In shelled marine gastropods the ability to withdraw deeply into the shell is an escape mechanism from the predators, such as crabs, that break the shell starting at the aperture: the deeper a snail can withdraw into its shell the more likely it is to survive an attack. Despite the obvious evolutionary importance of retractability, not much has been written about it. The only malacologist who seems to have given serious thought to the subject is Geerat Vermeij. In his Evolution & Escalation (1987), he gives 2 advantages to a snail of being able to withdraw deeply into its shell. 1. Avoidance of detection by predators; if a snail can hide within its shell, a predator would be less likely to see it or detect it by chemical means; 2. Avoidance of capture or injury even if a predator breaks away the aperture and portions of the body whorl.
Vermeij also gives an example:
In laboratory trials at Guam, for example, I have found that the crab Calappa hepatica frequently removes a half whorl or more of the shell of Terebra affinis and then stops, even though the foot [of the snail] has not been reached. Probably the shell opening becomes so small that the crab's claws cannot gain a purchase on the shell to continue peeling.I demonstrated the survival advantage of deep withdrawal in the case of the intertidal snail Batillaria minima in this paper and briefly discussed it in this post.
A snail can withdraw deep into its shell only if its shell is larger than the minimum size necessary to fully accommodate the snail’s body. This creates an evolutionary trade-off, because building a larger shell also has disadvantages. Vermeij mentions 2 in his A Natural History of Shells (1993): 1. Extra energy and resources required to build a larger shell; 2. Extra energy required to carry a larger shell.
I will add a 3rd disadvantage. Building a larger shell may also take time away from other activities, most importantly from reproduction, especially in species with determinate growth, which normally do not start reproducing until after they have completed shell growth.
I am trying to organize my ideas here in conjunction with a manuscript I am working on. Expect more posts along these lines in the near future.