Crabeater seal (Lobodon carcinophaga)

Description

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Crabeater seal fact file

• Description
• Biology
• Range
• Habitat
• Status
• Threats
• Conservation
• Find out more
• Glossary
• References
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Crabeater seal description

With an estimated population of around 15 million individuals, the crabeater seal (Lobodon carcinophaga) is by far the most numerous pinniped and is possibly the most abundant large mammal in the world ^{(2) (3) (4)}. Somewhat misleadingly, the crabeater seal does not actually eat crabs, instead feeding almost exclusively on Antarctic krill (Euphausia superba) ^{(1) (2) (3) (4)}.

The crabeater seal has a slender, streamlined body ^{(2) (3) (4)}, a distinct neck ⁽⁴⁾, and a slim face with a relatively pointed snout ^{(2) (3)}. In general, the crabeater seal is brown or silvery-grey on the upperparts of the body, becoming lighter brown to almost blonde on the underside ⁽²⁾. The flippers are mainly brown, sometimes marked with spots, and are usually the darkest areas of the body ⁽³⁾. Chocolate-brown patterns or dark flecks and spots also mark the shoulders, sides, flanks ^{(2) (4)}.

The colouration of the crabeater seal varies depending on the time of year, being darkest after the summer moult in January and February. The colour of the coat fades and becomes progressively paler throughout the year, changing to almost blonde, golden or creamy-white by the following summer ^{(3) (4)}.

Newborn crabeater seal pups have dark flippers and a pale coffee-coloured, white or grey-brown coat which is soft and woolly. The belly and the muzzle may have a yellowish tinge ^{(1) (4)}.

As juveniles, crabeater seals experience astonishingly high mortality rates of up to 80 percent, usually as a result of frequent attacks by leopard seals (Hydrurga leptonyx). Of the seals that survive their first year, nearly all individuals end up with pairs of long, parallel scars on their bodies inflicted by leopard seals ^{(1) (2) (4)}. Adult crabeater seals also typically have small scars around their front and rear flippers as result of aggressive displays and fighting. Often, female seals will have scars on their backs caused by male seals during mating, while males usually have a series of scars on their lower jaw and throat due to fights with other males during the breeding season ⁽²⁾.

Synonyms

Lobodon carcinophagus.

Size

Length: up to 260 cm ^{(1) (2)}

Weight

200 - 300 kg ^{(1) (3)}

Crabeater seal biology

The crabeater seal has remarkable agility on snow and ice, and is one of the quickest seal species when it moves on land ⁽³⁾. It moves using a almost serpentine (snake-like) gait, thrusting its forelimbs alternately against the snow, moving its pelvis from side to side and keeping its hind limbs held together off the ground ^{(3) (4)}. The crabeater seal’s method of travelling leaves long, distinctive tracks along the ground as its body is dragged through the snow, with characteristic alternating flipper imprints on either side ⁽⁴⁾.

Contrary to its name, the crabeater seal very rarely feeds on crabs. Instead, Antarctic krill (Euphausia superba) makes up around 95 percent of the crabeater seal’s prey, while the remainder of its diet is comprised of cephalopods, fish and crustaceans ^{(1) (2) (4)}.

The crabeater seal has specialised, finely lobed teeth which are thought to be an adaptation to its almost exclusively krill-based diet. In fact the crabeater seal’s scientific name, Lobodon, is derived from Greek words meaning ‘lobed tooth’ ⁽²⁾. The teeth interlock to form a sieve through which the krill are filtered, and a ridge of bone fills the gap between the teeth and the back of the jaw, stopping prey from escaping from the mouth while feeding ^{(1) (2) (3) (4)}. The crabeater seal feeds mostly at night ^{(1) (3)}, making fairly deep dives in search of its prey ⁽¹⁾. During the day the crabeater seal will rest, hauled out on ice floes ^{(1) (2) (4)}.

Generally, the crabeater seal spends most of its time alone or in small groups. However, much larger groups of seals, sometimes numbering around 1,000 individuals, have been observed to haul out on ice floes. This is particularly common during the annual moult in January and February. Herds of up to 500 individuals have also been known to swim and dive together ⁽¹⁾. During the spring, mature and juvenile crabeater seals will segregate, with juveniles forming large aggregations on the land while the mature adults remain on the pack ice for the breeding season ⁽⁴⁾.

The breeding season of the crabeater seal is fairly short ⁽¹⁾. Mating takes place from October to December and occurs on the ice ⁽³⁾. The crabeater seal has a fairly long gestation period of around 11 months, which probably involves a period of delayed implantation ^{(3) (4)}.

The female gives birth to a single pup between September and November the following year, with a peak in births around mid-October ⁽¹⁾. The female is usually joined by a male just before the birth, who will protect the female and the newborn pup from other males and potential predators ^{(2) (3) (4)}. The pup remains in close contact with the female until it is weaned, which usually occurs around three or four weeks after birth. During this time, the female will prevent the male from attempting to mate with her by using aggressive vocalisations, biting the male around the mouth and flippers, and chasing it away ^{(1) (4)}. However, once the pup is weaned, the female will come back into oestrus and the male will forcefully drive the female and pup apart before mating with the female ⁽³⁾.

