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

true seal The earless seals, phocids or true seals are one of the three main groups of mammals within the seal lineage, Pinnipedia. All true seals are members of the family Phocidae (). They are sometimes called crawling seals to distinguish them from th ...

with a circumpolar distribution around the coast of Antarctica. They are medium- to large-sized (over 2 m in length), relatively slender and pale-colored, found primarily on the free-floating pack ice that extends seasonally out from the Antarctic coast, which they use as a platform for resting, mating, social aggregation and accessing their prey. They are by far the most abundant seal species in the world. While population estimates are uncertain, there are at least 7 million and possibly as many as 75 million individuals. This success of this species is due to its specialized predation on the abundant

Antarctic krill Antarctic krill (''Euphausia superba'') is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 ind ...

of the Southern Ocean, for which it has uniquely adapted, sieve-like tooth structure. Indeed, its scientific name, translated as "lobe-toothed (''lobodon'') crab eater (''carcinophaga'')", refers specifically to the finely lobed teeth adapted to filtering their small crustacean prey. Despite its name, crabeater seals do not eat crabs. As well as being an important krill predator, the crabeater seal's pups are an important component of the diet of

leopard seals The leopard seal (''Hydrurga leptonyx''), also referred to as the sea leopard, is the second largest species of seal in the Antarctic (after the southern elephant seal). Its only natural predator is the orca. It feeds on a wide range of prey inc ...

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

Taxonomy and evolution

The genus name of the crabeater seal, ''Lobodon'', derives from Ancient Greek meaning "lobe-toothed", and the species name ''carcinophaga'' means "crab eater." The crabeater seal shares a common recent ancestor with the other Antarctic seals, which are together known as the

lobodontine seals The true seal tribe Lobodontini, collectively known as the Antarctic seals or lobodontin seals, consist of four species of seals in four genera: the crabeater seal (''Lobodon carcinophaga''), the leopard seal (''Hydrurga leptonyx''), the Weddell ...

. These include the leopard seal (''Hydrurga leptonyx''), the Ross seal (''Ommatophoca rossii''), and the Weddell seal (''Leptonychotes weddelli''). These species, collectively belonging to the

Lobodontini The true seal tribe Lobodontini, collectively known as the Antarctic seals or lobodontin seals, consist of four species of seals in four genera: the crabeater seal (''Lobodon carcinophaga''), the leopard seal (''Hydrurga leptonyx''), the Weddell ...

tribe of seals, share teeth adaptations including lobes and cusps useful for straining smaller prey items out of the water column. The ancestral Lobodontini likely diverged from their sister

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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

Description

Adult seals (over five years old) grow to an average length of and an average weight of around . Females are on average longer and around 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. During summer, males typically weigh , and females . A molecular genetic based technique has been established to confirm the sex of individuals in the laboratory. Large crabeater seals can weigh up to . Pups are about in length and at birth. While nursing, pups grow at a rate of about a day, and grow to be around when they are weaned at two or three weeks. 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. Pups are born with a light brown, downy pelage (

lanugo Lanugo is very thin, soft, usually unpigmented, downy hair that is sometimes found on the body of a fetus or newborn. It is the first hair to be produced by the fetal hair follicles, and it usually appears around sixteen weeks of gestation and is ...

), until the first molt at weaning. Younger animals are marked by net-like, chocolate brown markings and flecks on the shoulders, sides and flanks, shading into the predominantly dark hind and fore flippers and head, often due to scarring from

. 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. 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.

Distribution and population

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

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

and New Zealand. They spend the entire year on the pack ice zone as it advances and retreats seasonally, primarily staying within the continental shelf area in waters less than 600 m deep.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. They colonized Antarctica during the late Miocene or early Pliocene (15-25 million years ago), at a time when the region was much warmer than today. The population is connected and fairly well mixed (

panmictic Panmixia (or panmixis) means random mating. A panmictic population is one where all individuals are potential partners. This assumes that there are no mating restrictions, neither genetic nor behavioural, upon the population and that therefore all ...

), and genetic evidence does not suggest any subspecies separations.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. A genetic survey did not detect evidence of a recent, sustained genetic bottleneck in this species, 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 millionScheffer, V. B. (1958). Seals, sea lions and walruses: A review of the Pinnipedia. Stanford University Press, Stanford, USA. to 50-75 million individuals.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. Genetic evidence suggests that crabeater population numbers may have increased during the Pleistocene. The most recent point estimate is 7 million individuals,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. but this, too, is considered a likely underestimate. 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.

Behavior

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

lumbar In tetrapod anatomy, lumbar is an adjective that means ''of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum.'' The lumbar region is sometimes referred to as the lower spine, or as an area of the back i ...

region. This method of locomotion leaves a distinctive sinuous body track and can be extremely effective. When not subject to overheating (i.e. on cold days), speeds on land of have been recorded for short distances.

Satellite tracking A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...

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. 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. Crabeater seals give birth during the Antarctic spring from September to December. Rather than aggregate in reproductive rookeries, females haul out on ice to give birth singly. Adult males attend female-pup pairs until the female begins 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, at which time they are also beginning to molt into a subadult coat similar to the adult pelage. Curiously, crabeater seals have been known to wander further inland than any other pinniped. Carcasses have been found over 100 km from the water and over 1000 m above sea level, where they can be mummified in the dry, cold air and conserved for centuries.

Ecology

Diet

Despite its name, the crabeater seal does not feed on crabs (the few crab species in its range are mostly found in very deep water). Rather, it is a

specialist predator A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources (for example, a heterotroph with a varied diet). A specialist species can thrive only in a narrow range of env ...

(''Euphausia superba''), which comprise over 90% of the diet. Their high abundance is a testament to the extreme success of Antarctic krill, the single species with the greatest biomass on the planet. There is little seasonality in their prey preference, but they may target adult and male krill. Other prey items include

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

and diverse Antarctic fish species. Although the crabeater seal is

sympatric In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...

with the other Antarctic seal species (Weddell, Ross and leopard seals), the specialization on krill minimizes interspecific food competition. Among krill-feeding whales, only blue whales (''Balaenoptera musculus'') and minke whales (''B. acutorostrata'') extend their range as far south as the pack ice where the crabeater seals are most frequent. While no reliable historical population estimates have been done,

population models Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

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.

Predation

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. 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. 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. Predation by killer whales (''Orcinus orca'') is poorly documented, though all ages are hunted. 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. 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.

Gallery

Image:Antarctic (js) 7.jpg Image:Phoque crabier - Crabeater Seal.jpg Two crab-eater seals on the ice, Weddell Sea (4792722735).jpg

References

External links

Voices in the Sea - Crabeater Seal
{{Taxonbar, from=Q626027 Lobodontins Mammals described in 1842 Taxa named by Jacques Bernard Hombron Taxa named by Honoré Jacquinot Pinnipeds of Antarctica Pinnipeds of South America Pinnipeds of Australia Mammals of Western Australia Mammals of South Australia Mammals of New South Wales Mammals of Victoria (Australia) Carnivorans of South America Least concern biota of Oceania Least concern biota of South America