Re: What's wrong with Elysia clarki?

Dear Angel,
Thanks for your response. The photo from Belize you have sent is probably from the same series as the one already on the Forum [message #1656]. I added a note in May this year to that message saying it may be E. clarki, but in that photo the parapodial gap is not clear. Your photo definitley shoa the gap. My information was that this was a mangrove specimen, which would fit with E. clarki, not a reef specimen as you suggest.

This brings up an important point in this discussion. The only photo I had seen of this Belize form before today did not show either of the two points which seem critical in identifying these species externally - the parapodial gap and the digestive gland in the sole of the foot as I have discussed in earlier messages. You mention there are many photographs available of this common animal[s] which could be examined, but I suspect most if not all available photos will not clearly show these two features. When this topic first arose I searched the Forum, and all the published literature and websites I could find looking for photographs to see these features without success. In fact for such a common animal, it was extremely difficult to find an illustration or written description clearly describing the external morphology of  E. crispata including that the rather vital point that the parapodia are fused anteriorly. So although it may be common, information on its salient points are not easy to find. One way would be to alert the many interested amateurs throughout the Caribbean to look out for these differences. I guess that is what I found disappointing about your decision in your excellent book, to effectivley censor E. clarki from the minds of the many readers your book will surely receive. It would have been a perfect way to encourage them to give Elysia 'crispata ' more than the normal passing glance they would give to something considered so common..

Concerning colour and parapodial folding - I specifically mentioned that these points were probably not that helpful in a discussion on these species because of the difficulty in quantifying them. Similarly, Pierce et al mention the way E. clarki rests with its parapodia open flat, a bit like descriptions of the mangrove dwelling species Elysia leucolegnote in the Indo-West Pacific. Although I think this behavioural difference and degree of parapodial folding are probably good characters, discussing them at present, would bog us down in trying to quantify the unmeasurable.

What I said we should do is try and test the two important external characters - the parapodial fusion and digestive gland ducts in the sole of the foot. Unfortunately none of your photos show both those features so we are no further ahead.  Concerning habitat specificity. My advice is that the Belize animal is from a mangrove habitat so one of us is wrong. Concerning the Florida animal which looks to me like a 'typical' well-folded E. crispata - as I am at pains to repeat often, I have no personal experience with these animals, or the Caribbean, so I cannot comment on whether that is an unusual find or not, but from the writings of Pierce and his co-workers, it would seem to be.  

Concerning the genetics. I remember a couple of years ago reading that there was more genetic variability in a troop of chimps in West Africa than in the whole world population of Homo sapiens. While these genetic analyses are very interesting, it seems we shouldn't defer to them when we have, what seem to me, to be good external morphological characters. I am not belittling genetics and DNA studies, but we have to be careful as morphologists not to consider them to be the answer to all our unanswerables. After all, geneticists and DNA workers spend as much time commenting on each other's work as morphologists do, so they can't have all the answers.

 I am happy to invite participants to send photos from anywhere in the Caribbean of  Elysia crispata and  Elysia clarki, but there is a rule. All photos would have to be in a set of 2 or 3 showing the whole animal from above, the sole of the foot and the front of the parapodia to show whether it is fused or separate. If there is only one species, then surely we will find animals with fused parapodia and green patches of digestive gland in the sole of the foot.

Best wishes,
Bill Rudman