24 November 2011

Morphological scaling in snails

Here is the relation between shell volume, foot sole area and shell length for an intertidal snail.

In juveniles the foot is already shorter than the shell. During ontogeny the shell grows faster than the foot and so in adult snails the foot ends up being even shorter relative to the shell.

Note how much the rate of growth of the sole area lags behind the rate of growth of the shell volume. This is because volume is proportional to the cube of one or more linear dimensions, while area is proportional to the square of linear dimensions.

The interplay of volume and surface area and the resulting scaling effects underlie many evolutionary processes. For example, one reason why the smallest animals are all aquatic is that they lose their water content very quickly outside of the water, because their surface areas are very large relative to their volumes. At the opposite end of the range, the inner surfaces of the lungs of the largest animals always have convoluted morphologies, because the area of a flat surface would not be enough to satisfy the gas exchange requirements of the relatively much larger volume of the animal itself.

In the case of the subject snail, the consequence of this scaling effect is somewhat more mundane. When its shell gets too large relative to the foot, the snail can't lift the shell up anymore; it simply drags it behind its tail. In an earlier post, I discussed how this happens in the land snail Euxina circumdata.

This subject has had me preoccupied during most of my waking hours for the last month or so. In fact, I even lost sleep thinking over it one night. But finally, I am beginning to fit all the available pieces of the puzzle together.

13 November 2011

Jaws of a different kind

One of the many live snails I photographed last June when I was in Turkey was this juvenile of the species formerly known as Helix aspersa. After the photo session, I released it back to the wild, but its photos are now in my records with the code name HA1.

HA1 was one subject in an ongoing study looking at the relations between the dimensions of snails' feet and shells. One way of getting that information is to take pictures of snails from below while they are crawling on glass. The ruler provides the necessary scale.

When I took a closer look at the above picture a few months later, I noticed that the snail's jaw was also visible. Here is the relevant detail from the same picture.

Despite its name, the gastropod jaw is only remotely analogous to the vertebrate jaw. The jaw is attached transversely to the upper margin of the mouth. The snail uses its jaw to scrape off food particles, for example, algae, from surfaces or to rip off pieces of food, for example, leaves, and to push them into the mouth cavity.

03 November 2011

In search of forgotten knowledge

I spent several hours today looking up ancient information in ancient books. Thanks to sites like Google Books and the Biodiversity Heritage Library, one can now do that sort of research right from one's computer. Who needs libraries anymore?

The keyword I used for my searches was, of course, snail.

Among the not-so-dusty tomes I discovered was The system of natural history (1800) compiled from the writings of Comte de Buffon.

Buffon's book was the newest of the books I skimmed. The oldest one was John Hill's An history of animals: containing descriptions of the birds, beasts, fishes, and insects of the several parts of the world from 1752. I'm not sure who John Hill was.

The most interesting find, so far, was Microscopic observations or, Dr. Hooke's wonderful discoveries by the microscope by Robert Hooke (1780).

More than 200 years ago Hooke gave a fairly accurate description of how most terrestrial gastropods move (spellings and capitalizations are original):
Its Way of moving from place to place, though destitute of Feet, is effected by two large muscular Skins, that are lengthned by letting out; after which, their Fore-part is shortned into Folds, and the hinder Part falls into the fame Contraction: Then the Fore-part extends, and draws along the Shell. A glutinous Slime emitted from the Body, enables it, at the fame time, to adhere firmly and securely to all Kinds of Surfaces, which is an Advantage few Animals that have Feet can pretend to.

Hooke also described the jaw of a snail. I will write about that in a future post.