Hi all

To continue this dual larval thread a little more, the following year
Philippe Bouchet added a contribution. (1989) A review of poecilogony in
gastropods. Journal of Molluscan Studies 55(1):67-68. Unfortunately I can't
double check the paper as me and it are in two very far apart places but
basically all cases thought to have dual larval development at the time of
the paper, were shown, on closer examination to be two or more species, eg
a one genus of the top of my head was a small red nudibranch, Rostanga,
which has very similiar (cryptic) species some of which have direct
development and the others had planktotrophic development.

But like Harry I hadn't heard of the Skenid example it could possible but I
would be a bit skeptical but hey I am supposed to be thinking about
terrestrial and freshwater snails right now not their marine cousins.

Stephanie


At 09:15 AM 10/10/2003, you wrote:
>Dear Andrew,
>
>This thread is straying into the controversial, but two provocative
>notions (in which I have placed substantial credulity) ensue.
>
>Time will probably tell, but I think it not unlikely that Epitonium
>blainei Clench and Turner, 1953, although possibly based on
>honestly-mislocalized material, has been expanded into a cruel hoax by
>unscrupulous collectors and dealers who can profit from the disparate
>(perceived) value.  See <http://www.jaxshells.org/blain.html>.
>
>The theoretical reversion from lecithotophy (complete or nearly complete
>direct development of larva to juvenile [crawl-away]) to planktotrophy is
>a consideration.  However, this process hasn't been demonstrated in the
>mollusca as far as I am aware.
>
>Your report of poecilogeny (more than one type of larval form in a single
>species) in the skeneid is news to me.  Hoagland and Robertson (1988)
>exhaustively reviewed instances of reported poecilogeny reported in the
>Mollusca (42 species) and essentially refuted all of them.  At best this
>duality is extremely rare.
>
>Ref. Hoagland, K. E. and R. Robertson, 1988.  An assessment of poecilogeny
>in marine invertebrates: phenomenon or fantasy? Biological Bulletin 174:
>109-125. April.
>
>Harry
>
>
>At 01:52 AM 10/9/2003, you wrote:
>>>I recently looked at a pair of closely-related western Atlantic snails
>>>(Rissoidae: Stosicia aberrans and S. houbricki; see
>>><http://www.jaxshells.org/907b.htm>.  The former has planktotrophic, the
>>>latter lecithotrophic development.  As I pondered the evolutionary
>>>origin of the taxa, I wrote:
>>>Verduin (1977) looked at European Rissoa species and concluded that the
>>>lecithotrophic species was the descendant (apomorphic), and the
>>>planktotrophic was ancestral (plesiomorphic).  Similar conclusions were
>>>reached in a wider taxonomic and/or paleontological context in Ficus
>>>(Smith, 1945), Trophon (Bouchet and Warén, 1985), Nassariidae (Martinell
>>>and Cuadras, 1977), Neogastropoda in the early Tertiary and Volutidae in
>>>the Cenozoic to Recent (Hansen, 1983), Terebridae (Bouchet, 1981), and
>>>Turridae (Bouchet, 1990). To my knowledge, no instances of the opposite
>>>pattern of evolution has been reported.  In fact, Strathman (1978)
>>>concluded that plankotrophic development was ancestral to the
>>>lecithotrophic condition among all marine invertebrates with such life
>>>histories, and that the reversal of this process was much less frequent,
>>>even unlikely. A relatively limited geographic distribution of
>>>lecithotrophic species and their tendency to be less long-lived (in a
>>>geological sense) than their planktotrophic congeners has been noted by
>>>several workers including Jablonski (1982, 1986) and Hansen (1983)."
>>
>>If the West Indian "Epitonium" blainei is a synonym of New Zealand's
>>Cirsotrema zelebori (and shell charcters are indistinguishable, so until
>>the animals are compared, they must be synonymized), then the American
>>colony must have migrated from NZ at some time in the past. Today C.
>>zelebori (please forget Boreoscala, it's a synonym) has a
>>nonplanktotrophic protoconch. So either the snails crawled across the
>>abyssal plain and along the continental shelves, or at the time it had
>>planktotrophic development and swam/drifted.
>>
>>Here we have evidence of a change within a species from planktotrophic to
>>nonplanktotrophic development. And if a species has genes for one type of
>>development, if it changes type, it can revert at a future time, as those
>>genes remain.
>>
>>Other species (a wood-inhabiting skeneid, if I remember correctly) have
>>alternative development... larvae from ONE eggcapsule can have more than
>>one type of protoconch.
>>
>>Andrew Grebneff
>
>Harry G. Lee
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>
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******************************************************************************
Stephanie A. Clark

Biodiversity & Systematics
Department of Biological Sciences
University of Alabama
Box 870345
Tuscaloosa, AL  35487
Phone: 205-348-5828 FAX: 205-348-6460