Species

Crabeater seal

May 21, 2011, 4:30 pm
Content Cover Image

Crabeater seals. Source: Jerzystrzelecki/Wikipedia

The Crabeater seal (scientific name: Lobodon carcinophaga), also known as the White Antarctic seal, is one of 19 species of marine mammals in the family of True seals. Together with the families of Eared seals and Walruses, True seals form the group of marine mammals known as pinnipeds.

The Crabeater seal is one of the most numerous seal species in the world. Its name, however, appears to be a misnomer, as there is no evidence that it eats crabs; rather it eats krill.

Physical Description

After the summer moult, the crabeater seal is dark brown dorsally and grades to blonde ventrally. It has darker brown markings on the back and sides over the paler brown pelage. The flippers are the darkest parts of the body. Its fur gradually changes to blonde throughout the year and it is almost entirely blonde by the summer. In fact it is also known by the common name, white Antarctic seal.

It has a long snout and a fairly slim body compared to other seals. Females are slightly larger on average than males with a length from 2.2 meters to 2.4 meters. Males range from 2.0 meters to 2.4 meters. The crabeater seal often has long scars running along the sides of its body. These are most likely inflicted by its major predator, the leopard seal, Hydrurga leptonyx.

Its teeth are very distinct and have been called "the most complex of any carnivore". There are several tubercles on each tooth with spaces between them that cut deeply into the tooth. The main cusps of upper and lower teeth fit perfectly together. When the Crabeater seal closes its mouth, the only spaces are those between the tubercles. This arrangement probably serves as a sieve through which to strain krill, their primary food source.

Reproduction

caption Crabeater seal. Source: Mike Cameron/NOAA
Conservation Status

Scientific Classification

Kingdom: Anamalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Carnivora (Carnivores)
Family:-------- Phocidae (True Seals)
Genus:--------- Lobodon Species:--------Lobodon carcinophaga (Hombron and Jacquinot, 1842)

 

Reproduction in the crabeater seal probably takes place on the pack ice surrounding Antarctica in the austral spring, from October to December. Starting in September, a pregnant female occupies a space on the ice floe in which she gives birth and cares for her single pup. A male joins the female in her chosen area just before or just after parturition. He defends the female and the newborn pup. He is, in all likelihood, not the father of the pups.

Females come into estrus just after weaning and it appears that the male's only apparent interest is in waiting for the female to be sexually receptive. Males aggressively defend females from other intruding males. It is not clear if the males join the females because of a female cue such as scent or because of the pup.

Pups are born weighing approximately 20 kilograms (kg) and gain weight while nursing at about 4.2 kg/day. Physical contact between the mother and pup during this period is necessary. If either the pup or the mother strays, the other immediately follows.

Pups are weaned at approximately three weeks of age. It is unclear if physical mechanisms in the mother, such as reduced milk production, cause the weaning or if the defending male drives the pup and mother apart. Throughout the lactation period the male is aggressive towards the female. She defends herself by biting him on the neck and sides.

By the end of lactation her body weight may be reduced by half, so she would be unable to defend herself adequately. She becomes sexually receptive shortly after weaning and, unlike most seals, copulation appears to occur on the ice floes instead of in the water.

Gestation lasts about 11 months and probably includes a period of delayed implantation. Crabeater seals become sexually mature between three and four years of age and females may have successful pregnancies between five and 25 years old.

Behavior

Crabeater seals may be found in large aggregations of up to 1000 individuals, but are usually solitary or in small groups.

They dive primarily at night and are reported to average 143 daily dives in late February. Once in the water, diving occurs nearly continually for approximately 16 hours. There appear to be several types of diving: foraging dives, traveling dives, and exploratory dives. Most dives are for traveling and are less than a minute long and less than 10 meters deep. Foraging dives are slightly deeper, up to 30 m, and appear to very throughout the day, with crepuscular dives being deeper. This is most likely in response to krill distribution. Exploratory dives are the deepest and presumably for navigation as they usually occur just before a traveling or foraging dive.

Crabeater seals may use breathing holes created by Weddell seals (Leptonychotes weddellii). Young Weddell seals may even be chased from breathing holes by adult Crabeater seals.

