Crabeater seal

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Crabeater seal

Least Concern  (IUCN 3.1)[1]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Clade: Pinnipedia
Family: Phocidae
Genus: Lobodon
Gray, 1844
Species:
L. carcinophaga
Binomial name
Lobodon carcinophaga
Distribution of crabeater seal
Synonyms

Lobodon carcinophagus

The crabeater seal (Lobodon carcinophaga), also known as the krill-eater seal, is a

leopard seals
(H. leptonyx). They are the only member of the genus Lobodon.

Taxonomy and evolution

The

Mirounga (elephant seals) in the late Miocene to early Pliocene, when they migrated southward and diversified rapidly in the relative isolation around Antarctica.[4]

Description

Illustration of the skull

Adult seals (over five years old) grow to an average length of 2.3 m (7.5 ft) and an average weight of around 200 kg (440 lb). Females are on average 6 cm (2.4 in) longer and around 8 kilograms (18 lb) heavier than males, though their weights fluctuate substantially according to season; females can lose up to 50% of their body weight during lactation, and males lose a significant proportion of weight as they attend to their mating partners and fight off rivals.[5] During summer, males typically weigh 200 kilograms (440 lb), and females 215 kilograms (474 lb). A molecular genetic based technique has been established to confirm the sex of individuals in the laboratory.[6] Large crabeater seals can weigh up to 300 kg (660 lb).[7] Pups are about 1.2 metres (3.9 ft) in length and 20 to 30 kilograms (44 to 66 lb) at birth. While nursing, pups grow at a rate of about 4.2 kilograms (9.3 lb) a day, and grow to be around 100 kilograms (220 lb) when they are weaned at two or three weeks.[3][8]

These seals are covered mostly by brown or silver fur, with darker coloration around flippers. The color fades throughout the year, and recently molted seals appear darker than the silvery-white crabeater seals that are about to molt. Their body is comparatively more slender than other seals, and the snout is pointed. Crabeater seals can raise their heads and arch their backs while on ice, and they are able to move quickly if not subject to overheating. Crabeater seals exhibit scarring either from leopard seal attacks around the flippers or, for males, during the breeding season while fighting for mates around the throat and jaw.

leopard seals. After molting, their fur is a darker brown fading to blonde on their bellies. The fur lightens throughout the year, becoming completely blonde in summer.[citation needed
]

Crabeaters have relatively slender bodies and long skulls and snouts compared to other phocids. Perhaps their most distinctive adaptation is the unique dentition that enables this species to sieve Antarctic krill. The postcanine teeth are finely divided with multiple cusps. Together with the tight fit of the upper and lower jaw, a bony protuberance near the back of the mouth completes a near-perfect sieve within which krill are trapped.[3]

Distribution and population

Crabeater seals have a continuous circumpolar distribution surrounding Antarctica, with only occasional sightings or strandings in the extreme southern coasts of

panmictic), and genetic evidence does not suggest any subspecies separations.[10] A genetic survey did not detect evidence of a recent, sustained genetic bottleneck in this species,[11]
which suggests that populations do not appear to have suffered a substantial and sustained decline in the recent past.

Currently, no reliable estimates of the total crabeater seal population are available. Past estimates relied on minimal opportunistic sighting and much speculation, ranging from 2 million[12] to 50–75 million individuals.[13] Genetic evidence suggests that crabeater population numbers may have increased during the Pleistocene.[14] The most recent point estimate is 7 million individuals,[15] but this, too, is considered a likely underestimate.[1] An international effort, the Antarctic Pack Ice Seal initiative, is currently underway to evaluate systematically collected survey data and obtain reliable estimates of all Antarctic seal abundances.[3]

Behavior

Two crabeater seals

Crabeater seals have a typical, serpentine gait when on ice or land, combining retractions of the foreflippers with undulations of the

Satellite tracking data have resulted in conservative estimates of swimming speeds of 66 km/day and 12.7 km/h. While swimming, crabeaters have been known to engage in porpoising (leaping entirely out of the water) and spyhopping (raising the body vertically out of the water for visual inspection) behaviors.[2]

The most gregarious of the Antarctic seals, crabeaters have been observed on the ice in aggregations of up to 1,000 hauled out animals and in swimming groups of several hundred individuals, breathing and diving almost synchronously. These aggregations consist primarily of younger animals. Adults are more typically encountered alone or in small groups of up to three on the ice or in the water.[3]

Crabeater seals give birth during the Antarctic spring from September to December.

estrus one to two weeks after the pup is weaned before mating. Copulation has not been observed directly and presumably occurs in water. Pups are weaned in about three weeks,[17] at which time they are also beginning to molt into a subadult coat similar to the adult pelage.[2]