At birth, the crabeater seal pups are around 1.1 to 1.5 metres long and weigh between 20 and 40 kilograms ^{(1) (3)}. They grow rapidly, weighing more than 100 kilograms by the time they are weaned ⁽²⁾. The young are nearly fully grown after two years, although sexual maturity is not attained until they are between three and six years old ⁽³⁾.

Crabeater seal range

The crabeater seal is distributed throughout the Antarctic region ^{(1) (2) (3) (4)}. It is found primarily along the coasts and pack ice of Antarctica ⁽³⁾, but some individuals are known to wander as far as South America, South Africa, Australia, Tasmania and New Zealand ^{(1) (2)}.

Crabeater seal habitat

Populations of the crabeater seal are generally concentrated around Antarctic pack ice ^{(1) (3)}, south of the Antarctic Convergence where cold Antarctic waters meet the warmer northern waters ⁽⁴⁾.

During the winter, the pack ice covers an exceptionally large area and the seal’s range may extend to over 22 million square kilometres. In the summer, the pack ice retreats, confining the crabeater seal and other Antarctic species to a much smaller area ⁽³⁾.

Crabeater seal status

The crabeater seal is classified as Least Concern (LC) on the IUCN Red List ⁽¹⁾.

Crabeater seal threats

In the past, the crabeater seal was targeted for commercial harvesting, although exploitation of this species ceased when it was determined to be economically unsuccessful. However, commercial harvest of krill may in future limit the availability of the crabeater seal’s primary prey ⁽¹⁾. The development of a major krill fishery could impact severely on crabeater seal populations and may potentially alter the whole Antarctic ecosystem if large-scale harvesting becomes established ^{(1) (3)}. The crabeater seal may also be particularly vulnerable to loss of krill as result of recovering whale populations in the Antarctic, as they compete with the seal for food ⁽³⁾.

Disease, particularly canine distemper virus, has been identified as a threat to seal populations in the Antarctic and has the potential to cause mass die-offs if outbreaks occur. The possibility of disease outbreaks may rise with increasing tourism to the region, as well as with the effects of climate change. The impact of climate change on populations of seals in Antarctica is currently poorly understood, although initial studies suggest that the number of crabeater seals may decline as temperatures increase and pack ice is reduced. The loss of pack ice is likely to have a major influence on crabeater seal populations as it is an important habitat for breeding, resting and avoiding predators. Changes in sea ice could also affect this species’ access to its preferred foraging areas ⁽¹⁾.

Tourism may further affect the crabeater seal due to the effects of disturbance and noise from passing vessels, and by the approach of tourists close to groups of these seals on land. There is a fairly small risk of injury to the crabeater seal as a result of collision with boats ⁽¹⁾.

Crabeater seal conservation

The crabeater seal is protected by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals, which aims to promote protection, scientific study and regulated use of seals in the Antarctic ⁽⁵⁾. Under these international agreements, any potential future commercial harvest of seals would have to be carefully regulated ^{(1) (5)}.

View information on this species at the UNEP World Conservation Monitoring Centre.

Find out more

Find out more about conservation in the Antarctic:

• British Antarctic Survey:
  http://www.antarctica.ac.uk/
• British Antarctic Survey - Convention for the Conservation of Antarctic Seals:
  http://www.antarctica.ac.uk/about_antarctica/geopolitical/treaty/update_1972.php
• Antarctic Treaty System:
  http://www.ats.aq/
• Discovering Antarctica:
  http://www.discoveringantarctica.org.uk/

Visit ARKive’s Antarctic ecoregion page:

• ARKive - Antarctic:
  http://www.arkive.org/eco-regions/antarctic/

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Glossary

Cephalopods

A group of marine molluscs with grasping tentacles and either an internal or external shell. Includes squids, octopuses, cuttlefish and nautiloids.

Crustaceans

Diverse group of animals with jointed limbs and a hard external skeleton, characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.

Delayed implantation

The process of a fertilised egg remaining unattached in the uterus for a period of time, therefore delaying the start of development.

Gestation

The state of being pregnant; the period from conception to birth.

Moult

Periodic shedding of (usually) the outermost body covering (such as feathers, fur or skin) during growth and development, or at specific times of the year.

Oestrus

The time of ovulation (release of an egg from the ovary) in female mammals, when the female becomes receptive to males. Also known as ‘heat’.

Pack ice

Sea ice that floats on the surface of the water. Often formed from large pieces of ice that consolidate into a single ice mass, pack ice typically moves with currents, tides and wind.

Pinniped

A carnivorous, aquatic mammal belonging to the Pinnipedia, which includes the seals, sea lions and walruses. All have four limbs modified into flippers.

References

1. IUCN Red List (March, 2012)
   http://www.iucnredlist.org/
2. Perrin, W.F., Würsig, B. and Thewissen, J.G.M. (Eds.) (2002) Encyclopedia of Marine Mammals. Academic Press, San Diego, California.
3. Nowak, R.M. (1991) Walker’s Mammals of the World. The Johns Hopkins University Press, Baltimore and London.
4. Adam, P.J. (2005) Lobodon carcinophaga. Mammalian Species, 772: 1-14. Available at:
   http://www.science.smith.edu/msi/pdf/772_Lobodon_carcinophaga.pdf
5. British Antarctic Survey (March, 2012)
   http://www.antarctica.ac.uk/