Dendronotus nanus rediscovered in California

Dear Mark,
When I first saw your photo it reminded me of a dark form of  Triopha maculatus but on a closer examination it turns out to be much more interesting. Unless I am very mistaken, it is indeed a species of Dendronotus, but it seems to me to be Dendronotus nanus, a species described from Sonora, Mexico, in 1967 but for which I can't find any other information. The original description by Marcus & Marcus (1967) was based on photographs and preserved animals, so I had assumed that the short branchial papillae in their description were a description of the contracted preserved specimens, but from your photo the papillae do seem to be very short, and arranged in an arch, as in the Marcus's drawings. It almost looks as though the common trunk or stalk found in most species of Dendronotus is absent in this species.

Gordon Robilliard ( 1972) compares this species with D. iris and suggests they should be considered distinct, but felt that the question should be re-examined if the geographical distributions were shown to overlap. Your find does suggest their distributions do overlap, but from your photo, the nature of the branchial papillae look rather different. However I will leave any serious comparison to local experts, familiar with variation in D. iris. The egg ribbon could well belong to this species, but as I have said before, identifying egg ribbons is very risky unless you can catch the animal in the act of laying.

Could you check the locality data please? "Depth: 46 feet. Intertidal sand flats". doesn't sound right unless there is an enormous tidal range.

• Marcus, Er. & Marcus, Ev (1967) American opisthobranch mollusks. Part 2, Opisthobranchs from the Gulf of California. Studies Tropical Oceanography, Miami: 6(1-2), 141-256. (Figs 1-95)
• Robilliard, G. A. (1972) A new species of Dendronotus from the northeastern Pacific with notes on Dendronotus nanus and Dendronotus robustus (Mollusca: Opisthobranchia). Canadian Journal of Zoology, 50(4): 421-432.

This is a very interesting find.
Best wishes,
Bill Rudman