At the end of the summer, crabeater seals disperse northward as the ice freezes. It appears that some seals, usually young ones, become disoriented and head farther south over the pack ice. As they are particularly mobile on land for pinnipeds, reaching speeds of up to 25 km/hour. When sprinting, it lifts its head high and swings its head from side to side in synchrony with its pelvis. Its foreflippers move alternately across the snow and its hind flippers are lifted off the ground and held together. They may travel hundreds of kilometers inland. These seals almost always die and are well preserved as "mummies" in the ice throughout Antarctica. Most seals, however, successfully travel north to oceanic islands, Australia, South America, and even South Africa.

caption Geogrpahic distribution of the Crabeater seal. Source: Mirko Thiessen

Distribution

The Crabeater seal, Lobodon carcinophagus, is primarily found on the coast and pack ice of Antarctica. In the winter months, it may be found on the shores of South America, Australia, South Africa, Tasmania, New Zealand, and various islands surrounding Antarctica. In the winter its range covers about 22 million square kilometers.

Habitat

The Crabeater seal lives chiefly on the pack ice and the near freezing water surrounding Antarctica.

Predators

When the Crabeater seal is approached it snorts, hisses, and bares its teeth. If caught, it rolls over several times. This is probably an avoidance tactic developed for its primary predators, the Orca or killer whale (Orcinus orca) and the leopard seal (Hydrurga leptonyx).

Food Habits

caption thumbTeeth of the Crabeater seal. Source: Uwe Kils/Wikipedia

The Crabeater seal appears to be a misnomer as there is no evidence that it eats crabs. Its primary food is krill, Euphausia superba. It probably also eats other invertebrates. The Crabeater seal feeds by swimming through a school of krill with its mouth open, sucking them in and then sieving the water out through its specialized dentition report that a captive crabeater seal was able to suck small fish into its mouth at distances of up to 50 centimeter (cm). They note that this prey is much larger than the krill that it would consume in the wild, and suggest that it could probably suck krill in from a much greater distance. The seal preferred fish smaller than 12 cm and swallowed everything whole, in contrast to many seals which tear their food up with their teeth before swallowing. It was often observed exploring the bottom of its pool and sucking up debris. Klages and Cockcroft suggest that this is an adaption to winter feeding on krill in the Antarctic. At this time of year, krill is mainly found in crevices and caverns. The seal may be able to suck the krill out from these unreachable areas. Feeding probably occurs prinicipally at night.

Conservation Status

The crabeater seal is the most numerous species of pinniped in the world, with a population estimated at between 15 and 40 million. Since its habitat is remote, the only concerns for conservation are indirect. Trace amounts of chemicals such as DDT have been found in populations of the Crabeater seal, and if the fishing industry decides to use the krill in the Antarctic seas, the major food source of these seals may be severly depleted. Now, however, its numbers appear to be stable.

Economic Importance for Humans

Since Crabeater seals occupy a habitat that is fairly inaccessible to humans, there has been very little contact between the two species. There is a report, however, that a young crabeater seal found on the coast of South Africa was easily tamed and trained. Commercial fisheries have expressed interest in exploiting Antarctica's krill resources. As this is the primary food of the crabeater seal, there are bound to be negative consequences associated with the crabeater seal in this budding industry.

Further Reading

  1. Lobodon carcinophaga (Hombron and Jacquinot, 1842) Encyclopedia of Life (accessed May 20, 2010)
  2. Lobodon carcinophaga, Batty, K., 2008., Animal Diversity Web (accessed May 20, 2010)
  3. Crabeater seal, Seal Conservation Society (accessed May 20, 2010)
  4. Lobodon carcinophaga IUCN REd List of Threatened Species (accessed May 20, 2010)
  5. The Pinnipeds: Seals, Sea Lions, and Walruses, Marianne Riedman, University of California Press, 1991 ISBN: 0520064984
  6. Encyclopedia of Marine Mammals, Bernd Wursig, Academic Press, 2002 ISBN: 0125513402
  7. Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, Suzanne Montgomery and Timothy J. Ragen, Johns Hopkins University Press, 2005 ISBN: 0801882559
  8. Walker's Mammals of the World, Ronald M. Nowak, Johns Hopkins University Press, 1999 ISBN: 0801857899
  9. Crabeater seal, MarineBio.org (accessed May 20, 2010)

 

Glossary

Citation

(2011). Crabeater seal. Retrieved from http://www.eoearth.org/view/article/151524

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