Curiously, crabeater seals have been known to wander further inland than any other pinniped. Carcasses have been found over 100 km (62 mi) from the water and over 1,000 m (3,300 ft) above sea level, where they can be mummified in the dry, cold air and conserved for centuries.[18]

Ecology

Diet

Illustration of the teeth of crabeater seals, which are used to strain krill from the water

Despite the common name of the species, the crabeater seal does not feed on crabs (the few crab species in its range are mostly found in very deep water

minke whales (B. acutorostrata) extend their range as far south as the pack ice where the crabeater seals are most frequent.[2]
While no reliable historical population estimates have been done, population models suggest crabeater seal populations may have increased at rates up to 9% a year in the 20th century, due to the removal of large baleen whales (especially the blue whale) during the period of industrial whaling and the subsequent explosion in krill biomass and removal of important competitive forces.[21]

Predation

Crabeater seal with visible scars on the neck

Young crabeater seals experience significant predation by leopard seals. Indeed, first-year mortality is exceedingly high, possibly reaching 80%, and up to 78% of crabeaters that survive through their first year have injuries and scars from leopard seal attacks.[1] Long scars and sets of parallel scars, visible on the otherwise pale and relatively unmarked pelage of crabeaters, are present on nearly all young seals. The incidence of visible scars falls off significantly after the first year, suggesting leopard seals primarily target the young of the year.[22] The high predation pressure has clear impacts on the demography and life history of crabeater seals, and has likely had an important role in shaping social behaviors, including aggregation of subadults.[2]

Predation by

killer whales (Orcinus orca) is poorly documented, though all ages are hunted.[23] While most predation occurs in the water, coordinated attacks by groups of killer whales creating a wave to wash the hauled-out seal off floating ice have been observed.[24] Orca packs generally pursue other seals, however, as crabeaters are known to be a tenacious species that make themselves more trouble to catch than they're worth.[citation needed
]

See also

References

  1. ^
    doi:10.2305/IUCN.UK.2015-4.RLTS.T12246A45226918.en
    . Retrieved 19 November 2021.
  2. ^
    S2CID 198968561
    .
  3. ^ a b c d e f g h Bengtson, J. A. (2002). "Crabeater seal Lobodon carcinophaga". In Perrin, W. F.; Wursig, B.; Thiewissen, J. G. M. (eds.). Encyclopedia of marine mammals. London, UK: Academic Press. pp. 290–292.
  4. ^
    S2CID 15187438
    .
  5. doi:10.1017/s0952836902003072
    .
  6. PMID 17548861
    .
  7. Archive-It
    (2011).
  8. doi:10.1111/j.1748-7692.1989.tb00214.x
    .
  9. ^ Burns, J., Costa, D., Fedak, M., Hindell, M., Bradshaw, C., Gales, N., McDonald, B., Trumble, S. & Crocker, D. (2004). Winter habitat use and foraging behavior of crabeater seals along the Western Antarctic Peninsula. Deep-Sea Research Part II: Topical Studies in Oceanography 51 (17-19), 2279-2303.
  10. ^ Davis C, Stirling I, Strobeck C (2000) Genetic diversity of Antarctic pack ice seals in relation to life history characteristics. In: Antarctic Ecosystems: Models for Wider Ecological Understanding (eds Davison W, Howard-Williams C, Broady P), pp. 56-62. The Caxton Press, Christchurch, New Zealand.
  11. S2CID 732351
    .
  12. ^ Scheffer, V. B. (1958). Seals, sea lions and walruses: A review of the Pinnipedia. Stanford University Press, Stanford, USA.
  13. ^ Erickson, A. W., Siniff, D. B., Cline, D. R. and Hofman, R. J. (1971). Distributional ecology of Antarctic seals. In: G. Deacon (ed.), Symposium on Antarctic Ice and Water Masses, pp. 55-76. Sci. Comm. Antarct Res., Cambridge, UK.
  14. S2CID 5323435
    .
  15. ^ Erickson, A. W. and Hanson, M. B. (1990). Continental estimates and population trends of antarctic ice seals. In: K. R. Kerry and G. Hempel (eds), Antarctic Ecosystems. Ecological change and conservation, pp. 253-264. Springer-Verlag, Heidelberg, Germany.
  16. S2CID 7565646
    .
  17. doi:10.1111/j.1365-2664.2007.01399.x
    .
  18. JSTOR 1378440
    .
  19. PMID 23843974
    .
  20. ^ Nicol, S.; Endo, Y. (1997). Fisheries Technical Paper 367: Krill Fisheries of the World. FAO.
  21. ^ Mori, M.; Butterworth, D. (2006). "A first step towards modelling the krill-predator dynamics of the Antarctic ecosystem". CCAMLR Science. 13: 217–277.
  22. JSTOR 1379341
    .
  23. doi:10.1093/icb/31.1.143
    .
  24. doi:10.1139/z81-167
    .

External links

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