Five or more arms. At least one adambulacral is fused into an adoral carina. The adambulacrals are wider than their length. Crossed and straight pedicellariae are present, the former usually in dense tufts around the spines. The aboral skeleton is meshlike. The tube feet are arranged in four rows.
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Evasterias troschelii has a fairly distinctive colour, usually mottled green, red, brown or orange, with white spines and a lighter oral surface. It has five arms up to 30 cm long, and its arm-to-disc ratio ranges from 5.0 to 7.6. This is one of the most variable sea stars in this region (the area from Glacier Bay to Puget Sound to a depth to 200 metres), especially in the pattern of aboral spines, which varies from numerous small spines in a netlike pattern to longitudinal rows of large blunt spines on the arms. Fisher (1930) described three forms to cover this range of variability, but they are not geographically separated so are not considered subspecies. Papulae occur abundantly between the spines. A regular series of superomarginals curves upward at the junctions of the arms and meets the series from the adjoining arm. The first two rows of similar spines below the superomarginals are inferomarginals, the next four rows are oral intermediates. The adambulacrals alternately bear 1 and 2 spines. Three to five pairs of adambulacrals from adjoining arms fuse together to form an adoral carina. The mouth plates at the proximal end of the adoral carina bear two unequal marginal spines and one longer suboral spine. Lanceolate pedicellariae occur on the oral and adambulacral spines. Crossed pedicellariae occur on the aboral and marginal spines and also in clusters on the oral and adambulacral spines.
Evasterias troschelii is similar to Pisaster ochraceus, but it has a greater arm-to-disc ratio, more rows of spines between the marginals and ambulacral furrow, and a less sunken mouth. The six-armed star, Leptasterias hexactis,
Evasterias troschelii has a varied diet, including bivalves (especially the rock oyster Pododesmus machrochisma around the San Juan Islands), barnacles (in Gabriola Passage), sea squirts, chitons, gastropods and lamp shells. On rocky shores, E. troschelii eats mostly molluscs and barnacles, while rejecting tunicates; but on soft bottoms, it eats tunicates with a preference for those with thinner tunics. It digests tough-skinned tunicates, like Styela gibbsii and Pyura haustor, through a hole in the tunic. To eat a bivalve, E. troschelii everts its stomach and inserts it between the shells while the tube feet pull from the outside. A specimen with arm length of 22.5 cm can exert an average force of 4500 grams for six hours and a maximum force of 5500 grams. A small concentration (0.12 parts per million) of the water soluble fraction of crude oil reduces the feeding rate of E. troschelii; above 0.97 ppm, feeding ceases. In Southeast Alaska, an Alaska King Crab (Paralithodes camtschatica) was reported to attack and eat E. troschelii.
Around Vancouver Island and in Southeast Alaska this species breeds from April to June. Eggs are numerous and small (150 micrometres). Gastrulation takes place on the second or third day after fertilization. By the fourth day a bipinnaria larva forms.
A parasitic ciliate Orchitophrya stellarum, endemic to the North Atlantic, was discovered recently in E. troschelii. It was first discovered in 1988 in the Purple Sea Star (Pisaster ochraceus) in British Columbia, but appears to have spread to this other member of the Family Asteriidae. It infests the male gonad causing castration and loss of sperm production. The scale worm Arctonoe fragilis is commensal on E. troschelii.
The east coast of Kamchatka and the Pribilof Islands to Monterey, California. Usually found on rock or cobble, but also on sand, from the intertidal zone to 75 metres deep. Common on the northwest coast of North America, especially in the more sheltered waters of inlets. Evasterias troschelii tends to replace Pisaster ochraceus in these localities.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2017. 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:
19/11/2017 8:10:49 AM]
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