18 February 2012

Suspended malacology

Suspending snail shells from pieces of string while sipping wine is a good way to spend a quiet Saturday evening at home. Only a few would disagree, I'm sure.

There is a deeper motive behind this activity, of course. I am trying to figure out where the centers of gravity of the shells are located.

Here is a marine gastropod shell hanging from a string with the columella (central axis) of the shell approximately horizontal (I don't know what the species is).

Here is another unidentified marine gastropod in suspension.

Finally, a land snail, a Cerion sp.

From the photographs, I've estimated the relative location of the center of gravity for each shell. The numbers from the base of the aperture and expressed as a fraction of the total shell length were as follows: 1st species, 0.37; 2nd species, 0.46; Cerion, 0.52.

Keep in mind that these numbers are from a single specimen for each species. There will undoubtedly be some within species variation.

I don't yet know what these number mean in terms of the shape of each shell. Various decorations present on some shells, for example, the spines along the lip of the 2nd shell, must also influence the location of the center of gravity. Also, the location of the center of gravity will probably be different when a shell is occupied by a snail.


Fred Schueler said...

for the conical species you could ask what's the relationship between the angle of the spire and the whorl where the c.o.g. falls - and you could work out fractional whorls by the angular position of the aperture.

Snail said...

A simple yet elegant method. And the wine is essential.

I think the tall brown shell is a turritellid and the pale one a strombid, maybe Tibia. But I would not be at all surprised if someone with a better knowledge of marine shells gives you a completely different answer.

Anonymous said...

Elegant! Looks like an art exhibit! I think it's Tibia sp, Turritella sp, and Cerion sp, or something like that.

Susan J. Hewitt

Will Chapman said...

Fascinating. I found this site by accident so forgive me if my comment is naive.

It occurs to me that the point of central gravity is probably related to where the animal itself 'resides' and how it feeds.

Perhaps that states the obvious but if one thinks about how the shell of a newly born creature grows, isn't it a given that evolutionary pressure will have made sure that it will grow in such a way that its mouth is where the source food is likely to be. If the shell grew in a random way, those snails with, for example, a top heavy rear, would have a struggle feeding and not survive many breeding cycles. If I am correct, the cog ratio could be species specific.


Since a shell starts out small & gets wider as it grows, its COG will always be closer to the aperture than to the apex. The COG of a shell with a snail in it will be different depending on if the snail's head-foot is in or out of the shell. But I suspect the COG is probably always nearer to the aperture.