonnetjes of de zwaardschede (zie onderstaande foto). Ze graven zich in de bodem, soms tot wel 30 centimeter diep! [4]  

      Hoe komen ze dan aan eten? Dat doen ze met behulp van hun sifo’s. Dat zijn een soort slangetjes waarmee ze met de ene sifo water ‘inslikken’ en fytoplankton opeten, en met de andere sifon het water weer uit hun lichaam halen. 


      Op de bodem

      Zeesterren en krabben zijn voorbeelden van soorten die óp de bodem van de Waddenzee leven. Zeesterren klinken misschien erg tropisch, maar ze komen ook zeker in de Waddenzee voor. Zij eten bijvoorbeeld schelpdieren zoals oesters en mosselen us.  

      Did you know...

      Zeesterren op een hele bijzondere manier mossels eten? Met de vele zuignappen die aan de armen vastzitten trekt een zeester de schelp open, duwt zijn maag uit zijn lichaam, de schelp in en eet zo de mossel op. 

      In de Waddenzee leven verschillende soorten krabben. Krabben zijn alleseters: ze eten zeewieren, plankton, wormen, kleine schelpdieren en garnalen. Ze eten ook (delen van) dode dieren op. Hun voorste poten zijn de grote scharen. Met deze scharen kunnen ze hun prooi te pakken krijgen, in kleine stukjes scheuren of een schelp kraken.

      Een krab op het strand


      Er komen zo’n 140 soorten vissen voor in de Waddenzee. [5] Bekende vissoorten als kabeljauw en haring ken je waarschijnlijk wel. Maar ken je ook platvissen, zoals tong en schol? Zij leven op de bodem van de zee. Zij hebben een goede camouflage, zodat ze niet opvallen. Sommige vissen blijven hun hele leven lang in de Waddenzee, anderen gebruiken deze zee tijdelijk omdat ze op trektocht zijn of ze komen daar alleen voor als ze nog jong zijn. 

      Did you know...

      De Waddenzee ook een leefgebied is voor haaien? De gevlekte gladde haai en de ruwe haai zijn hier voorbeelden van.

      Watch this video

      In deze video zie je hoe wij in 2016 een gevlekte gladde haai hebben gered.


      Het Waddengebied is van onschatbare waarde voor miljoenen vogels. Veel vogels maken gebruik van de Waddenzee als:

      • pitstop tijdens hun trekvlucht 
      • broedgebied in de zomer 
      • overwinteringsgebied 
      • algemeen leefgebied

      Did you know...

      De Waddenzee door miljoenen vogels wordt bezocht? 

      Waarom kiezen ze voor de Waddenzee om te eten, te rusten en om te broeden? Dat heeft te maken met het feit dat de Waddenzee een getijdengebied is. Tijdens laagwater is de Waddenzee een lopend buffet! Als het eb wordt stroomt het water uit de Waddenzee weg en ligt de zeebodem open en bloot. Met hun dunne lange snavel kunnen de wadvogels hun eten uit de bodem halen.  

      Scholeksters hameren bijvoorbeeld met hun snavel de schelp open om die te eten. Zodra het weer vloed wordt kunnen de vogels rusten op hoogwatervluchtplaatsen (hvp’s), de plekken die dan nog droog blijven liggen.  

      Onderzoek naar de Waddenzee


      In de Waddenzee zwemmen drie soorten zeezoogdieren rond. Een daarvan is de bruinvis. De bruinvis is een kleine walvissoort die jaagt op vis. De andere twee kun je misschien al wel raden: de common and grey seal. Zij staan aan de top van de voedselketen in de Waddenzee. De zeehond is een vleeseter en hun dieet bestaat uit vissen, garnalen, inktvissen en krabben. Er zijn zelfs berichten bekend van volwassen grijze zeehonden die bruinvissen aanvallen [6] en soms zelfs jonge grijze en common seals [7]. 

      Zeehond onder water

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      Harp seal

      Knowledge repository

      Scientific name: Pagophilus groenlandicus
      Family: Phocidae
      Size: male: 1.90 meter; female: 1.80 meter
      Weight: male 140 kilos; female 130 kilos
      Habitat: Northwest Atlantic Ocean and the Arctic region
      Endangered status: not endangered

      See also

      • zadelrob

      • zadelrob

      • Zadelrob in Zeehondencentrum

      • Zadelrob in zeehondenopvang

      "Swallowing stones helps the harp seal with going for a quick deep dive."

