The Good, the Bad and the Ugly. It's not hard to figure out which category this critter falls into. Mike told me that when, returning from a night dive, he first spotted this guy, he had no clue what it was, sponge, a cucumber or a glob of something, let alone a nudibranch. It wasn't until he starred at it for several moments did he notice what appeared to have a dorsal gill tuft, and took this shot. Animal was approx. 25 mm in size and residing in about 10 ft. of water.
I had never seen this species either and it wasn't til old buddy, Dr. Terry Gosliner ID'd the Branch as Actinocyclus, a genus closely related toHallaxa. Originally described from Japan, this species has been observed throughout the tropical Indo- West Pacific, including the Red Sea. Once you figure out, sure enough, it's a dorid nudibranch, things become a little clearer. It has perfoliate rhinophores, and the branchial plume is made up of a bubble-like circlet of unipinnate gills. That's the brown sphere shaped thing on the top right in this photo. The texture of the notum is firm and leathery to the touch and obviously cryptic colored allowing it to blend protectively on the sponges it feeds upon. Some specimens even have marking that resemble the oscula of the sponge. This species varies widely in color.
Specimens may reach 12 cm in length, and according to Rudman feed on spicule-less, Dendroceratid sponges which are tough and leathery themselves forming slimy layers over hard surfaces. Wayne Ellis has documented the color of the egg mass from Australia as a deep purple color.
Even ugly can be interesting. Thanks Mike.
Millen, S.V.; Gosliner, T.M. 1985. Four new species of dorid nudibranchs belonging to the genus Aldisa (Mollusca: Opisthobranchia), with a revision of the genus. Zoological Journal of the Linnean Society 84(3):195-233; 21 figs; tbl 1. [Actinocyclus japonicus, Aldisa albomarginata n.sp., Aldisa banyulensis, Aldisa benguelae n.sp., Aldisa berghi, Aldisa binotata, Aldisa cooperi, Aldisa expleta, Aldisa nhatrangensis, Aldisa pikokai, Aldisa sanguinea cooperi, Aldisa sanguinea sanguinea, Aldisa sanguinea, Aldisa smaragdina, Aldisa sp., Aldisa tara n.sp., Aldisa trimaculata n.sp., Aldisa zetlandica, Awuka, Boreodoris, Doris (Asteronotus) alabastrina, Doris (Asteronotus) sanguinea, Doris alabastrina, Doris zetlandica, Halgerda rubra, Jorunna, Rostanga, Sclerodoris, Thordisa Bimaculata, Thorunna]
Valdes, Angel. 2002. Review of the Genus Actinocyclus Ehrenberg, 1831 (Opisthobranchia: Doridoidea). The Veliger 45(3):193-202.
David W. Behrens
Pacific Coast Nudibranchs
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