The Ross seal (''Ommatophoca rossii'') is a
true seal
The earless seals, phocids or true seals are one of the three main groups of mammals within the seal lineage, Pinnipedia. All true seals are members of the family Phocidae (). They are sometimes called crawling seals to distinguish them from th ...
(family Phocidae) with a range confined entirely to the
pack ice
Drift ice, also called brash ice, is sea ice that is not attached to the shoreline or any other fixed object (shoals, grounded icebergs, etc.).Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer-Verlag. Unlike fast ice, which is "fasten ...
of
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
. It is the
only species of the genus ''Ommatophoca''. First described during the
Ross expedition
The Ross expedition was a voyage of scientific exploration of the Antarctic in 1839 to 1843, led by James Clark Ross, with two unusually strong warships, HMS ''Erebus'' and HMS ''Terror''. It explored what is now called the Ross Sea and discov ...
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. Ross seals are
brachycephalic
Brachycephaly (derived from the Ancient Greek '' βραχύς'', 'short' and '' κεφαλή'', 'head') is the shape of a skull shorter than typical for its species. It is perceived as a desirable trait in some domesticated dog and cat breeds, ...
, as they have a short broad muzzle and have the shortest fur of any other seal.
Taxonomy and evolution
The Ross seal shares a recent common ancestor with three other extant Antarctic seals, which are together known as the
lobodontine seals
The true seal tribe Lobodontini, collectively known as the Antarctic seals or lobodontin seals, consist of four species of seals in four genera: the crabeater seal (''Lobodon carcinophaga''), the leopard seal (''Hydrurga leptonyx''), the Weddell ...
. The other species are the
crabeater seal
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 medium- to large-sized (over 2 m in length), relatively slender and pale-co ...
(''Lobodon carcinophaga''),
leopard seal
The leopard seal (''Hydrurga leptonyx''), also referred to as the sea leopard, is the second largest species of seal in the Antarctic (after the southern elephant seal). Its only natural predator is the orca. It feeds on a wide range of prey incl ...
(''Hydrurga leptonyx'') and
Weddell seal
The Weddell seal (''Leptonychotes weddellii'') is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. The Weddell seal was discovered and named in the 1820s during expeditions led by British sealing ...
(''Leptonychotes weddelli'').
These species, collectively belonging to the seal
tribe
The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide usage of the term in English language, English is in the discipline of anthropology. This definition is contested, in p ...
Lobodontini, 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
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
, ''
Mirounga'' (elephant seals) in the late
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
to early
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...](_blank)
of New Zealand.
Description
Ross seals reach a length of about and weight of ; females are slightly larger at .
A molecular genetic based technique has been established to confirm the sex of individuals in the laboratory. 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 its close relatives Weddell seal
The Weddell seal (''Leptonychotes weddellii'') is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. The Weddell seal was discovered and named in the 1820s during expeditions led by British sealing ...
s, crabeater seal
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 medium- to large-sized (over 2 m in length), relatively slender and pale-co ...
s and leopard seal
The leopard seal (''Hydrurga leptonyx''), also referred to as the sea leopard, is the second largest species of seal in the Antarctic (after the southern elephant seal). Its only natural predator is the orca. It feeds on a wide range of prey incl ...
s are ubiquitous in Antarctic waters, the Ross seal is an uncommon and relatively unknown animal, considered to be the least common pack ice
Drift ice, also called brash ice, is sea ice that is not attached to the shoreline or any other fixed object (shoals, grounded icebergs, etc.).Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer-Verlag. Unlike fast ice, which is "fasten ...
seal. It almost never leaves the Antarctic Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
, with the very rare exception of stray animals found around subantarctic islands, and uniquely, off the south coast of Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
. 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
In frequentist statistics, a confidence interval (CI) is a range of estimates for an unknown parameter. A confidence interval is computed at a designated ''confidence level''; the 95% confidence level is most common, but other levels, such as 9 ...
range from 20,000 to 227,000). Thus, very little is known about trends in the population. A genetic survey did not detect evidence of a recent, sustained genetic bottleneck in this species, which suggests that populations do not appear to have suffered a detectable and sustained decline in the recent past.
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
Ross seal feeds primarily on squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...
and fish
Fish are aquatic, craniate, gill-bearing 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. Approximately 95% of li ...
, primarily Antarctic silverfish
The Antarctic silverfish (''Pleuragramma antarctica''), or Antarctic herring, is a species of marine ray-finned fish belonging to the family Nototheniidae, the notothens or cod icefishes. It is native to the Southern Ocean and the only truly pela ...
, in the pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
zone. Ross seals are presumed to be preyed upon by killer whale
The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus ''Orcinus'' and is recognizable by its black-and-white pa ...
s (''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 is the process of gradually introducing an infant human or another mammal to what will be its adult diet while withdrawing the supply of its mother's milk.
The process takes place only in mammals, as only mammals produce milk. The infan ...
. 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 thought to live around 20 years in the wild.[
]
See also
References
External links
University of Michigan museum of zoology
Fisheries Global Information System factsheet
{{Taxonbar, from=Q837610
Lobodontins
Mammals described in 1844
Mammals of South Australia
Pinnipeds of Antarctica
Taxa named by John Edward Gray
Vulnerable fauna of Australia