      External features of the harp seal

      The harp seal is a medium-sized seal species. Their body shape is somewhat elongated. The species has a pointed snout with eyes that are close together. But the most recognizable thing about the species is the saddle-shaped marking on their back. The species is therefore also called saddleback seal. In addition to the dark marking on the back, the harp seal also has a dark head.

      Gender differences

      Males are slightly larger than the females and have a more distinct color difference in their fur. In males, the saddle-shaped marking is dark and very noticeable, because the rest of the body is white. In females, the color sometimes differs from dark to gray and from white to light gray. Furthermore, there is little difference in appearance between males and females.

      Distribution and status

      Although they all belong to the same species, scientists distinguish three different populations of harp seals in the North Atlantic and the Arctic Ocean. The difference between these populations is mainly in the location where they birth their pups. The three populations are those of the Northwest Atlantic, the Northeast Atlantic and the Barents Sea.

      The Northwest Atlantic population is further divided into two major groups: the 'Front' herd which births their pups off northern Newfoundland and southern Labrador, and the 'Gulf' herd that gives birth in the southern Gulf of St. Lawrence.

      With over 7 million animals wordwide, the harp seal is not considered endangered. In fact, populations are growing in certain areas. Traditionally, the harp seal was also found in the Baltic Sea, but they have been eradicated there.

      Diet and foraging

      Like most seal species, the harp seal is "opportunistic". That means they eat whatever food is best to grab at the time. They don't make a big deal about it.

      Did you know...

      Harp seals in migration can cover as much as 5,000 kilometres a year? That's as far as you would walk from the Netherlands to Egypt.

      Harp seals make long migrations during the year. Immediately after mating, harp seals go on a migration, eventually always returning to the mating and suckling grounds.

      It is very typical of this species to swim long distances during the year. They follow the edge of the ice and the prey they can find there. So depending on where they are, their main prey also varies. Research has sometimes shown that they eat at least 67 species of fish and 70 species of invertebrates.

      The first food for young harp seals are usually swimming crustaceans, such as krill and razor clams. Once the seals are older and can dive deeper, crustaceans, squid and fish are eaten.

      Behaviour of the harp seal

      Harp seals are very social. They can always be found in small groups on the ice, but also are together in the water. It is not known whether they also hunt in groups.

      In the first period after suckling, the pups are alone for a while, but later they join the older animals. At that age, they also have to be very careful not to be caught by polar bears or arctic foxes. Once they become adults, they are no longer bothered by these, but they are still hunted by orcas and large shark species, such as the greenland shark.

      Reproduction in harp seals

      Mating behaviour

      When the female finishes suckling the pup, she can mate immediately. The males know this. Hence, they fight with other males on the ice during this period for the females. Eventually, she mates in the water with the male of her choice. Immediately after mating, migration starts.

      Diapause and pregnancy

      The fertilised egg is not transferred to the uterus until three to four months after fertilisation. This is called diapause. After that, a harp seal female is pregnant for eight months.


      When harp seals are born, they weigh an average of 11 kilos and are about 75 centimetres long. Newborn harp seals have a white coat (the lanugo coat), which keeps them warm on the ice. After the mother finishes suckling, the white fur falls out and they develop a silvery fur with a few dark spots. This is called beater-fur.

      Did you know...

      That when harp seal pups learn to swim, they hit the water with their tails?

      The name beater does not refer to the fur, however, but to the fact that at this stage, puppies learn to swim and they hit the water with their tails. They keep this fur for the rest of their first year. After that, the fur becomes more spotted and the seal is called a "bedlam". As the seal ages, it develops more spots, until they reach adulthood. Then the spots disappear. However, some females will keep spots all their lives.

      Birth and nursing period

      Harp seals take advantage of the brief moment in the year when there is a thick layer of pack ice. On this layer of ice they have the "land" they need to have their young.

      The suckling period lasts about 10 to 12 days. During this time, the pup gains more than 20 kilos. Then the mother leaves the pup and it is left alone on the ice. Here the process of moutling to the beaterfur begins. They may be left like this for up to 6 weeks without food. In these extreme cases, they lose half their body weight again. Eventually, hunger causes them to search for food in the cold arctic waters.

