Ross Seal
Encyclopedia
The Ross seal is a true seal (family Phocidae) with a range confined entirely to the pack ice of Antarctica. It is the only species
of the genus Ommatophoca. First described during James Clark Ross
' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic pinnipeds. Its distinctive features include disproportionately large eyes, whence its scientific name (Ommato- meaning "eye", and phoca meaning "seal"), and complex, trilling and siren-like vocalizations.
(Lobodon carcinophaga), leopard seal
(Hydrurga leptonyx) and 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. The ancestral Lobodontini likely diverged from its sister clade
, the Mirounga (elephant seals) in the late Miocene
to early Pliocene
, when they migrated southward and diversified rapidly in relative isolation around Antarctica.
The Ross seal is able to produce a variety of complex twittering and siren-like sounds that are performed on ice and underwater, where they carry for long distances. The underwater siren sound can be composed of two harmonically unrelated superimposed tones that are pulsed with the same rhythm. Uniquely, the vocalizations, whether on ice or in water, are made with a closed mouth - emitting no air. The purpose of these sounds is unknown, though their distinctive nature and long range are likely to facilitate either encounters or avoidance of individuals.
s, crabeater seal
s and leopard seal
s are ubiquitous in Antarctic waters, the Ross seal is an uncommon and relatively unknown animal, considered to be the least common pack ice seal. It almost never leaves the Antarctic Ocean, with the very rare exception of stray animals found around sub-Antarctic islands, and uniquely, off the south coast of Australia
. Nonetheless, its distribution is circumpolar, with individuals found in low densities - usually singly - in very thick pack ice in all regions of the continent.
The total Ross seal population is estimated at around 130,000 individuals, but there is great uncertainty in this estimate (reported 95% confidence intervals range from 20,000 to 227,000). Thus, very little is known about trends in the population.
Interactions with humans have been limited. They have been collected historically by Antarctic expeditions and for scientific collections. Their range does not generally overlap with commercial fishing.
and fish
, primarily Antarctic silverfish
, in the pelagic zone. Ross seals are presumed to be preyed upon by killer whales (Orcinus orca) and leopard seals, large predators that share their Antarctic habitat, though there are no documented observations of predation.
Females give birth to their young on the ice in November. Pups are nursed for only four weeks before weaning
. Mating is thought to occur underwater shortly after the pup is weaned, but has never been observed. Ross seals mature sexually at approximately three years of age, and are though thought to live around 20 years in the wild.
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
of the genus Ommatophoca. First described during James Clark Ross
James Clark Ross
Sir James Clark Ross , was a British naval officer and explorer. He explored the Arctic with his uncle Sir John Ross and Sir William Parry, and later led his own expedition to Antarctica.-Arctic explorer:...
' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic pinnipeds. Its distinctive features include disproportionately large eyes, whence its scientific name (Ommato- meaning "eye", and phoca meaning "seal"), and complex, trilling and siren-like vocalizations.
Taxonomy and evolution
The Ross seal shares a recent common ancestor with the other Antarctic seals, which are together known as the lobodontine seals. These include the crabeater sealCrabeater Seal
The crabeater seal, Lobodon carcinophagus, is a true seal with a circumpolar distribution around the coast of Antarctica. They are medium to large-sized , relatively slender and pale-colored, found primarily on the free floating pack ice that extends seasonally out from the Antarctic coast, which...
(Lobodon carcinophaga), leopard seal
Leopard Seal
The leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...
(Hydrurga leptonyx) and Weddell seal
Weddell Seal
The Weddell seal , is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound...
(Leptonychotes weddelli). These species, collectively belonging to the Lobodontini tribe
Tribe (biology)
In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...
of seals, share teeth adaptations, including lobes and cusps useful for straining smaller prey items out of the water column. The ancestral Lobodontini likely diverged from its sister clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, the Mirounga (elephant seals) in the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
to early Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
, when they migrated southward and diversified rapidly in relative isolation around Antarctica.
Description
Ross seals reach a length of about 1.68–2.09 m (5.5–6.9 ft) and weight of 129 kilogram; females are slightly larger at 1.96–2.5 m (6.4–8.2 ft). Pups are about 1 m and 16 kg at birth. The coat is colored dark-brown in the dorsal area and silvery-white beneath. At the onset of the Antarctic winter, the coat fades gradually to become light brown. At close range, the Ross seal can be easily identified by its large eyes, which are up to 7 cm in diameter.The Ross seal is able to produce a variety of complex twittering and siren-like sounds that are performed on ice and underwater, where they carry for long distances. The underwater siren sound can be composed of two harmonically unrelated superimposed tones that are pulsed with the same rhythm. Uniquely, the vocalizations, whether on ice or in water, are made with a closed mouth - emitting no air. The purpose of these sounds is unknown, though their distinctive nature and long range are likely to facilitate either encounters or avoidance of individuals.
Range and population status
Although Weddell sealWeddell Seal
The Weddell seal , is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound...
s, crabeater seal
Crabeater Seal
The crabeater seal, Lobodon carcinophagus, is a true seal with a circumpolar distribution around the coast of Antarctica. They are medium to large-sized , relatively slender and pale-colored, found primarily on the free floating pack ice that extends seasonally out from the Antarctic coast, which...
s and leopard seal
Leopard Seal
The leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...
s are ubiquitous in Antarctic waters, the Ross seal is an uncommon and relatively unknown animal, considered to be the least common pack ice seal. It almost never leaves the Antarctic Ocean, with the very rare exception of stray animals found around sub-Antarctic islands, and uniquely, off the south coast of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Nonetheless, its distribution is circumpolar, with individuals found in low densities - usually singly - in very thick pack ice in all regions of the continent.
The total Ross seal population is estimated at around 130,000 individuals, but there is great uncertainty in this estimate (reported 95% confidence intervals range from 20,000 to 227,000). Thus, very little is known about trends in the population.
Interactions with humans have been limited. They have been collected historically by Antarctic expeditions and for scientific collections. Their range does not generally overlap with commercial fishing.
Feeding and reproductive behavior
The Ross seal feeds primarily on squidSquid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, primarily Antarctic silverfish
Antarctic silverfish
The Antarctic Silverfish, Pleuragramma antarcticum is a member of the suborder Notothenioidei of the Perciform fishes.Pleuragramma antarcticum is a keystone species in the ecosystem of the Southern Ocean....
, in the pelagic zone. Ross seals are presumed to be preyed upon by killer whales (Orcinus orca) and leopard seals, large predators that share their Antarctic habitat, though there are no documented observations of predation.
Females give birth to their young on the ice in November. Pups are nursed for only four weeks before weaning
Weaning
Weaning is the process of gradually introducing a mammal infant, either human or animal, to what will be its adult diet and withdrawing the supply of its mother's milk.The process takes place only in mammals, as only mammals produce milk...
. Mating is thought to occur underwater shortly after the pup is weaned, but has never been observed. Ross seals mature sexually at approximately three years of age, and are though thought to live around 20 years in the wild.