Re: nudibranch extinction? (1)

May 2, 2001
From: Cynthia Trowbridge

Dear Bill,

Your message about this potential nudibranch extinction is provocative. The problem of many species of opisthobranchs is their very low population abundance. Many taxa are known from only one or a few records. In some cases, once we know where to look (i.e., the specific prey food or habitat), the problem of apparent scarcity is resolved. In other cases, the species are indeed uncommon. To conclude that a species has gone extinct, I think that we need more specific and conclusive information than we have for the nudibranch. Lack of published reports does not always reflect the absence of information. Please let me give two examples from my experience.

First, at Oregon State University, we commonly show undergraduate students a pulmonate 'limpet' (Trimusculus reticulatus) on the shore during the spring Marine Biology class. It was not until I came across Jeff Goddard's paper in The Veliger that I learned our population was outside the known geographic range; in 1997, Jeff et al. extended the species' range north to Washington. So, what I knew to be a common taxon, other malacologists considered to be unusual or absent. So, by soliciting for details about your nudibranch, you may indeed hear of more recent records. On the other hand, not everyone who dives necessarily reads the Sea Slug Forum so you cannot validly interpret the negative information as species absence.

Second, in 1990, I found several specimens of an unknown mollusk that I sent to Terry Gosliner. He subsequently described it as a new species (Runcina macfarlandi Gosliner, 1991). According to Terry's paper, runcinids have been collected from Pacific Grove in 1899 (illustration but no photo or specimen) and one other time; so my record from Oregon represented the 3rd record in a century and the only verified record that was photographed (both Terry and I took pictures) and dissected. Does that mean this thing is rare or endangered? I would argue that we do not have sufficient evidence to support that interpretation. I have not found any more specimens but, then again, I have not looked.

When we look at a species' abundance curve for any community, the vast majority of species in a community are uncommon to rare. There are at least 7 fundamentally different types of “rarity” that have been defined by ecologists; many of these are not at all endangered. To conclude that something that is very, very uncommon is actually extinct is very nebulous. I think it is wonderful to request additional information about your nudibranch species but would be cautious about speculating that it is extinct. Thank you for considering my comments!

• Goddard, J.H.R.; Wayne, T.A.; Wayne, K.R. 1997. Opisthobranch mollusks and the pulmonate limpet Trimusculus reticulatus (Sowerby, 1835) from the outer Washington coast. The Veliger, 40(4): 292-297.
• Gosliner, T.M. 1991. Four new species and a new genus of opisthobranch gastropods from the Pacific coast of North America. The Veliger, 34(3): 272-290.


Trowbridge, C., 2001 (May 2) Re: nudibranch extinction? (1). [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Cynthia,
I agree that we are very short of information on the distribution, natural history, abundance etc etc of most nudibranch species. That is why I encourage 'non-scientists' to help us 'scientists' out. As you know there are very few professionals studying these fascinating animals, but there are thousands of interested amateurs, divers, photographers and naturalists who are observing them and photographing them. The Forum is a way to encourage this vast army of helpers to share their information.

Chromodoris petechialis is an excellent example of the importance and value of all these extra eyes. This animal is relatively large and Hawaii has been fairly well studied, compared with many tropical localities. By making people aware of the potential extinction of this species, I am hoping that a lot more people will go looking for this species than have in the last 130 years. I realise there are many species which have been recorded only once or twice, but when a large, distinctly coloured species of chromodorid, has apparently escaped the fairly good efforts of Hawaiian collectors in the last 30 years, we must begin to wonder whether it is gone for good.

As we well know here in Australia, where many land animals have become extinct in the last 200 years, it is very difficult to prove an animal as gone for ever. Hopefully, by raising the subject of extinction, with a real example, more of us will become aware that habitat destruction can effect marine animals as well as though on land. despite all this my main aim is to find C. petechialis alive somewhere.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (May 2). Comment on Re: nudibranch extinction? (1) by Cynthia Trowbridge. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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