Evasterias troschelii

The description of this species was first formally published in 1862 by William Stimpson under the name Asterias troschelii (Stimpson first presented the new species in a lecture at the Boston Society of Natural History in 1861).[3] The holotype was collected in Puget sound, Washington.[3][4] Only part of one arm still remains of this smallish specimen at the United States National Museum.[4]

It was moved to the new genus Evasterias by Addison Emery Verrill in 1914, with as the type species. Verrill published five new varieties (alveolata, densa, parvispina, rudis and subnodosa) besides the nominate variety, and also published E. acanthostoma in 1914.[4]

Alexander Michailovitsch Djakonov synonymised all of Verrill's varieties, but retained alveolata as a forma, and also made Verrill's E. acanthostoma a form of E. troschelii, leaving the species with three forms including the nominate.[2]

Evasterias troschelii (orange variant)

The mottled star is a large starfish with a radius of up to 28 centimetres (11 in). It has a small disc and five long narrow arms often turned up at the tip. The widest point of the arms is a little distance away from the edge of the disc. The aboral (upper or dorsal) surface of the disc is covered with a network of calcareous plates with spines about 2 mm (0.1 in) long surrounded by smaller spines and crossed and straight pedicellariae, minute pincer-like structures with two jaws. There is an irregular line of white-tipped spines running down the centre of the arms and the whole upper surface is rough to the touch. On the oral (lower or ventral) surface a long ambulacral groove stretches from the central mouth to the tip of each arm with four rows of tube feet and clumps of pedicellariae and spines on either side. The colour is very variable and includes plain or mottled shades of orange, brown, greenish-grey, bluish-grey and pale purple. The outer edges of the arms often have a contrasting coloured rim and the underside is pale brown.[5][6]

Evasterias troschelii is found on the west coast of North America. Its range extends from Pribilof Islands, Alaska southwards to Monterey Bay, California but it is rarely seen south of Puget Sound. It also occurs in Kamchatka (Bechevinskaya Bay).[2][5] It is usually found on rocks and pebbles and occasionally on sand, at depths down to about 75 metres (246 ft)[6] or 70 metres (230 ft).[2] In bays and other sheltered locations it largely replaces the other common species of the area, the purple sea star (Pisaster ochraceus).[6]