>Rather than asking why Conus (and others with tiny opercula) has an
>operculum, ask what reason it might have to NOT have one. What
>selection force might cause them to loose it? Obviously there is no
>great force acting in that manner, since so many snails have
>vestigial opercula. It's still there because there's it's not
>lowering the animal's fitness (ability to produce young), but no
>longer large because having one doesn't increase the animal's
>fitness either.
>
>How about that for a possible answer.

Not bad. There will be little "selective pressure" to lose an
unneeded structure unless it is interfering with some other function
(eg as a heavy shell would do in prosobranch animals adopting a
plaktonic existence eg Carinaria, Pterotracheas, Atlantidae).

Wghat occurs odd to me is obviously monophyletic families which
contain both operculate and nonoperculate species (Turbinellidae,
Olividae, Volutidae)... heck, some SPECIES contain animals which bear
opercs and some don't (Volutomitra springs to mind).

The neritoid pseudolimpets Septaria (Neritidae, freshwater &
brackish) and Phecolepas (Phenacolepadidae) both have internal
opercs. The latter's is apparently tiny and vestigial, but the
former's is large, well-developed and I bet is used as a muscular
attachment/lever to elevate the center of the foot, increasing
adhesion at minimal metabolic cost.
--
Andrew Grebneff
Dunedin, New Zealand
64 (3) 473-8863
<[log in to unmask]>
Fossil preparator
Seashell, Macintosh & VW/Toyota van nut
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I want your sinistral gastropods!
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Opinions in this e-mail are my own, not those of my institution
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A: Because it reverses the logical flow of conversation.
Q: Why is top posting frowned upon?