PRICE, R. M.: Evaluating a putative neogastropod adaptation: is there a functional relationship between columellar folds and the columellar muscle?

Malacologists traditionally assume that columellar folds, ridges inside a gastropod shell, offer a functional advantage. Two functional hypotheses for the existence of folds are frequently cited in the literature, but neither has been rigorously tested. One suggests that folds increase the surface area of columellar muscle attachment, increasing the animal’s ability to maneuver its shell. The other states that folds guide the columellar muscle during retraction and protraction, as a railroad guides a train. To test these hypotheses, I have qualitatively described the muscle attachment in 20 neogastropod species with and without folds. I have also quantified attachment morphology in half of those species. The attachment is far more complex than assumed by most malacologists. It is not parallel to or coincident with the columellar folds, so folds cannot substantially increase the surface area of attachment. Furthermore, in all but one species, the muscle hardly contacts the folds, implying that folds may not guide the muscle. There is a range of muscle attachment morphology among all species studied, but there are no obvious differences in the attachment between species with and without folds. Thus, a variety of muscle attachment types is functionally suitable for neogastropods; if columellar folds are adaptive, their function is probably not directly related to columellar muscle attachment. Future work will document convergence in columellar folds to determine whether independent derivations of similar columellar folds corroborate the supposition that columellar folds are adaptive.