26 April 2006

You are what you digest

A recurring theme on this blog has been what terrestrial mollusks eat. Certain foods (e.g. indigestible fiber) notwithstanding, what an animal eats will be useful only if the animal has the means to extract nutrients from it. This is done by breaking down the food first mechanically and then enzymatically. In many cases, symbiotic gut bacteria contribute to the process. Terrestrial mollusks are no exceptions from these generalizations.

Yesterday, while searching thru the free online archives of the Journal of Experimental Biology, I found an old paper1 on the digestive enzymes of the land snail Helix pomatia. The authors determined the following enzymatic activities in the extracts of the gastrointestinal tract of H. pomatia.

Proteolytic enzymes (proteinases). All animals, including strict herbivores, need to have enzymes to hydrolyze proteins to release amino acids. Slugs would definitely need proteolytic enzymes to digest the earthworms they have eaten.

Lipases. These enzymes hydrolyze fats into fatty acids. Probably they are also present in all animals.

Carbohydrases. These enzymes hydrolyze anything from a disaccharide (e.g. sucrose) to a polysaccharide (e.g. starch) into smaller and simpler carbohydrates. Helix pomatia had enzymes that hydrolyzed numerous carbohydrate substrates, including sucrose, maltose, xylan, mannan, starch, snail glycogen, chitin and cellulose. Xylans are polymers of xylose and are constituents of plant cell walls. Mannans are plant polysaccharides that are polymers of mannose. The ability of snails to hydrolyze plant polysaccharides (xylan, mannan, starch and cellulose) as well as chitin, an animal and fungal polysaccharide, is probably an indication of their omnivorous diet.

1. FAY L. MYERS and D. H. NORTHCOTE. A Survey of the Enzymes from the Gastro-Intestinal Tract of Helix Pomatia. J. Exp. Biol. 1958 35: 639-648. pdf


pascal said...

I'm curious how the ability to hydrolize animal chitin has evolved - presumably several times over the course of animal history.

It seems to be a kind of chicken-and-egg problem: did the ability to hydrolize chitin (animal bits) allow carnivory to evolve, or did carnivory dictate the ability to hydrolize chitin?


Chitinases are probably very old enzymes.

After I put this post up I remembered that snail radula also contains chitin. So chitinase may also function in the maintenance of the radula.