Introduction to Crabeater Seals
|Crabeater seal on iceberg|
The crabeater seal (Lobodon carcinophaga) is the most abundant seal species on Earth. Some scientists believe they are more numerous than all other seal species, combined. It is a "true" seal or "earless" seal and is a perfectly adapted denizen of Antarctica, living almost exclusively amidst the Antarctic pack ice. Ironically, crabeater seals do not eat crabs (there are no crabs in Antarctic waters), but consume more krill (shrimp-like crustaceans) than any other species on the planet. Misnamed by the early Antarctic whalers and sealers, crabeaters might more accurately be called the "krilleater" seal. The crabeater seal’s abundance is due, in large part, to the slaughter of the great baleen whales in Antarctic waters, which reduced whale feeding pressure on the krill and thus made krill more available to seals and penguins.
Identifying Crabeater Seals—Physical Characteristics
Crabeater seal adults are relatively slender and pale-colored, with an average length of 7–8 feet (2.5m) and weight of 450 pounds (200kg). Females are slightly larger. The crabeater seal’s skull and snout are longer than those of other Antarctic seals, often giving their face a dog-like appearance.
The fur of the seal lightens throughout the year, becoming uniformly blonde in summer, and lacks any spots or blotches. They molt in January–February. Older animals become progressively paler in color and may appear almost white—they were given the name "white seal" by some sealers and naturalists in the late 1800s. Adult crabeater seals are frequently heavily scarred, having survived attacks from leopard seals and orcas, especially when they are young.
Distribution and Migration of Crabeater Seals
|Crabeater seal entering the water|
Crabeater seals dwell along the circumpolar pack ice at the edge of the Antarctic continent and adjacent islands. They are remarkably agile on land and are sometimes found far inland—juveniles will accidentally travel towards the interior of the Antarctic continent on occasion. Their remains, mummified by the drying winds of Antarctica’s remarkable dry valleys, have been found over 60 miles inland. Presumably, crabeater seals migrate in search of food during the Antarctic winter but their pattern of movement is still unknown. Infrequently, wandering crabeaters are seen at sub-Antarctic islands and, rarely, a few end up on the southern coasts of South America, Africa, Australia and New Zealand.
Crabeater Seal Habitat
Crabeater seals spend the entire year on and around the Antarctic ice pack, an enormous ice boundary that advances and retreats around the continent seasonally.
Crabeater Seal Diet
Krill comprise 90 percent of the crabeater seal’s diet, with cephalopods and Antarctic fish making up the rest of their diet. While many Antarctic birds and mammals eat krill, crabeater seals are krill specialist feeders, with a unique adaptation for filtering the tiny crustaceans from seawater. Their ornate teeth are five-pointed with interlocking upper and lower rows that form a strainer, a structure that whose function (but not form) is similar to the baleen plates of whales.
The seals can dive continuously for up to 16 hours a day, in short 5-minute dives for food. They feed by swimming through massive krill swarms with their mouth open wide, sieving the seawater out using their complex teeth. Most of the seven Antarctic krill species rise near the ocean surface at night and retreat to deeper water during the day. Crabeaters have adapted to this behavior by feeding mainly at night. Threats to Antarctic krill stocks due to overfishing and climate change may have a future impact on the population of crabeater seals.
Crabeater Seal Behavior
|Hauled out crabeater seals|
Using their foreflippers and by undulating their lumbar region, crabeater seals attain a distinctive gait, and their ability to traverse ice or land allows them to reach speeds of 12–16 mph (19–26km/h). They can outpace humans for short distances. In the water, crabeater seals can be seen porpoising and spyhopping. They usually haul out on the pack ice and rest midday.
Crabeater seals experience significant predation by leopard seals, losing up to 80 percent of the seal pups their first year. Away from the pupping area, leopard seals usually attack sub-adult crabeater seals, avoiding the adults. Sometimes crabeaters can be found in aggregations of over 1,000 animals. Their significant scars from leopard seal attacks may explain why they haul out together in larger numbers than other seals—the "safety in numbers" strategy may make leopard seals less likely to attack. Orcas are another significant crabeater predator. Orcas have been observed creating shock waves to swamp an ice floe where a crabeater seal is hauled out, knocking the seal into the water.
Crabeaters are relatively shy and tend to retreat into the water if a boat approaches.
Crabeater Seal Breeding and Mating
Crabeater seals give birth during the spring (late September–early November), with most births occurring during October. Unlike many other seals that give birth in colonies, the crabeater seal hauls out on the ice to give birth alone. Following the birth an adult male will attend the female–pup pair. The pup nurses for three weeks. At birth, newborn crabeater seals weigh approximately 44 pounds (20kg) and by the time they wean have grown more than five times in size to 240 pounds (110kg). Pups are born with a light brown downy "lanugo" coat, until the first molt at weaning.
|A pair of crabeater seals at ice edge|
Females fast throughout the nursing period, losing up to 50 percent of their body weight. Males, likewise, lose a significant proportion of their weight as they attend to their mating partner and fight off rivals. Males become very aggressive in defending their female.
The female is ready to mate shortly after weaning. Though most ice-breeding seals mate in the water, research has showed most crabeater seals mate on the ice. After mating the male leaves and goes to find another receptive female.
Females and males achieve sexual maturity at 2–6 years; the actual breeding age may be linked to food abundance.
Most adult crabeater seals live 20–25 years, though they have been known to live as long as 40 years.