''Mediaster aequalis'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

sea star Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...

in the family

Goniasteridae Goniasteridae (the biscuit stars) constitute the largest family of sea stars, included in the order Valvatida. They are mostly deep-dwelling species, but the family also include several colorful shallow tropical species. Description Goniast ...

. It is native to the west coast of North America, ranging from Alaska to California. It is found in various habitats including beaches during very low tides, and at depths down to about . Also known as the vermilion sea star, it is the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

of the genus '' Mediaster'' and was first described in 1857 by the American zoologist

William Stimpson William Stimpson (February 14, 1832 – May 26, 1872) was an American scientist. He was interested particularly in marine biology. Stimpson became an important early contributor to the work of the Smithsonian Institution and later, director o ...

.
Pl. 23
figs. 7–11.

Description

''M. aequalis'' has five (occasionally four or six) stumpy arms and grows to a diameter of up to . The aboral (upper) surface is bright red while the oral (under) surface is an orangey-red. The tube feet are red. There is a row of conspicuous marginal plates along the edge of the arms on the aboral surface, and the central disc bears many flat-topped ossicles (platelike calcareous structures); these consist of a central group of granules surrounded by a ring of about twenty-five marginal granules.

Distribution and habitat

''M. aequalis'' is native to the western coast of North America, its range extending from Chignik Bay in

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

southwards to

Baja California Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northwesternmost of the 32 federal entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of B ...

. It often inhabits rocky substrates, at depths ranging from the low intertidal to around .

Ecology

''M. aequalis'' is both an

omnivore An omnivore () is an animal that regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize t ...

and a

predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

and will scavenge for dead animals and

detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...

. As well as feeding on algae, it preys on

tunicate Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

s (such as sea pork),

sea pen Sea pens are marine cnidarians belonging to the superfamily Pennatuloidea, which are colony-forming benthic filter feeders within the order Scleralcyonacea. There are 14 families within the order and 35 extant genera, and it is estimated a ...

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns,

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

s and

polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...

worms. In its turn it is hunted by the larger

morning sun star ''Solaster dawsoni'', the morning sun star, is a species of starfish in the family Solasteridae. It is found on either side of the northern Pacific Ocean. It has two subspecies: *''Solaster dawsoni arcticus'' Verrill, 1914 *''Solaster dawsoni daw ...

. It can move at the rate of per minute, which is fast for a starfish. Juvenile ''M. aequalis'' often congregate among the tubes of ''

Phyllochaetopterus prolifica ''Phyllochaetopterus prolifica'' is a species of marine polychaete worms that live in a tube that it constructs. It is native to shallow waters in the eastern Pacific Ocean and forms colonies of tubes on rocks and submerged objects. Description ...

'', a tube-dwelling worm, and research in

Washington state Washington, officially the State of Washington, is a state in the Pacific Northwest region of the United States. It is often referred to as Washington State to distinguish it from the national capital, both named after George Washington ...

has shown that the larvae are highly selective in where they settle, exclusively choosing to do so on these tubes.

Description

Distribution and habitat

Ecology

References

Further reading