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Psolidium bidiscum was misidentified for many years as Psolidium bullatum Ohshima, but a detailed examination showed it to be an undescribed species. P. bullatum is known only from the Aleutian Islands and Gulf of Alaska. P. bidiscum is a small (1 to 3 cm), dome-shaped sea cucumber with mauve to pink, shinglelike plates on the top side; about 10 rows of scales between the mouth and the anus. Each plate has up to 13 knoblike bumps and up to 6 tiny tube feet protruding through pores in the plate. The flat sole has three rows of tube feet around the edge, made up of two rows of robust tube feet and one outer row of minute tube feet. A staggered row of tube feet runs down the centre of the sole. The 10 feeding tentacles, 8 equal and 2 smaller, are translucent white, with reddish-brown blotches. Many specimens have prominent reddish-brown bands near the bases of the two smaller tentacles.
Skin ossicles: two sizes of perforated plates in the ventral sole and cups in the dorsal membrane that covers the plates.
There are two species similar to Psolidium bidiscum in our waters: Psolus chitonoides and Psolus squamatus. The most common, P. chitonoides, is bright red to orange, with red tentacles; it does not have minute tube feet piercing the dorsal scales. P. squamatus has white dorsal scales and white tentacles. Psolidium bidiscum and Psolus squamatus both occur in deeper water. P. bidiscum is also accessible by scuba, but the sediment-covered body is difficult to spot.
bidiscum = referring to two types of plates in the sole
Psolidium bidiscum feeds like Psolus chitonoides. Sticky tentacles catch particulate matter and are placed into the mouth individually and cleaned off.
Spawning occurs from late March to early May. The anterior end of the animal lifts slightly off the substratum during spawning. The female may spawn up to 3,000 golden yellow to light brown-orange eggs (mean diameter 330 μm). When ripe, the tan ovary or white testis is visible through the thin ventral sole. The larva is a pelagic lecithotrophic vitellaria with three ciliary rings.
Psolidium bidiscum is known mostly from dredged specimens attached to rocks or shells. However, I have seen it at 10 m in Saanich Inlet, British Columbia, on sediment-covered rock surfaces. The pale tentacles are exposed, but sediment usually covers the body. When the tentacles are retracted it can easily be overlooked. It may be more common in shallow water than the number of specimens in collections suggests, because of the difficulty in sampling solid rock with a dredge. In San Juan Channel, Washington, it is found in densities of 10 per 0.1 square metres. It has only been recorded inter-tidally in one locality in Hood Canal, Puget Sound.
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:
2022-08-14 3:29:43 PM]
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