What's wrong with Elysia clarki?

August 30, 2006
From: Bill Rudman

I have been waiting with anticipation for the publication of Caribbean Sea Slugs [Valdes, Hamann, Behrens & DuPont, 2006] so when it arrived I dropped everything and spent an hour thumbing through it. As I state in my review, it is an excellent contribution to our knowledge. However, the treatment of Elysia clarki puzzles me. I noticed that it was not there and presumed that since it was only recently described, it had not made the cut off date. I was a bit surprised a couple of days later, when looking at what treatment they had given Marcus's problematic Elysia patina, to find that Elysia clarki was in fact in the book, but as very junior synonym ofElysia crispata. Synonymised within a month of publication must be some sort of record - and hardly time for reasonable reflection.

The authors reason for the synonymy was:
'Elysia clarki was described for the Florida Keys populations without comparison to other Caribbean populations except for specimens from the Virgin Islands. Morphological differences between Florida and Virgin Islands populations fit within the intraspecific variability of E. crispata.'

My reading of the original description of E. clarki was that the comparison with Virgin Islands specimens was included because that was the type locality of E. crispata - a sensible thing to do considering the possiblility of more than one species. To say that new species cannot be valid if based on single populations may be a goal we can aspire to, but it is far from normal practice. It is certainly desirable to have specimens from many localities when describing new species, but often such material is unavailable and all of us have described species from one or two specimens at times. In this case the species was based on populations which have been studied extensively by the authors for a number of years both in the field and in the lab. I find it strangely inconsistent for the authors to effectively dismiss E. clarki in this book and yet accept many Marcus' species which were often based on single preserved animals - species we don't know the colour or shape of one living animal let alone even a single population.

It is certainly true that there are similarities between E. crispata and E. clarki, that's presumably the reason E. clarki hasn't been recognised before. It is also important to realise that both species are found in Florida, but E. clarki is found only in mangrove areas and similar canal habitats while E. crispata is found in open reef localities. Are the differences then intraspecific - perhaps the result of environmental differences? I don't think so, and I certainly have not been told of any intermediates. I thought I had discussed these two species, and E. diomedea, quite clearly in an earlier message [#16728].

Let's forget colour variation and even the degree of folding to the parapodial edge, which are difficult to quantify, and concentrate on two major morphological differences.

Fused parapodia: Elysia crispata is unique amongst the elysiids in having the parapodia fused in the anterior midline. On the other hand, in E. clarki and E. diomedea, like all other species of Elysia, the parapodia do not fuse anteriorly. To me this is a major anatomical difference.To say that fused and unfused parapodia are part of normal intraspecific variation is not credible without some pretty convincing evidence.

Digestive gland in foot: Elysia clarki and E. diomedea both have branches of the digestive gland ramifying through the sole of the foot. These are easily seen as green lines or a network colouring the sole of the foot. E. crispata on the other hand does not. These are not variations. In starved animals the ducts may not be clearly visible in the first two species, but  E. crispata, even when well fed, cannot grow such ducts in its foot.

These differences are well illustrated in my earlier message [#16728 ]. Although I am far from the Caribbean, these differences certainly appear real to me. I understand there is some genetic argument against E. clarki. I don't want to muddy the waters with a discussion on the efficacy of genetics and DNA in species determination; suffice to say that since morphology is controlled by DNA, if the genetics and DNA studies do not reflect the morphological differences then the genetical tests must be wanting. The first thing that needs to be addressed is why these clear morphological differences are being ignored. I think the onus is on those who do not accept E. clarki to show why major morphological differences which we would normally use to differentiate species don't apply in this case. Perhaps I am missing something? If so I would be grateful if someone could explain it to me.

Bill Rudman

Rudman, W.B., 2006 (Aug 30) What's wrong with Elysia clarki?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/17670

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