The crabeater seal ( Lobodon carcinophaga ), also known as the krill-eater seal, is a true seal with a circumpolar distribution around the coast of Antarctica. They are the only member of the genus Lobodon. The crabeater seal is a medium-sized, light-coloured seal with a gentle, dog-like muzzle. Beneath their unassuming appearance lies a remarkable adaptation: specialised, sieve-like teeth perfectly designed for filtering krill from the frigid Antarctic waters. Crabeater seals are just one of the many species that make Antarctica such an unforgettable destination.
Appearance and Identification
Adult seals (over five years old) grow to an average length of 2.3 m (7.5 ft) and an average weight of around 200 kg (440 lb). Large crabeater seals can weigh up to 300 kg (660 lb). Females are on average 6 cm (2.4 in) longer and around 8 kilograms (18 lb) 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 200 kilograms (440 lb), and females 215 kilograms (474 lb). Pups are about 1.2 metres (3.9 ft) in length and 20 to 30 kilograms (44 to 66 lb) at birth.
Its fur ranges from silver-grey to creamy white, often patterned with subtle spots or rings along the sides. 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), 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, their fur is a darker brown fading to blonde on their bellies.
Distribution and Habitat
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 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 (2,000 ft) 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. Crabeater seals live throughout the Antarctic region and are perfectly adapted to living in Antarctica, almost exclusively amidst the pack ice.
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Evolutionary History and Taxonomy
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. 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 tribe of seals, share teeth adaptations including lobes and cusps useful for straining smaller prey items out of the water column.
Diet and Feeding Behavior
Despite its name, the crabeater seal does not eat crabs. The name “crabeater” is a misnomer, although there are crabs in Antarctica, the crabeater seals do not eat them. It is believed that their name was given by the whalers and sealers of the past centuries, who, somehow, were under the misconception that these seals consumed crabs. Other sources suggest that the name originates from the German term “krebs”, which translates to crabs and other crustaceans such as krill. No matter where the name originated or the reason behind it, the name has stuck.
Crabeater seals are carnivores, highly specialised for feeding on krill which makes up 95% of their diet. Though krill is the primary food source they also consume fish and squid. 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. Crabeater seals are highly specialised feeders, relying mainly on Antarctic krill ( Euphausia superba ) for sustenance. This krill-rich diet gives their faeces a distinctive pink-red colour, which often stains the ice or snow where they haul out. Although krill dominates their menu, around 10% of their diet also includes fish and squid.
These seals possess some of the most specialised dentition in the animal kingdom with interlocking tricuspid teeth uniquely adapted for filter feeding. This remarkable dental structure enables them to engulf krill laden mouthfuls of water and efficiently strain out their prey in a manner to the baleen plates of the great whales. They are accomplished divers, typically hunting at depths of 20-50 metres, but capable of descending to an impressive 600 metres when needed. Most dives last under 10 minutes, though they can extend to 24 minutes in search of prey.
Behavior and Social Structure
While they are often seen alone or in small groups of 1-3 individuals, crabeater seals are also known to gather in vast numbers, with haul-out groups occasionally reaching 1,000 seals. 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. They spend most of their time alone or in a small group. Much larger groups, however, sometimes with as many as about 1,000 individuals, have been seen hauling out on ice floes, particularly during the annual molt, which takes place in January and February. Up to 500 in a herd have been seen swimming and diving together.
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Crabeater seals have a typical, serpentine gait when on ice or land, combining retractions of the foreflippers with undulations of the lumbar 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. Despite the fact true seals typically struggle to support their body weight on land, Crabeater seals are surprisingly agile and are among the fastest movers of any Antarctic animal. Using a serpentine snake-like motion, they can reach impressive speeds of 19 to 26 km/h .
Curiously, crabeater seals have been known to wander further inland than any other pinniped. Crabeater seals travel long distances and likely hold the record for the greatest distance travelled inland by any seal; with carcasses discovered up to 113km from open water. Ninety-six mummified crabeater seals were found on Seymour Island, in the Graham Land area of the Antarctic Peninsula.
Reproduction and Life Cycle
Crabeater seals give birth during the Antarctic spring from September to December. Between September and December, pregnant females haul out alone on the ice to give birth. A single pup is usually born in the second half of October. 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. A male usually joins the female just before birth takes place, and protects her and her newborn from other males and from predators. Mating is believed to take place in the water. Copulation has not been observed directly and presumably occurs in water.
The gestation period is eleven months, which is due to a phenomenon called delayed implantation. The embryo remains in a dormant state for approximately three months before being implanted in the female’s uterus. Pups are weaned after just three weeks, having quickly built up a thick layer of blubber for insulation. Newborn pups can weigh between 20-30 kilograms, and are nursed for approximately 3-4 weeks; growing rapidly due to the high fat and protein content within their mother’s milk. At times males will haul out on the ice alongside the females, not necessarily the biological father. The pup stays close by its mother until weaning takes place, which usually three to four weeks after birth. Adults and subadults undergo their annual moult in January and February.
Their maximum lifespan can be 40 years, though the average lifespan is between 20-25 years.
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Predation and Mortality
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. Crabeater seals are vulnerable to two main natural predators; leopard seals and orca. Leopard seals are the most significant threat; particularly to inexperienced pups less than a year old. Among the orca populations that inhabit the frigid Antarctic waters, Type B orcas are known to include seals in their diet. They have developed highly specialised hunting strategies, working together swimming in a synchronised manner to create waves, washing the seals off of the ice floes, a technique known as ‘wave washing’. When threatened, a Crabeater seal will snort, hiss, show its teeth, and roll over many times, which is probably a tactic evolved to evade Leopard seals and Killer whales.
Conservation Status and Threats
The international Union for Conservation of Nature (IUCN) classifies Crabeater seals as “least Concern” due to their large population size and their widespread distribution; with a current population estimate of at least 7 million crabeater seals. The actual number is thought to be in excess of 10 million with some figures indicating 75 million making them one of the most numerous large mammals on earth. The exact population size of crabeater seals remains unknown. Currently, no reliable estimates of the total crabeater seal population are available. The IUCN Red List records the total Crabeater seal population as 8,000,000, including 4,000,000 adults for the area surveyed, with major areas of pack ice around Antarctica unsurveyed. This species is widespread, but the NOAA Fisheries resource states that there is currently no reliable approximate number of Crabeater seals. Currently, an international group of scientists is collaborating to provide a good estimate. Disease, primarily canine distemper virus, is a threat to Antarctic seal populations, and if outbreaks occur, it can cause mass die-offs.
Another emerging threat for these seals is the combination of global warming and ocean pollution. The warming temperatures of the ocean water are resulting in a decline in krill populations, threatening the survival of the seals in the coming decades. However, initial studies suggest that the number of Crabeater seals may decline as temperatures increase and pack ice is reduced, which is an important habitat for breeding, resting and avoiding predators.