Re: Is Elysia viridis a single species?

August 10, 1999
From: Cynthia D. Trowbridge

Dear Bill,

Thank you for your response. Yes, in ecology, "cryptic species" could refer to those that match their background (which is pretty straight-forward) to those that are cryptic in the sense that they are difficult to distinguish from closely related species in the same or different regions. With the development of molecular techniques, we are finding many true species that were previously considered color morphs or variants.

Bill, you mentioned Placida dendritica, a species which has long fascinated me. In my mind, P. dendritica should be subdivided into many species; in fact they were originally described as regionally distinct species in New Zealand, South Africa, NE Atlantic, Japan, etc. As an ecologist, the animals in these different areas look and behave quite differently....VERY differently in my view but this is a rather controversial topic. I will send photos and drawings so you can judge the extent of geographic variation.

Elysia viridis is one of the best studied sacoglossans in the world...in all attributes: ecology, reproductive biology, physiology, etc. It is found in the NE Atlantic and Mediterranean; there are more questionable records of China, Japan, and South Africa. I believe the latter 3 records have now been been changed to other species .... I would welcome any information about the details.

Many sacoglossan "species" eat more than one algal food, often in different taxonomic groups. For example, Elysia viridis eats the filamentous algae Cladophora and Chaetomorpha as well as coenocytic algae Codium and Bryopsis. The slugs acquire chloroplasts from their food so their color is algal-derived, hence the different color "morphs"...not really comparable to color morphs in nudibranchs where the animal has different amounts of different pigments. Kathe Jensen has done excellent work on E. viridis with regard to Chaetomorpha vs. Codium diet...some individuals can learn to switch whereas others cannot (J. Moll. Stud. 1989). She has beautiful SEMs in more recent papers illustrating that there are differences in radular tooth shape and size associated with diet AND she demonstrated that tooth shape was inducible (i.e., by holding slugs from Codium on a Chaetomorpha and vice versa).

Bleakney has found a similar pattern with Placida dendritica (Veliger 1989, 1990) but he did not do the experimental switching of diets to see if radular morphology changed.

At what point is such variation enough to be species-level differences? I welcome discussion on this topic.

Thanks!!

Cordially, Cynthia

trowbric@bcc.orst.edu

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