20 April 2005

Spacious shells 1

Most pulmonate land snails seem to have quite a bit of extra space in their shells. One way to demonstrate this is to hold a snail that has been dormant for a while against a bright light and to try to make out the outline of the snail’s body within the body whorl (this will work only if the shell is translucent). The picture on the right shows an individual of Neohelix albolaris that I collected in Maryland in February 2001. The snail had been dormant for a while as indicated by the multiple membranous epiphragms (short arrows) it had formed as it withdrew deeper and deeper into its shell possibly as the weather got colder and colder. The long arrow is pointing at the edge of snail's body.

Considering that the body whorl makes up the bulk of the shell volume, it is obvious from this picture that the volume of extra space in the body whorl is a significant fraction of the total volume. Building a shell is both a time consuming and a costly process in terms of the raw materials and energy it takes. Why don't the snails build shells that are just big enough for their bodies?

I can think of three advantages in having a shell that is larger than its occupant. First, a larger shell allows a snail to carry water in its mantle cavity1. This water in turn allows the snail to be temporarily independent of the water fluctuations in its environment and to visit places that may be on the dry side.

Second, when the snail is dormant the extra space in the body whorl provides additional security against both biotic (predators, parasites) and physical dangers by letting the snail withdraw from the shell aperture and fill the intervening space with multiple epiphragms.

Third, if a snail with extra space in its shell loses a portion of its shell starting at the lip, it may still have enough shell left to withdraw its entire body into. In another post, I will demonstrate with examples that this indeed happens.

Spacious shells 2

1. Blinn, W.C. 1964. Water in the mantle cavity of land snails. Physiological Zoology 37:329-337.


Tim Pearce said...

Land gastropods can take up and lose a lot of water. I suspect the "extra space" is not extra at all, but accommodates the fully hydrated snail adequately, with a little breathing room. Terrestrial slugs readily lose 35-50% of body weight by water loss, and in one case, a Limax tenellus recovered after losing 80% body weight (Runham & Hunter 1970: 71-72).

Runham, N.W. & Hunter, P.J. 1970. Terrestrial Slugs. Hutchinson U. Library. 184 pp.

Frank Anderson said...

I'd noted that some snails have a lot of space in their shells, but as Tim notes, maybe I'm just seeing snails at different hydration levels.

What about the ones that maybe don't have so much space? I can't think of land snails that lack space -- other than semislugs -- but surely there's appreciable variation among species (I was mainly thinking of geoducks, which can't even remotely pack their siphons between their large, but still insufficient, valves). And how costly is it to make shells? Calcium isn't common in all terrestrial habitats, and some snails have very thin shells, so it seems like "extra" space should confer some advantage...

Frank Anderson said...

And another thing...in marine gastropods, much of the space near the top of the spire is loaded with gonads. Lots of critters, particularly those with spacious coeloms like some echinoderms and annelids, have substantial internal space that gets filled with gametes prior to spawning. Is the same also true of land snails?

Anonymous said...

I am over five years late in weighing in on this, but surely for pulmonate land gastropods when they are active, a very significant amount of space is taken up by the pallial lung? if you look at this image


you will see that a large part of the body whorl is taken up by the pallial lung. I am assuming that when a snail is simply resting temporarily it still needs a certain amount of internal space for breathing, but when it is totally dormant, I would think it uses significantly less oxygen and can make do with a much smaller "breathing space". Also of course, as Tim Pearce said, a land gastropod can lose a very significant percentage of its water content during a long hibernation (or estivation), and yet it can rehydrate successfully when exposed to a suitable temperature and plenty of rain water at the end of dormancy.

Susan J. Hewitt