      Harp seal in Sealcentre Pieterburen

      In October 2016, we caught a harp seal. Very exceptional, because normally this seal species is are not found in the Netherlands. She was found severely weakened near Den Oever. She weighed only 60 kilos, while an adult harp seal should weigh between 140-150 kilos. Fortunately, she was eventually able to recover and be released. And that was quite a special moment. Read the story of Summer the harp seal here.

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      Knowledge repository

      In its first year of life, a seal is susceptible to infection by lung worms. After being weaned, young seals will go out and start hunting for prey by themselves. This is when they often contract lung worm infections. A number of them will get so sick, that they would not be able to survive without help. 

      See also

      • Longwormpatient

      • Longwormen bij gewone zeehond

      • Longwormen bij zeehonden

      Lung worms are parasites that can severely damage the lungs – they eat the tissues in the lungs and reproduce there, causing more and more damage in the process.

      The seal will experience shortness of breath and the damaged lungs are more susceptible to pneumonia or other bacterial infections. The shortness of breath also prevents a seal from staying under water long enough to hunt. This leads to starvation, weakness, and possibly death.

      Seals are exposed to lung worm through the food they eat. For example, a fish might have been infested with lung worm when it was eaten by a seal. From the stomach, the lung worms will travel through the blood stream to the lungs, where they grow and produce larvae. Infected seals will start coughing, expelling the microscopic larvae from their lungs.

      These larvae are then ingested by the seal, thus entering the host’s digestive tract. When the seal defecates, the larvae are released into the sea, where they can infect small sea creatures and grow. The infected sea creatures are eaten by seals, at which point the infection cycle restarts. If an infected animal is found on time, it can be treated. A seal with lung worm infection can be recognised by symptoms such as laboured breathing, a high back, and blood around the mouth from coughing.

      Usually a seal with a lung worm infection will need to be rehabilitated for two to three months; once a seal has been cured, it will be resistant to this parasitic infection for the rest of its life.

      On this page


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      Knowledge repository


      Laura Verga, Marlene G. U. Sroka, Mila Varola. Stella Villanueva and Andrea Ravignani. (2022). Spontaneous rhythm discrimination in a mammalian vocal learner. Biology Letters, 18:20220316

      David Ebmer, Stephan Handschuh, Thomas Schwaha, Ana Rubio‑García, Ulrich Gärtner, Martin Glösmann, Anja Taubert and Carlos Hermosilla. (2022). Novel 3D in situ visualization of seal heartworm (Acanthocheilonema spirocauda) larvae in the seal louse (Echinophthirius horridus) by X-ray microCTScientific Reports, 12:14078

      Anna Salazar-Casals; Koen de Reus; Nils Greskewitz; Jarco Havermans; Machteld Geut; StellaVillanueva; Ana Rubio-Garcia. Increased Incidence of Entanglements and Ingested Marine Debris in Dutch Seals from 2010 to 2020. (2022) Oceans, Vol 3, Issue 3, 389-400.

      Jörg Hirzmann, David Ebmer, Guillermo J. Sánchez‑Contreras, Ana Rubio‑Garcia, Gerd Magdowski, Ulrich Gärtner, Anja Taubert and Carlos Hermosilla. The seal louse (Echinophthirius horridus) in the Dutch Wadden Sea: investigation of vector-borne pathogens (2021) Parasites & Vectors 14:96

      Abbo van Neer, Ana Rubio-Garcia , Stephanie Gross, Anna Salazar-Casals, Alberto Arriba-Garcia2 Peter Wohlsein and Ursula Siebert. An innovative approach for combining marking of phocid seals with biopsy sampling using a new type of livestock ear tags. (2020) Journal of Marine Animals and Their Ecology Vol 12, Issue 1, 2020.

      Anna Salazar-Casals, Klaas Marck, Tijmen de Jong, James Collins, Joost Dorgelo, Pier Prins, and Ana Rubio-Garcia Retrospective study of surgical treatment of refractive osteomyelitis and infectious arthritis in the flippers of seals in The Netherlands. (2020) Journal of Zoo and Wildlife Medicine 51(3), 598-605, (16 November 2020).

