Recently I received a large collection of photos of opisthobranchs from the Aleutian Islands collected by fellow biologist Roger Clark. I was amazed by the diversity. One species that confused me was Colga pacifica, which I had ID'ed as a Triopha until Sandra Millen set me straight.
I shared these photos with Drs. Alexander Martynov and Olga Baranets at the Laboratory of Marine Research in Saint Petersburg, Russia who have been studying this genus and they confirmed the identification as C. pacifica. Both expressed excitement in seeing Roger's photos of this species because they had only seen preserved specimens from trawl samples and had never seen living material. Alexander replied to me - "All three specimens from the photos have the typical pattern of pre-branchial notal processes (very similar to Atlantic C. villosa , but the later species often has more rows on the back). A main row of elongate processes runs down the middle of notum and usually a pair of rows of similar processes accompanied it from the both sides. In some specimens these lateral rows are almost same length as the middle row."
The importance of Roger's photos is we can see the previously unpublished variation in color found in this species. Specimens vary from pure white with yellow gills and rhinophores shown above, and as described originally by Bergh, to specimens that are completely orange . Alexander also replies - "Especially interesting the colour variation, and certainly most marvelous is the yellow specimen with pink notal processes , which seem to compare with rests anatomically. Because the Colga pacifica probably eats practically all arborescent bryozoa along with encrusting species, the simplest idea that comes to mind is that the source of such colour variation is the different food and also that more paler examples can be from deeper waters."
Drs. Martynov and Baranets have recently described a second species from the cold waters of the North Pacific. Colga minichevi from Aniva Bay, South Sakhalin (Okhotsk Sea) differs externally by having many more randomly and more densely distributed notal processes.
Thank you Roger for bringing this interesting species to our attention and thank you Drs. Millen, Martynov and Baranets for your help with the identification of a species from waters most of us will never dive.
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.
David W. Behrens
Pacific Coast Nudibranchs
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