For Crabeater seals, the head and muzzle are moderately long and slim relative to the animal's overall size. The eyes are set fairly far apart and the head tapers to the base of the straight muzzle, forming a slight forehead in profile. The nostrils are on top of the muzzle, just back from the end, and in profile can look slightly enlarged, giving the animal a slight tipped up appearance to the end of the muzzle. The effect of this appearance is enhanced by the crabeater’s tendency to raise the head and end of the muzzle to the level of the eye or higher when disturbed. The line of the mouth is virtually straight from the gape to the end of the muzzle. The vibrissae are short, pale to clear, and inconspicuous.
The fore flippers are long, wide, and somewhat sickle-shaped, tapering to a rounded end, and are similar to those of otariids, but are fully furred. The first digits are elongated and robust, and the fifth digit is reduced. Crabeater seals can spread the digits of the fore flippers while swimming and stretching, greatly enlarging the surface area. Many crabeaters bear long dark scars, either singly or as a parallel pair that are from attacks by leopard seals. Many older animals have numerous smaller scars and injuries to the face and sides of the mouth and head, presumably from intraspecific fighting.
The coat of a freshly molted crabeater has a rich sheen, and can be light to dark shades of colors ranging from silvery gray to tawny brown. Irregular patches of spots and rings can be on the shoulders, sides, tops of the flippers, and around the insertion of the flippers. These markings produce a reticulated, or web-like, pattern on the sides of many crabeaters. The flippers can be so heavily marked with spots and rings that they appear darker than the rest of the body. As time progresses from the molt, crabeaters fade dramatically, becoming very pale gray, tan, or whitish. As these seals become older they become paler overall, and some look faded all year long. Pups are born with a soft woolly coat that is grayish-brown in color and has been described as light, milk coffee brown, with darker coloring on the flippers. Molt begins in about 2-3 weeks and the pup sheds into a subadult pelage similar to that of the adult.
Adults reach 2.6 m in length and weigh an estimated 200-300 kg, although little data are available on this species. Neonates are thought to be at least 1.1 m and 20-40 kg.
The dental formula is I 2/2, C 1/1, PC 5/5. All of the post-canine teeth are ornate, with multiple accessory cusps. Upper and lower teeth interlock to form a network for straining krill from the seawater. A ridge of bone on each mandible fills the gap in the mouth behind the last upper postcanine teeth and helps prevent the loss of krill from the back of the mouth when feeding.
Can be Confused With
Crabeater seals are most likely to be confused with leopard, Weddell, Ross, or young elephant seals. Leopard seals have a much larger, almost reptilian-looking head, with a very broad muzzle. Weddell seals have a small round head with a proportionately long and heavy-set body, and distinctive spots over the entire body. Ross seals have a wide head with a short muzzle, giving the snout a blunt appearance. Also, distinctive stripes originating from the head, lower jaw, and thick neck run posteriorly along the length of the body. The presence of long scars and sets of parallel scars are more common and readily visible on the pale, relatively unmarked pelage of crabeaters than on the other Antarctic phocids. Crabeater feces are routinely a pinkish red, from their diet of predominantly krill, and reddish stains are frequently seen on the ice near where they are hauled-out. Also, of all the Antarctic phocids, only crabeaters occur routinely in groups in the water or on ice floes. Young southern elephant seals of similar size are medium to dark gray or brownish, lack any markings, and have large wide heads with large eyes and prominent dark vibrissae.
The distribution of crabeater seals is tied to seasonal fluctuations of the pack ice. They can be found right up to the coast of Antarctica, as far south as McMurdo Sound, during late summer ice break-up, and as vagrants as far north as New Zealand and the southern coasts of Africa, Australia, and South America. Crabeaters have been known to wander far inland and die in the dry valleys adjacent to McMurdo Sound. A live animal was found 113 km from open water at an elevation of 920 m above sea level, and carcasses have been found as high as 1,100 m above sea level.
Ecology and Behavior
Pups are born from September to December, and weaned in approximately three weeks. There are no specific rookeries; females haul-out on ice singly to give birth. Adult males attach themselves to female pup pairs and stay with the female until her estrous one to two weeks after the pup is weaned. Mating has not been witnessed and presumably occurs in the water. Females are reported to bite males around the mouth and flippers and this may account for the abundant small scars on the faces of older males. Mortality is high in the first year and may reach 80%. Much of this mortality is attributed to leopard seal predation, and up to 78% of crabeaters that survive through their first year have injuries and scars from leopard seal attacks. Leopard seal attacks appear to fall off dramatically after crabeaters reach one year of age.
Crabeaters are frequently encountered alone or in small groups of up to three on the ice or in the water. However, much larger groups of up to 1000 hauled-out together have been observed. They can be seen swimming together in herds estimated to be up to 500 animals, breathing and diving almost synchronously. The molt is in January and February. They have a pattern of feeding from dusk until dawn, and hauling-out in the middle of the day. A large proportion of the animals in an area routinely haul-out during the annual molt. Crabeaters frequently use their fore flippers for propulsion, and surface and roll forward to begin a dive. They are known for their ability to move rapidly on ice, with sinuous serpentine motions of the back, aided by the flippers. When agitated, their response is to arch their back and raise their neck and head in an alert posture.
Recent research has revealed that crabeater seals can dive to 430 m and stay submerged for 11 minutes, although most feeding dives were to 20-30 m, and shorter. Foraging occurs primarily at night, and instrumented seals have been recorded to regularly dive continuously for periods up to 16 hours. Dives at dawn and dusk are deeper than at night, and indicate that crabeater feeding reflects the daily vertical migrations of krill.
Feeding and Prey
Crabeater seals feed primarily on Antarctic krill, Euphausia superba, and 95% of their diet may be made up of this species. Small amounts of fish and squid are also part of the diet.
Threats and Status
Crabeater seals are considered to be the most abundant seal species and one of the most numerous large mammals on earth. Population estimates range from two to 75 million animals. There is no recent population estimate available, but the most widely-used estimates are of 10-15 million animals. If correct, this would mean that there are more crabeater seals than the populations of all other pinniped species combined.
A mass die-off was reported from an area near a base on the Antarctic Peninsula in 1955. About 3,000 animals were trapped in areas 5-25 km from open water and most died over a two to three month period. None of the animals examined appeared to be starving, and numerous abortions of fetuses were noted. A disease outbreak was suspected, but never identified. In the 1980s a study revealed antibodies to a distemper-like virus in crabeater seals from this area. Outbreaks of distemper virus in the North Atlantic led to mass die-offs of harbor and gray seals in the 1980s.
Several brief episodes of commercial harvesting ended when they were determined to be economically unsuccessful. Commercial harvest of krill may pose a threat to crabeater seals, if conducted on a large scale. There are currently no direct threats from human activity throughout most of the species’ normal range. The effect of global climate change on the formation and extent of Antarctic pack ice, and crabeater seal prey, poses an unknown threat to this ice dependent species. Crabeater seals are protected by the Antarctic Treaty, and the Convention for the Conservation of Antarctic Seals.