North Pacific - from Alaska to Shikotan Island (sthn Kurile Islands).
Aleutian Islands. Photos: Roger Clark
The genus Colga is known basically from preserved specimens. Apart from Lemche's illustration (Just & Edmunds, 1985) and Roger Clark's photos, we know nothing about living animals. The genus has been recently reviewed (Martynov & Baranets, 2002) and the authors consider there are 3 species, C. villosa [NW Atlantic - Barents Sea} C. pacifica [Nth Pacific: Alaska - Kurile Ids], and C. minichevi [Nth Pacific: Kurile Ids - nth Japan Sea]. From their published illustrations there seems to be no unambiguous way to distinguish the species externally on body shape and arrangement of papillae, other than that C. minichevi has a smooth edge to the rhinophore pocket while the other two species have a papillate edge. C. pacifica and C. villosa also have very similar radular morphology, with a large central tooth, while C. minichevi differs in having a very small, or sometime absent, central tooth. The three species are reported to differ in the arrnagemnt of their penial armature.
In general shape, species of Colga are similar in shape to Palio but differ in having simple, but elongate papillae arranged all around the edge of the mantle. There are also similar tapering papillae on the dorsum. In C. pacifica and C. villosa there is a distinct row of papillae down the midline, and there can be up to 3 or 4 parallel rows of papillae on either side of the midline. In C. minichevi it seems that the dorsal papillae are more irregularly scattered. Preserved specimens of C. pacifica reach up to 40mm in length. The genus names Issa Bergh, 1880 and Issena Iredale & O'Donoghue, 1923 are considered synonyms of Colga.
• Martynov, A. V. & O. N. Baranets. 2002. A revision of the genus Colga Bergh (Opisthobranchia, Polyceridae), with description of a new species from the North Pacific. Ruthenica, 12(1): 23-43.
• Just H. & M. Edmunds. 1985. North Atlantic nudibranchs (Mollusca) seen by Henning Lemche. Ophelia, Supp. 2: 1-150.
Rudman, W.B., 2003 (September 17) Colga pacifica (Bergh, 1894) . [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/colgpaci