Crabeater Seal
Encyclopedia
The crabeater seal, Lobodon carcinophagus, is a true seal 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
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 (carcinophagus)", refers specifically to the finely-lobed teeth adapted to filtering their small crustacean
prey. As well as an important krill predator, the crabeater seal is an important component of the diet of leopard seals (Hydrurga leptonyx), which consume about 80% of all crabeater pups.
(Ommatophoca rossii), leopard seal
(Hydrurga leptonyx) and Weddell seal
(Leptonychotes weddelli). These species, collectively belonging to the Lobodontini 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 its sister clade
, the Mirounga (elephant seals) in the late Miocene
to early Pliocene
, when they migrated southward and diversified rapidly in relative isolation around Antarctica.
, Africa
, Australia
and New Zealand
. They spend the entire year on the pack ice zone as it advances and retreats seasonally, primarily staying within in the continental shelf area in waters less than 600m deep. 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), and genetic evidence does not suggest any subspecies separations.
There are no current reliable estimates of the total crabeater seal population. Past estimates relied on minimal opportunistic sighting and much speculation, ranging from 2 million to 50-75 million individuals. The most recent point estimate is 7 million individuals but this, too, is considered a likely underestimate. An international effort, the Antarctic Pack Ice Seal initiative (APIS), is currently underway to evaluate systematically collected survey data and obtain reliable estimates of all Antarctic seal abundances.
Pups are born with a light brown, downy pelage (lanugo
), 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 leopard seals. After molting, the fur of the crabeater seal is a darker brown fading to blonde on its belly. The fur lightens throughout the year, becoming completely blonde in summer. Older animals become progressively paler, even when freshly molted, and may appear almost white.
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.
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 19–26 km/h (12-16 mph) have been recorded for short distances. 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.
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 estrous 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 approximately 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.
Despite its name, the crabeater seal does not feed on crabs, which are not found in its Antarctic habitat. Rather, the crabeater seal is a specialist predator on Antarctic krill
(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 and diverse Antarctic fish species. Although the crabeater seal is sympatric 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 seal are most frequent.
While there are no reliable historical population estimates, 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.
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.
Image:Antarctic (js) 7.jpg
Image:Crabeater Seals (js).jpg
Image:Antarctic, Crabeater Seal (js) 9.jpg
Image:Phoque crabier - Crabeater Seal.jpg
Antarctic krill
Antarctic krill, Euphausia superba, is a species of krill found in the Antarctic waters of the Southern Ocean. It is a shrimp-like crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre...
of the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
, for which it has uniquely adapted sieve-like tooth structure. Indeed, its scientific name, translated as "lobe-toothed (lobodon) crab eater (carcinophagus)", refers specifically to the finely-lobed teeth adapted to filtering their small crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
prey. As well as an important krill predator, the crabeater seal is an important component of the diet of leopard seals (Hydrurga leptonyx), which consume about 80% of all crabeater pups.
Taxonomy and evolution
The crabeater seal shares a common recent ancestor with the other Antarctic seals, which are together known as the lobodontine seals. These include the Ross sealRoss Seal
The Ross seal is a true seal with a range confined entirely to the pack ice of Antarctica. It is the only species of the genus Ommatophoca. First described during James Clark Ross' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic...
(Ommatophoca rossii), leopard seal
Leopard Seal
The leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...
(Hydrurga leptonyx) and Weddell seal
Weddell Seal
The Weddell seal , is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound...
(Leptonychotes weddelli). These species, collectively belonging to the Lobodontini tribe
Tribe (biology)
In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...
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 its sister clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, the Mirounga (elephant seals) in the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
to early Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
, when they migrated southward and diversified rapidly in relative isolation around Antarctica.
Distribution and population
Crabeater seals have a continuous circumpolar distribution surrounding Antarctica, with only occasional sightings or strandings in the extreme southern coasts of South AmericaSouth America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. They spend the entire year on the pack ice zone as it advances and retreats seasonally, primarily staying within in the continental shelf area in waters less than 600m deep. They colonized Antarctica during the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
or early Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
(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), and genetic evidence does not suggest any subspecies separations.
There are no current reliable estimates of the total crabeater seal population. Past estimates relied on minimal opportunistic sighting and much speculation, ranging from 2 million to 50-75 million individuals. The most recent point estimate is 7 million individuals but this, too, is considered a likely underestimate. An international effort, the Antarctic Pack Ice Seal initiative (APIS), is currently underway to evaluate systematically collected survey data and obtain reliable estimates of all Antarctic seal abundances.
Physical description
Adult seals (over five years old) grow to an average length of 230 cm (7.6 ft) and an average weight of around 200 kg (440 lbs). Females are on average 6 cm (2.5 in) longer and around 8 kg heavier than males on average, 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. Large crabeater seals can weigh up to 300 kg (660 lbs). Pups are about 120 cm in length and 20 and 30 kilograms at birth. While nursing, pups grow at a rate of about 4.2 kilograms a day, and can weigh as much as 100 kg by the time they are weaned after only two to three weeks.Pups are born with a light brown, downy pelage (lanugo
Lanugo
Lanugo is fine, downy hair as a type of fur. It is often found in teratomas .-Fetal development:Lanugo grows on fetuses as a normal part of gestation, but is usually shed and replaced by vellus hair at about 33 to 36 weeks of gestational age...
), 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 leopard seals. After molting, the fur of the crabeater seal is a darker brown fading to blonde on its belly. The fur lightens throughout the year, becoming completely blonde in summer. Older animals become progressively paler, even when freshly molted, and may appear almost white.
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.
Behavior
Crabeater seals have an atypical, serpentine gait when on ice or land, combining retractions of the foreflippers with undulations of the lumbarLumbar
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 ...
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 19–26 km/h (12-16 mph) have been recorded for short distances. 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.
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 estrous 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 approximately 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.
Diet
Antarctic krill
Antarctic krill, Euphausia superba, is a species of krill found in the Antarctic waters of the Southern Ocean. It is a shrimp-like crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre...
(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 and diverse Antarctic fish species. Although the crabeater seal is sympatric 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 seal are most frequent.
While there are no reliable historical population estimates, population models
Population dynamics
Population dynamics is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes...
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
Whaling
Whaling is the hunting of whales mainly for meat and oil. Its earliest forms date to at least 3000 BC. Various coastal communities have long histories of sustenance whaling and harvesting beached whales...
and the subsequent explosion in krill biomass and removal of important competitive forces.
Predation
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.
Gallery
Image:Antarctic (js) 7.jpg
Image:Crabeater Seals (js).jpg
Image:Antarctic, Crabeater Seal (js) 9.jpg
Image:Phoque crabier - Crabeater Seal.jpg
See also
Other Antarctic seal species include:- Leopard sealLeopard SealThe leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...
(Hydrurga leptonyx) - Ross sealRoss SealThe Ross seal is a true seal with a range confined entirely to the pack ice of Antarctica. It is the only species of the genus Ommatophoca. First described during James Clark Ross' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic...
(Ommatophoca rossii) - Weddell sealWeddell SealThe Weddell seal , is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound...
(Leptonychotes weddellii)