The true sealtribeLobodontini, collectively known as the lobodontine seals, consist of four species of seals in four genera: the crabeater seal, the leopard seal, the Weddell seal, and the Ross seal. All lobodontine seals have circumpolar distributions surrounding Antarctica. They include both the world's most abundant seal and the only predominantly mammal-eating seal. While the Weddell seal prefers the shore-fast ice, the other species live primarily on and around the off-shore pack ice. Thus, though they are collectively the most abundant group of seals in the world, the combination of remote range and inaccessible habitat make them among the least well studied of the world's seals.
Adaptations
The Lobondontini are thought to have diverged from the elephant seals in the late Miocene or early Pliocene after moving southward from the Atlantic to the Southern Ocean. All species share teeth adaptations including lobes and cusps useful for straining smaller prey items out of the water. Nonetheless, they have diversified into specialized prey ecological niches, thereby illustrating the radiating sympatric speciation associated with colonization of a novel environment with multiple available niches. Thus, the crabeater seal, with the most specialized sieve-like dental features, is the only seal that feeds predominantly on Antarctic krill, while the leopard seal is the only seal which actively preys on other seals and penguins, while still retaining the ability to filter-feed on krill.
Abundance
The lobodontine seals in aggregate are among the most successful of all marine mammal groups, collectively accounting for at least 50% of all seals on Earth and about 80% of the global biomass of pinnipeds. The extremely high abundance of crabeater seals in particular, with possibly over 30,000,000 individuals, is a testament to the high productivity of the Southern Ocean, especially with respect to krill. High numbers of seals may also be the indirect result of the wide-scale extermination of large baleen whales in the Antarctic due to commercial whaling in the 19th and 20th centuries, and the subsequent increase in krill densities. Genetic evidence suggests that Weddell and crabeater seal populations may have increased in size during the Pleistocene. None of the four species is currently thought to be declining in numbers.