      Anna Salazar-Casals, Alberto Arriba-Garcia, Antonio A. Mignucci-Giannoni, John O’Connor, Ana Rubio-Garcia. Hematology and serum biochemistry of harbor seal (Phoca vitulina) pups after rehabilitation in the Netherlands (2020) J. of Zoo and Wildlife Medicine, 50(4):1021-1025

      Rubio-Garcia, A., Rossen, JWA., Wagenaar, JA., Friedrich, AW., van Zeijl, JH. Livestock-associated meticillin-resistant Staphylococcus aureus in a young harbour seal (Phoca vitulina) with endocarditis (2019) Veterinary Record Case Reports 7: e000886.

      Ravignani A, Kello CT, de Reus K, Kotz SA, Dalla Bella S, Méndez-Aróstegui M, Rapado-Tamarit B, Rubio-Garcia A, de Boer B. Ontogeny of vocal rhythms in harbor seal pups: an exploratory study (2019) Current Zoology, Volume 65, Issue 1, Pages 107–120,

      Maarten J. Gilbert*, Aldert L. Zomer, Arjen J. Timmerman, Mirlin Spaninks, Ana Rubio-Garcia, John Rossen, Birgitta Duim, and Jaap A. Wagenaar. Campylobacter blaseri sp. nov., isolated from common seals (Phoca vitulina) (2018) International Journal of Systematic and Evolutionary Microbiology. DOI 10.1099/ijsem.0.002742

      Ravignani A*, Gross S*, Garcia M, Rubio-Garcia A, de Boer B. How small could a pup sound? The physical bases of signalling body size in harbour seals. (2017) Current Zoology, 2017, 1–9. Doi: 10.1093/cz/zox026

      Ulrich SA, Lehnert K, Rubio-Garcia A, Sanchez-Contreras GJ, Strube C, Siebert U. Lungworm seroprevalence in free-ranging harbour seals and molecular characterisation of marine mammal MSP. (2016) International journal for parasitology: parasites and wildlife. Doi:10.1016/j.ijppaw.2016.02.001

      Bodewes R*, Rubio García A*, Brasseur SM*, Sanchez Conteras GJ, van de Bildt MWG, Koopmans MPG, Albert D. M.E. Osterhaus, Thijs Kuiken. Seroprevalence of Antibodies against Seal Influenza A(H10N7) Virus in Harbor Seals and Gray Seals from the Netherlands. (2015) PLoS ONE 10(12): e0144899. doi:10.1371/journal. pone.0144899

      Rubio García A, Sánchez Contreras GJ, Juliá Acosta C, Lacave G, Prins P, Marck K. Surgical treatment of osteroarthritis in harbor seals (Phoca vitulina).(2015) Journal of Zoo and Wildlife Medicine 46(3):553-559.

      Woodman S, Gibson A.J, Rubio Garcia A, Sanchez Contreras G, Rossen J.W, Werling D, Offord V. (2015) Structural characterisation of Toll-like receptor 1 (TLR1) and Toll-like receptor 6 (TLR6) in elephant and harbor seals. Veterinary Immunology and Immunopathology 169, DOI: 10.1016/j.vetimm.2015.11.006

      Bodewes R, Sánchez Contreras GJ, Rubio García A, Hapsari R, van de Bildt MWG, Kuiken T, Osterhaus ADME. (2015) Identification of DNA sequences that imply a novel gammaherpesvirus in seals. Journal of General Virology, 96, 1109–1114 DOI 10.1099/vir.0.000029

      Reichel M, Muñoz-Caro T, Sánchez Contreras GJ, Rubio García A, Magdowski G, Gärtner U, Taubert A, Hermosilla C. (2015) Harbour seal (Phoca vitulina) PMN and monocytes release extracellular traps to capture the apicomplexan parasite Toxoplasma gondii. Developmental and Comparative Immunology (2015), doi: 10.1016/j.dci.2015.02.002

      Bodewes R, Hapsari R, Rubio García A, Sánchez Contreras GJ, van de Bildt MWG, de Graaf M, Kuiken T, Osterhaus ADME. (2014) Molecular epidemiology of seal parvovirus, 1988-2014. PLoS ONE 9(11): e112129. doi:10.1371/journal.pone.0112129

      Bodewes R, Rubio García A, Wiersma LCM, Getu S, Beukers M, Schapendonk CME, van Run PRWA, van de Bildt MWG, Poen MJ, Osinga N, Sánchez Contreras GJ, Kuiken T, Smits SL, Osterhaus ADME. (2013) Novel B19-Like Parvovirus in the Brain of a Harbor Seal. PLoS ONE 8(11): e79259. doi:10.1371/journal.pone.0079259

