Costasiella ocellifera - one species or two?

March 3, 2004
From: Bill Rudman

Marina Poddubetskaia's recent message [#12277] which includes a photo of an egg ribbon laid by Costasiella ocellifera raises a very interesting topic. Miles & Clark (2002) suggest that what we call C. ocellifera is in fact two species which seem identical externally but have quite different larval development which can be clearly seen in their egg ribbons. They also suggest the two 'species' can be distinguished on their habitat preference, although they both feed on Avrainvillea nigricans.

They describe two egg ribbons, one which gives rise to planktotrophic veliger larvae and one in which direct development occurs, small crawling slugs hatching from the egg capsules. The egg ribbon photographed by Marina alongside, is identical to their photo of the ribbon of the planktotrophic larval type. Its eggs hatch in about 6 days and there are about 10 times as many eggs as in the direct-developer's egg ribbon, which takes 12 days to hatch. The direct developer's egg ribbon is of similar diameter to the other but the individual eggs are about 40% larger [106 and 76 microns respectively].

The authors consider the habitats of the two 'species' are also significantly different, with the direct developer being found in the Upper Florida Keys, occupying a mangrove fringe habitat where their algal food, Avrainvillea nigricans, is tall and abundant. The planktotrophic species by comparison was found in the Lower Florida Keys in an exposed coral sand habitat where A. nigricans is shorter and less densely spaced.

Miles & Clarke consider the planktotrophic developer is the true Costasiella ocellifera while the direct developer is a distinct but unnamed species. Since we don't know the development type of Simroth's original material this could become a nomenclatural problem. Also Costasiella lilianae Marcus, 1969 has been considered a synonym of C. ocellifera but I note that Clark & Jensen (1981) describe its development type as direct [capsular metamorphic]. As Marcus didn't describe the development type for this species either it could become quite confusing.

The first question though is whether this is a case of poecilogony in which one species can have more than one development type, or whether we are dealing with two species which can only be distinguished by watching them lay their eggs.

• Miles, C.M. & Clark, K.B. (2002) Comparison of biochemical composition and developmental mode in two populations of Costasiella [Opisthobranchia: Ascoglossa (=Sacoglossa)]. Journal of Molluscan Studies, 68: 101-109.

Best wishes
Bill Rudman

Rudman, W.B., 2004 (Mar 3) Costasiella ocellifera - one species or two?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/12361

Copyright © Australian Museum, 1998-2010