Crabeater Seal (marine mammals)


The crabeater seal may be the most abundant pinniped in the world, existing in the millions around Antarctica. The scientific name is derived from Greek and means “lobed tooth” (Lobodon) “crab eater” (carcinophaga).

I. Diagnostic Characters and Comments on Taxonomy

Adult crabeater seals are generally about 205 to 240 cm long, with some older individuals reaching up to 260 cm in length. Females may attain a slightly larger size than males. During the summer molting period, adults typically exhibit weights in a range of about 180 to 225 kg. Pups weigh about 35 kg at birth, but can grow to more than 100 kg by the time they are weaned.

The pelts of crabeater seals usually have medium brown to silver hair over most of their body, although darker coloration and spotting are not uncommon on the front and rear flippers and flanks (Fig. 1). Because hair fades in color throughout the year, recently molted seals may appear darker than those about to begin their molt, whose pelts can appear silvery white. The body form is relatively slender compared to other phocids. and crabeater seals’ faces have a somewhat pointed snout. Crabeater seals have a high incidence of obvious scarring on their bodies, mostly caused by leopard seal (Ht/dmrgn leptonyx) attacks (Fig. 2). Adults typically also have small scars from bites around their front and rear flippers (both sexes) and around their lower jaws and throat (mostly males) from intraspecific interactions during the breeding season.

Crabeater seals are highly mobile on ice and, when disturbed, often raise their heads and arch their backs. They can move surprisingly quickly over ice and snow, and on a cold day (when not subject to overheating) they may be capable of outrunning a fit human.

II. Distribution/Range Map

Crabeater seals have a circumpolar Antarctic distribution, spending the entire year in the pack ice zone as it seasonally advances and retreats. Occasionally, crabeater seals are found along the southern fringes of South America, Australia, New Zealand, and Africa, but such sightings or strandings are rare. Genetic analyses suggest that the circumpolar crabeater seal population is panmictic; there are no known subspecies.

Crabeater seal head and shoulders illustrating spotting around front flippers.

Figure 1 Crabeater seal head and shoulders illustrating spotting around front flippers.

Most crabeater seals possess long, raking scars on their torsos resulting from attacks in their first year of life by leopard seals.

Figure 2 Most crabeater seals possess long, raking scars on their torsos resulting from attacks in their first year of life by leopard seals.

III. Ecology

There is presently no reliable estimate of the total abundance of crabeater seals. Estimates have ranged from 2 to 75 million individuals, although many scientists currently consider a population estimate in the range of 10-15 million may be reasonable. The observed densities of crabeater seals censused in 1983 were lower than densities observed in the late 1960s and early 1970s (4.3 versus 11.4 seals per nm” in the Weddell Sea and 1.9 versus 4.9 seals per nm” in the Pacific Ocean Sector, respectively). However, it is unclear whether these differences in densities reflected a change in population abundance or a shift in distribution within the sea ice zone. An international research initiative, the Antarctic Pack Ice Seals (APIS) Program, was undertaken in the late 1990s to refine estimates of the abundance and distribution of crabeater seals.

In their first year, crabeater seals experience a surprisingly high mortality rate that may be as high as 80%, which is perhaps double that which might normally be expected. For the approximately 20% of crabeater seals that survive past their first birthday, as many as 78% exhibit large, raking scars on their bodies resulting from previous attacks by leopard seals, suggesting that leopard seals may have a significant negative impact on crabeater seal populations. Most attacks by leopard seals on crabeater seals occur in the crabeater seals” first vear; fresh wounds, indicating a recent attack, are rarely seen on crabeater seals that are older than 1 year.

Studies of crabeater seal diet have shown that these seals depend almost exclusively on Antarctic krill (Euphausia superba). Most investigators have reported that krill comprise over 95% of the crabeater diet, with the remainder being made up of small quantities of fish and squid. As specialist krill predators, crabeater seals do not seem to switch their prey seasonally.

IV. Behavior

Crabeater seals migrate over large distances in association with the annual advance and retreat of the pack ice. Although they can be found anywhere within the pack ice zone, it is typical to find higher densities of crabeater seals in the marginal ice zone as well as in pack ice that is present near or over the continental shelf. In a peculiar behavioral twist, crabeater seals likely hold the record for any pinniped wandering inland from the coast. Carcasses have been found up to 113 km from open water and as high as 1100 m above sea level. Seals that wander inland become lost and die, may eventually become mummified in the cold, dry Antarctic air, and can remain in this “freeze-dried” state for many decades or centuries.

Similar to other Antarctic pack ice seals, crabeater seals exhibit a daily haulout pattern that generally involves hauling out on ice floes during the middle of the day. However, only about 80% of crabeater seals haul out simultaneously on the ice, even during the height of the molting period in January and February. Haulout patterns also vary markedly among seasons, with as few as 40% of seals hauling out at the peak of daily haulout during winter months.

During daily foraging periods, which normally occur during the night, crabeater seals dive nearly continuously for periods of up to 16 hr. In one study, a single crabeater seal continued diving for 44 hr without interruption. Foraging dives made during crepuscular periods are often deeper than those made during the darkest hours, suggesting that the seals may prefer dark conditions when catching their principal prey, Antarctic krill.

V. Anatomy, Physiology, and Life History

Crabeater seals have finely divided, lobed teeth, presumably an adaptation to their specialized diet on krill. The multiple cusps of upper and lower postcanine teeth interlock to form a sieve that can be used to filter crustaceans from seawater. A bony protrusion on the lower jaw behind the most posterior postcanine tooth fills the gap in this sieve so that prey cannot escape at the rear of the mouth.

During the breeding season, crabeater seals form “family groups,” consisting of a female, her pup, and an attendant male who guards the female from other males until she completes lactation. Pups are born in September and October, with a light brown lanugo that is molted at about 2 weeks of age. Following weaning, the attendant male and the female form a “mated pair” and remain together for an estimated 1 to 2 weeks or until copulation. Females without pups also form mated pairs as they come into estrus. Crabeater seals can live up to 40 years, but adults dying at about 20-25 years is more typical.

VI. Interactions with Humans

Crabeater seals experienced an incident of mass mortality in 1955 in the vicinity of an Antarctic base where sledge dogs were active. Up to 97% of mixed-age aggregations died during that event. It was speculated that a viral infection may have been associated with the die-off, and circumstantial evidence suggests that it may have been caused by a distemper-like virus. Blood samples taken from crabeater seals in the late 1980s confirmed that populations of crabeater seals along the Antarctic Peninsula had antibodies similar to those related to canine distemper and phocine distemper, viruses that were responsible for major epizootic die-offs of harbor seals in the Northern Hemisphere in the late 1980s.

Crabeater seals were harvested commercially twice during the past century: in 1964/1965 by Norway and in 1986/1987 by the former Soviet Union. In both cases, the sealing ventures were judged to be economically unsuccessful. However, the concern generated by the earlier harvest was sufficient to mobilize an international effort to prevent potential overexploita-tion of the seals. This concern resulted in the Convention for the Conservation of Antarctic Seals, which came into effect in 1978 and provides international oversight for the conservation and management of crabeater seals throughout their range.

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