Onchidoris muricata (Miller, 1776)
Onchidoris muricata is a small white, tuberculose dorid nudibranch, which can be distinguished from many similar animals because it is oval, almost round and not as elongate as many. The tubercles are also rounder. It reaches a size of 14 mm, although it is usually smaller and it looks a lot like Adalaria jannae (Bill Rudman's Sea Slug Forum). It can be distinguished because Onchidoris muricata has larger, mushroom shaped tubercles and lacks a large specialized post branchial tubercle. It does however, have a low, white, triangular mound in the same position. Further details to separate the two look-alike species can be found in Millen (1985, 1987). If they are spawning, it is easy because Onchidoris muricata has spawn with a simple coil and Adalaria jannae has a sinuous coil. The radula has 20-44 rows of teeth with the formula (188.8.131.52.1). The central tooth is an elongate rectangle; the first lateral tooth is large, with a triangular base and long hook with 8-18 tiny denticles. The outer lateral (marginal) tooth is small with a single hook.
Onchidoris muricata has a disjunct boreal distribution on both sides of the Atlantic and down the Pacific coast from the Bering Sea to California. It feeds upon a variety of encrusting bryozoans, usually on rocks or under boulders from the low intertidal to the shallow subtidal. It is an annual species, found year-round but is most abundant from January to April in the Pacific. It lays a white or pale orange spawn mass with a thick, inward leaning ribbon of 1.5-2 whorls, which have been found in January, February, April and October in the Pacific.
Millen, Sandra V. 1985. The nudibranch genera Onchidoris and Diaphorodoris (Mollusca, Opisthobranchia) in the northeastern Pacific. Veliger 28: 80-93.
Millen, Sandra. 1987. The nudibranch genus Adalaria with a description of a new species from the northeastern Pacific. Can. J. Zool. 65: 2696-2702.
Photos - Photo by Ron Long, taken near Vancouver, British Columbia, tank shot, Pentax and a macrolens. Scanning Electron Microphotograph of the radula taken by Sandra Millen.
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