03 November 2010

Volume of a snail shell

A subject high in my list of current interests is the measurement and the biological applications of the volumes of snail shells. Recent relevant posts are here and here.

The volume of a shell is a direct indicator of the amount of space the occupant snail requires. For that reason, I consider it a biologically significant property. However, shell volume is rarely used in research, probably because there is no easy way to measure it. One method that immediately comes to mind is to fill a shell with water and then to determine the volume of the water, preferably by weighing the shell before and after. However, this is easier said than done, because air tends to get trapped in the apexes of shells, which makes it very difficult to fill them completely with water. That's why empty snail shells float in water. Moreover, many field-collected shells have debris trapped in them, which also prevents them from getting filled completely. And because most adult shells are opaque, one can't see if a shell is clean or filled completely.

Here is a rare example of the use of shell volumes from the literature (Kemp and Bertnes, 1984).

This graph shows the relationship between shell volumes and shell lengths in 2 morphs of the intertidal snail Littorina littorea. The authors state that shell volumes were determined by "measuring the water holding volume" without additional details.

From general geometric considerations, volumes of snail shells (V) are expected to follow a power law in the form, V=cL3, where c is a constant and L is a linear shell dimension (for a relevant discussion see, Schmidt-Nielsen, 1984). If you take the logarithm of both sides of that equation, you'll get, logV=logc+3logL. In the graph above, the equation for line C was logV=-4.04+3.09logL. The equation for line E was similar.

Schmidt-Nielsen, K. 1984. Scaling: Why is animal size so important? Cambridge University Press.
Paul Kemp & Mark D. Bertnes. 1984. Snail shape and growth rates: Evidence for plastic shell allometry in
Littorina littorea. Proc. Natl. Acad. Sci. USA. 81:811–813.


MK said...

Here's another paper that uses shell volume for some ecological pattern study:


Fred Schueler said...

Maybe the thing to do is to draw some heated glass tubing down to the dimensions of the shells you're working with, and as twisty as possible, and see how your methods of filling shells with water work on the glass surrogates.

As for methods, I'd put the shells in water and than draw a vacuum above the water to draw the air out, and release the vacuum to let the water in. Or you could boil the shells and get the same effect.

I've contemplated both of these for sinking the shells in drift, while leaving the vegetable debris floating.

Anonymous said...

Two suggestions that may be easier to apply than earplugs: pour melted wax into the shell until it is overflowing, after it cools just scrape off the extra. If you want to remove it, just heat the shell up and it will drip out. You can also try expanding foam insulation, I tested it on a shell here and it seems to work fine, but it isn't removable.

Anonymous said...

Up here in North Carolina, we use sand (sieved until uniform). Fill up the shell, level the sand at the aperture, then pour out and weigh. Of course, you'll have to weigh different amounts of sand to figure out the conversions.

Nick D Waters said...

Inspecting various species, shell morphology does not seem to be taken into account on how volume is calculated.

Shells of Sonorella, Eremarionta, and Maricopella (Sonoran Desert, Arizona), my thoughts are to calculate the volume using the standard cone formula, and then deduct a certain propertion, say 10%, by use of a second cone to arrive at a more conservative estimate.

Growth of their shells proceeds on a moderately depressed form to ~4 whorls, but the aperture growth is incomplete. Looking directly into the aperture, the growth is more akin to "c" rather than "o".

Hope I make sense. I'd appreciate feedback.