      Bodewes R, Morick D, van de Bildt MWG, Osinga N, Rubio García A, Sánchez Contreras GJ, Smits SL, Reperant LAP, Kuiken T & Osterhaus ADME. (2012). Prevalence of phocine distemper virus antibodies: bracing for the next seal epizootic in north-western Europe. Emerging Microbes and Infections (2013) 2, e3; doi:10.1038/ emi.2013.2

      Anichini M, de Reus K, Hersh TA, Valente D, Salazar-Casals A, Berry C, Keller PE, Ravignani A. 2023 Measuring rhythms of vocal interactions: a proof of principle in harbour seal pups. Phil.Trans. R. Soc. B 378: 20210477.  

      Ravignani, A., Anichini, M., Sroka, M., Varola, M., Salazar Casals, A., de Reus, K., & Verga, L. (2022). Vocal learning, chorusing seal pups, and the evolution of rhythm. The Journal of the Acoustical Society of America, 152(4), A275-A275.

      Koen de Reus, Daryll Carlson, Alice Lowry, Stephanie Gross, Maxime Garcia, Ana Rubio-Garcia, Anna Salazar-Casals, Andrea Ravignani; Vocal tract allometry in a mammalian vocal learner. J Exp Biol 15 April 2022; 225 (8): jeb243766. doi:

      Ravignani, A., Torres Borda, L., Rasilo, H., Salazar Casals, A., & Jadoul, Y. (2022). Parselmouth for bioacoustics: Analysis pipelines for seal vocalizations. The Journal of the Acoustical Society of America, 151(4), A29-A29.

      Torres Borda, L., Jadoul, Y., Rasilo, H., Salazar-Casals, A., & Ravignani, A. (2021). Vocal plasticity in harbour seal pups. Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences, 376(1840): 20200456. doi:10.1098/rstb.2020.0456.

      Hoeksema, N., Verga, L., Mengede, J., Van Roessel, C., Villanueva, S., Salazar-Casals, A., Rubio-Garcia, A., Curcic-Blake, B., Vernes, S. C., & Ravignani, A. (2021). Neuroanatomy of the grey seal brain: Bringing pinnipeds into the neurobiological study of vocal learning. Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences, 376: 20200252. doi:10.1098/rstb.2020.0252.

      Gilbert MJ, IJsseldijk LL, Rubio-García A, Gröne A, Duim B, Rossen J, Zomer AL, Wagenaar JA. 2020 After the bite: bacterial transmission from grey seals (Halichoerus grypus) to harbour porpoises (Phocoena phocoena). R. Soc. Open Sci. 7:192079. 

      Ravignani, Andrea, Christopher T. Kello, Koen de Reus, Sonja A. Kotz, Simone Dalla Bella, Margarita Méndez-Aróstegui, Beatriz Rapado-Tamarit, Ana Rubio-Garcia, and Bart de Boer. “Ontogeny of vocal rhythms in harbor seal pups: an exploratory study.” Current Zoology 65, no. 1 (2019): 107-120.

      Jo WK, Pfankuche VM, Lehmbecker A, et al. Association of Batai Virus Infection and Encephalitis in Harbor Seals, Germany, 2016. Emerging Infectious Diseases. 2018;24(9):1691-1695. doi:10.3201/eid2409.171829.

      Melero, Mar, Víctor Rodríguez-Prieto, Ana Rubio-García, Daniel García-Párraga, and José Manuel Sánchez-Vizcaíno. “Thermal reference points as an index for monitoring body temperature in marine mammals.” BMC research notes 8 (2015): 1-8.

      Lees verder

      Pup roept

      Sound of a seal

      Knowledge repository

      See also

      • Sound of a seal

      • Hoe communiceren zeehonden?

      • Opname geluid zeehond

      • Zeehonden communicatie

      Postdoctoral research into vocal learning in animals

      Grey seals and Common seals are apparently part of a very specific group of animals. Animals that, just like humans, have the ability to develop their voice over the course of their life. This phenomenon is called ‘vocal learning’. Other animals that possess this ability are parrots, passerine birds, and bats. An extraordinary example of vocal learning in seals is a seal called Hoover, who was kept in Boston’s New England Aquarium. Hoover was raised by humans and learned to copy human speech. Andrea Ravignani did his postdoctoral research at the Sealcentre and investigated the ‘crying’ of seal pups.

