Mediaster aequalis Stimpson, 1857

Common name(s): Vermilion star, Red sea star

Phylum Echinodermata
 Class Asteroidea
  Order Valvatida
   Suborder Granulosina
    Family Goniasteridae
Mediaster aequalis from 100 m depth, San Juan Channel
(Photo by: Dave Cowles, July 2000)
Description:  This 5-rayed star has conspicuous marginal plates (picture).  Aboral surface is covered with circular to oval or haxagonal, flat-topped plates (ossicles), each consisting of about 25 marginal granules and a central group of slightly different granules (picture).  Up to 20 cm diameter.  Orange or vermilion in color.  Oral surface is lighter colored than is the aboral surface.

How to Distinguish from Similar Species: Pteraster tesselatus, Dermasterias imbricata, and Asterina miniata have similar shapes but do not have the large marginal plates.  The aboral surface of Asterina miniata has ossicles but they are crescent-shaped.  Pteraster tesselatus and Dermasterias imbricata have a smooth aboral surface.

Geographical Range: Chignik Bay, Alaska to Baja California

Depth Range: Low intertidal to 503 meters.  In our area almost exclusively subtidal (deeper than 15 m).  May be seen in fairly shallow, protected waters.

Habitat: Varies.  Especially on rocky areas.

Biology/Natural History: An omnivorous predator.  Prey include sponges, bryozoans, sea pens,tunicates such as Aplidium californicum and Corella spp., algae, and detritus.  Predators include the seastar Solaster dawsoni.  Moves fairly fast for a seastar, up to 40 cm/minute.  May have an internal parasitic barnacle Dendrogaster sp.  In British Columbia spawns from March to May.  Juveniles often congregate subtidally among tubes of the tubedwelling polychaete Phyllochaetopterus prolifica.

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Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Gotshall and Laurent, 1979
  Kozloff, 1993
  O'Clair and O'Clair, 1998

Scientific Articles:

Emlet, Richard B., 1994. Body forms and patterns of ciliation in nonfeeding larvae of echinoderms: functional solutions to swimming in the plankton? American Zoologist 34: pp. 570-585

Knott, K. Emily, and Gregory A. Wray, 2000. Controversy and consensus in Asteroid systematics: new insights to Ordinal and Familial relationships. American Zoologist 40:3 pp. 382-392

McEdward, Larry R. and Benjamin G. Miner, 2006.  Estimation and interpretation of egg provisioning in marine invertebrates.  Integrative and Comparative Biology 46:3 pp 224-232

General Notes and Observations:  Locations, abundances, unusual behaviors:

Closeup of the aboral surface of Mediaster aequalis.  Note the round clusters of ossicles and the madreporite.  Photo by Dave Cowles, July 2000

A view of the underside.  Photo by Dave Cowles, July 2000

Authors and Editors of Page:
Dave Cowles (2005):  Created original page