Mediaster aequalis
Vermilion sea star, type species of Mediaster, described in 1857.
Mediaster aequalis, also called the vermilion sea star, belongs to the family Goniasteridae. It serves as the type species for the genus Mediaster, and American zoologist William Stimpson first described it in 1857.
This sea star usually has five arms, though occasionally four or six, and can reach a diameter of up to 20 centimeters. Its upper side is bright red, while the underside is a more orange-red, and its tube feet are red. A row of noticeable marginal plates runs along the upper edge of each arm, and the central disc is covered with flat-topped ossicles—platelike calcareous structures—each made up of a central cluster of granules surrounded by a ring of about twenty-five marginal granules.
The species lives along the west coast of North America, from Chignik Bay in Alaska south to Baja California. It occupies a variety of habitats, from beaches exposed during very low tides down to depths of about 500 meters, and it often prefers rocky substrates.
An omnivore and predator, *M. aequalis* also scavenges dead animals and detritus. Its diet includes algae, tunicates (such as sea pork), sea pens, sponges, bryozoans, brachiopods, and polychaete worms. In turn, it is preyed upon by the larger morning sun star. It can move at a speed of 40 centimeters per minute, which is fast for a starfish. Juveniles often gather among the tubes of the worm *Phyllochaetopterus prolifica*, and research in Washington state has found that the larvae are highly selective about where they settle, choosing only these tubes.
Quick Facts
- Genus
- Mediaster
- Species
- aequalis
Facts from the source article.
Lore & Background
Mediaster aequalis has five stumpy arms, occasionally four or six, and grows to a diameter of up to 20 cm. Its aboral (upper) surface is bright red, while the oral (under) surface is orangey-red, and its tube feet are red. A row of conspicuous marginal plates runs along the edge of the arms on the aboral surface, and the central disc bears many flat-topped ossicles consisting of a central group of granules surrounded by a ring of about twenty-five marginal granules.
It is native to the western coast of North America, ranging from Chignik Bay in Alaska southwards to Baja California, and inhabits rocky substrates from the low intertidal to depths around 500 m. The species is both an omnivore and a predator, scavenging for dead animals and detritus, feeding on algae, and preying on tunicates (such as sea pork), sea pens, sponges, bryozoans, brachiopods, and polychaete worms. It is hunted by the larger morning sun star and can move at 40 cm per minute, which is fast for a starfish.
Juveniles often congregate among the tubes of the tube-dwelling worm Phyllochaetopterus prolifica. Research in Washington state has shown that the larvae are highly selective in where they settle, exclusively choosing to do so on these tubes.
Reader's Guide
Mediaster aequalis holds significance as the type species of the genus Mediaster, established by William Stimpson in 1857. Its description contributed to the taxonomic foundation of the family Goniasteridae. The species is notable for its bright red aboral surface and orangey-red oral surface, with distinctive marginal plates and ossicles. Its ecological role as both an omnivore and predator, feeding on a wide range of organisms including algae, tunicates, sea pens, sponges, bryozoans, brachiopods, and polychaete worms, highlights its importance in benthic food webs. It is also prey for the larger morning sun star. The species' relatively fast movement of 40 cm per minute is noteworthy among starfish. A key aspect of its life history is the highly selective larval settlement exclusively on the tubes of Phyllochaetopterus prolifica, as documented by research in Washington state. This specificity underscores the species' dependence on a particular microhabitat for juvenile development. Its range from Alaska to Baja California and its occurrence from the low intertidal to depths of 500 m demonstrate its adaptability across a broad geographic and depth gradient.
Did You Know?
- Mediaster aequalis is the type species of the genus Mediaster.
- It was first described in 1857 by American zoologist William Stimpson.
- Its larvae are highly selective, settling exclusively on the tubes of the worm Phyllochaetopterus prolifica.
- It can move at a rate of 40 cm (16 in) per minute, which is fast for a starfish.
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