E-Fauna BC: Electronic Atlas of the Wildlife of British Columbia

Mediaster aequalis Stimpson, 1857
Vermilion Star
Family: Goniasteridae
Photo of species

© Aaron Baldwin  Email the photographer   (Photo ID #6731)

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Distribution of Mediaster aequalis in British Columbia
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Family Description:

Flat body. Prominent marginals; the supero- and inferomarginals are similar. The aboral plates are flat or paxilliform, bearing granules, low stumps or spines. Only the radial areas have papulae. Pedicellariae are generally present. The tube feet have suckers.

Species Information

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Mediaster aequalis is a flat, broad-armed sea star with five arms up to 10 cm long. It is vermilion aborally and more orange orally, with red to flesh-coloured tube feet. The ratio of arm to disc is from 2.4 to 3.0. The aboral tabulate plates are hexagonal to round and bear up to 24 flat-topped, roundish central granules and up to 25 slightly longer peripheral granules. Two or three papulae occur between the plates, except in a triangular area in the interradial region where there are none. M. aequalis has 28 conspicuous superomarginals that are not bulging or separated from the inferomarginals by an obvious groove, as in Gephyreaster swifti. The inferomarginals are similar to the superomarginals, and there is a slight zigzag line between the two series. The oral interradial area is extensive, with oral intermediates nearly to tip of the arm, each one oval, with roundish groups of 2 or 3 central granules and 5 to 10 peripheral. The adambulacrals are squarish with 3 to 5 blunt prismatic furrow spinelets and two longitudinal series of 3 shorter blunt spines on the oral surface. The mouth plates have a marginal series of 5 to 7 prismatic spinelets, and 3 to 5 thicker prismatic suboral spinelets and granules on the outer part of the plate.

Similar Species

Mediaster aequalis may be confused with Gephyreaster swifti or Pseudarchaster alascensis, but it differs mainly in the form and arrangement of the marginal plates. Compare the descriptions of marginal plates for the three specie


The diet of Mediaster aequalis varies with the substrate and season. It eats encrusting sponges, bryozoans, the sea pen Ptilosarcus, loose algae, detritus and dead animals. In Gabriola Passage in December, 41 per cent were feeding and, of those, 56 per cent ate detritus.

M. aequalis reaches sexual maturity in about four years. It breeds from March to May. A specimen with a radius of 65 mm can produce about 1,800 eggs per year. The eggs, 1.0 to 1.2 mm in diameter, are bright opaque orange. The larvae are lecithotrophic. In the laboratory, the presence of the tubes of the polychaete worm Phyllochaetopterus caused larvae to settle to bottom after 30 days. In nature, these worm tubes are a nursery area for juvenile M. aequalis as well as Crossaster papposus, Luidia foliolata, Pteraster tesselatus, Henricia leviuscula, Solaster stimpsoni and S. dawsoni. M. aequalis can move at speeds of 27 to 40 cm per minute.



Chignik Bay, Alaska Peninsula, to southern California; intertidal to 293 metres. Common in shallow subtidal waters on rocks, shells, sand, gravel, pebbles and mud.

Status Information

Origin StatusProvincial StatusBC List
(Red Blue List)
BC Ministry of Environment: BC Species and Ecosystems Explorer--the authoritative source for conservation information in British Columbia.

Additional Range and Status Information Links

Additional Photo Sources

General References

Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2021. E-Fauna BC: Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab for Advanced Spatial Analysis, Department of Geography, University of British Columbia, Vancouver. [Accessed: 2024-07-23 7:31:33 AM]
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