      Read more about his research group.

      Onderzoeker Andrea Ravignani

      Why do people talk? And why do seal pups cry?

      Andrea Ravignani is doing postdoctoral research at Sealcentre Pieterburen that is funded by the prestigious Marie Curie Scholarship. Andrea is an Italian scientist who is fascinated by the question of how animals – and humans – learn to produce sounds. As a part of his research in Pieterburen, he records the sounds of seal pups every day; the so-called ‘crying’. Andrea uses these recordings to then see if there is a development in the production of these sounds.

      Have you ever noticed how babies (human pups) begin with crying, then transition to incoherent babbling, and how this slowly changes into language? Something similar happens in seals. The sound – or voice – of each seal is very different, and their voices also change as they age. In fact, it looks like Grey and Common seals are members of a special group of animals that, like humans, have the ability to shape their voices over the course of their lives. This ability is called ‘vocal learning’. Other animals that have this ability are parrots, passerine birds and bats.

      An exceptional example of this ability in seals is the aforementioned seal named Hoover. Hoover was an orphaned pup who was taken in and raised by humans in Boston. Before he was relocated to the New England Aquarium, he had learned to mimic human speech. Hearing Hoover speak his former owner’s catchphrases is truly bizarre. You can hear how he sounds in the YouTube video “Hoover the Talking Seal”. Seals learn to make sounds through imitation, which they then adapt and change into their very own voice. Because of that, they might be the animal closest to us humans when it comes to ‘vocal learning’.

      Andrea’s research is important for two reasons. On the one hand it helps us to gain a better understanding of seals and what is important for them in the first phases of their life. On the other hand it gives us a glimpse into the evolution of humans. Human evolution is very difficult to reconstruct without a time machine, especially when it comes to the evolution of speech and language. Communication in seals is a completely new field of study.

      Special research findings vocal learning

      December 2018 - Seal pups communicate just like us

      In December 2018, Dr Andrea Ravignani published his discovery that seal pups adjust their communication based on the sounds of other pups. A sense of rhythm and timing had never before been demonstrated in seals. His work gives an insight into the vocal communication of humans, as well as seals.

      Research conducted by Dr Andrea Ravignani from the Artificial Intelligence Lab of the Vrije Universiteit Brussel at Sealcentre Pieterburen was the first to show that seal pups exhibit complex communication behaviour. He observed that seal pups adjust their sounds and especially their rhythm to what another pup is doing. Said simply: they take turns making sounds. However simple this may seem, it has never before been demonstrated in seals and it characterises animals that use complex communication methods. The study will be published in the Journal of Comparative Psychology.

      Ravignani: “Us humans often consider our communication to far more complex than that of other animals. However, what we are seeing in seal pups is astonishing. Even at four weeks old, seals seem to demonstrate a very precise and flexible timing in their communication. To an extent this is very comparable to the alternation we see in human conversations or in a musical canon.”

      The discovery fits in well with a study conducted by the Sealcentre into the behaviour of mothers and pups in the wild, which showed that pups suckle with several different mothers. It is therefore important that pups stand out between their peers and adjust their calls.

      Ravignani: “This finding was relatively unexpected and even seems counterintuitive at first. Communication in Common seals is usually relatively vertical – between mother and pup, not between different pups. However, what we see here is horizontal communication: the rhythm of one pup determines the rhythm of another pup. While this is surprising within the established knowledge of mother-pup interactions in Common seals, my findings fit very well in the behavioural research that the Sealcentre is conducting with the University of Groningen.”

      The study is part of a two-year research programme that Ravignani is conducting with the support of the Marie Curie Scholarship.

      October 2021 - Seals and the evolution of human speech

      Seals are good at learning sounds. The 'talking seal' Hoover could even imitate human speech. But can young seals already adapt their sounds to the environment? Researchers from the Max Planck Institute for Psycholinguistics, the Vrije Universiteit Brussel and the Pieterburen Seal Centre studied seal pups a few weeks old. When the seal pups heard louder ambient sounds, they themselves called out with a lower pitch. This makes seals very suitable for research into the evolution of human speech.

      The seal Hoover had been taken in as a pup by an American family. Even after he had already been moved to an aquarium, he continued to mimic human speech: he barked at visitors in his gruff voice (“Come over here”). Seals therefore belong to the small group of mammals that can learn to imitate sounds, also called 'vocal learning'.

      It is even more special if animals can adjust the pitch of their voices: an important feature of human speech. Senior researcher Andrea Ravignani says: "By studying these extraordinary mammals, we hope to eventually better understand how humans evolved speech and why we ourselves are such a talkative species." What Ravignani and his colleagues particularly wanted to know: could seals adjust their pitch from an early age?

      Sounds of the Wadden Sea

      The researchers decided to study eight seal pups from 1 to 3 weeks old. The seals were already staying at seal centre Pieterburen to gain strength. After 2-3 months in the sanctuary, they were released into the wild. To investigate whether the seal pups could adapt their sounds to ambient noise, the biologists first made recordings of natural ambient sounds of the Wadden Sea. These sounds were played in the seals' enclosure for a few days, at three different sound levels (from no sound to 65 dB). The pitch of the ambient sounds was similar to that of the seal sounds. The researchers also made recordings of the seal sounds. Would the pups adapt to the ambient noise and call higher or lower?

      When ambient sounds were louder, the seals called with a lower pitch. At the loudest sounds, their pitch also remained the most stable. One seal also clearly showed the 'Lombard' effect: it started calling louder when ambient sounds were louder. This effect is also typical of human speech: people start talking louder when there is ambient noise, so they can be better understood. But the seals did not call out more often or longer at different noise levels.

      Brain pathways

      So even very young seals can already adapt their sounds to the environment by calling at a different pitch. That ability they share with humans and bats is unusual for a mammal. Other animals only call louder in similar experiments.

      "The seal pups have much better control over their vocalisations than we thought,", says Ravignani "And they already have control over their voice when they are only a few weeks old. That is unique in the animal world. We thought only humans had a direct connection between the cerebral cortex and the larynx. But seals therefore seem to have these connections too. That brings us another step closer to unravelling the mystery of human speech."

      Veterinarian and researcher at seal centre Pieterburen Anna Salazar Casals added: "As a rehabilitation centre, we are happy to collaborate on research, in order to understand the animals better and protect them even better. We can use these new insights, for example, when setting up new shelter facilities or determining what resting areas in the wild should comply with."

      April 2022 - Anatomical studie confirms: seals learn to make sounds

      Seals are good at learning sounds. The 'talking seal' Hoover could even imitate human speech. But can young seals already adapt their sounds to the environment? Researchers from the Max Planck Institute for Psycholinguistics, the Vrije Universiteit Brussel and the Pieterburen Seal Centre studied seal pups a few weeks old. When the seal pups heard louder ambient sounds, they themselves called out with a lower pitch. This makes seals very suitable for research into the evolution of human speech.

      Most animals make sounds that fit their body size. A larger animal will sound lower because the larynx is longer: the air-filled canal in the neck where the vocal cords are located. But seals do not always sound the way they look. Sometimes they sound lower and therefore bigger, for example to impress a rival. Or higher and thus smaller, for instance to get more attention from their mother. Are these animals good at learning sounds (vocal learning) or has their larynx adapted to be more flexible?

      Sealcentre Pieterburen

      To answer this question, PhD student Koen de Reus and senior researcher Andrea Ravignani from the MPI worked with researchers from Sealcentre Pieterburen. The team measured the larynxes and body size (length and weight) of 68 young seals (up to 12 months old) that had died before or after a rescue attempt. The researchers also analysed previously recorded seal sounds, confirming the seal's wide range in pitch.

      De Reus and Ravignani found that the length of the seal larynx matched their body size exactly. So there were no anatomical explanations for their vocal talents. According to the researchers, only the vocal learning ability of seals can explain why they do not always sound the way they look.

      Vocal learning ability

      "Animals with vocal learning ability will sound differently than expected based on their body size, but the length of their larynx matches their body size. This combined acoustic and anatomical data can help us find more animals like this,", says de Reus "Studying multiple animal species with this ability is also going to help us find the biological basis of vocal learning. And possibly it will also provide insights into the evolution of complex communication systems such as speech.".

      "The more we study seals, the more we see that they can tell us something about human speech,", Ravignani adds.

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