A trick of the tail: How snails avoid snail-eating snakes

Cross-posted to Bioscience Writing & Editing

There's not much chance of a land snail outrunning a predator, so a slow-moving gastropod must rely on other measures to stay out of trouble. The shell is an effective defence, but it's not perfect. Some predators can crush them. Others, such as the song thrush and pitta, smash them open on rocks. Carabid beetles and firefly larvae are small enough to breach the defences by slipping through the only gap in the armour — the shell aperture. Even the best protection has a weak spot.

Not only do snails in some parts of Asia have to survive these usual suspects — brute force crushers and smashers and those sneaky nibblers — but they also have to deal with specialist predators: snail-eating snakes. Iwasakii's snail-eater, Pareas iwasakii, has asymmetrical jaws that are adapted to extract snails from their shells. (Click here to to watch a video of the snake in action.)

Researcher Masaki Hoso of the Naturalis Biodiversity Centre in the Netherlands has discovered that Satsuma caliginosa, a camaenid land snail from Japan, has evolved counter-measures to escape its serpentine hunter.

The range of Satsuma caliginosa partly overlaps that of Iwasaki's snail-eater. Where the two are separate, the interior of the Satsuma shell aperture is smooth, but where predator and prey occur together, the shell aperture is lined with several low barriers that narrow the opening and prevent the snake grabbing hold of the snail and dragging it out. Unfortunately for Satsuma, these barriers only develop in adults.

Masaki Hoso found that Satsuma employs an equally effective but more drastic method of staying alive until the shell barriers develop. When snagged by a snake, the snail sheds its tail. (Really, the hind part of the foot.) Result: predator gets a bite-sized morsel of prey and prey gets to live another day.

(a-c) Foot regeneration in S. caliginosa in the wild.
(a) S. caliginosa caliginosa with an intact, (b) a regenerating and
(c) regenerated foot. (d) proportion of S. caliginosa with
a regenerating or regenerated foot in the wild.
(Image from Hoso, 2012)

But this process of autotomy is costly. Regrowing the shed tail requires a lot of energy and while that is taking place, resources are diverted from shell growth. For immature snails, this involves a trade off between defence mechanisms. Dropping the tail is a life-saver, but delays the development of apertural barriers. Still, dropping the tail means that the snail can survive long enough to grow those apertural barriers.

Reference 
Hoso, Masaki (2012). Cost of autotomy drives ontogenetic switching in antipredator mechanisms under developmental constraints in a land snail. Proceedings of the Royal Society B DOI: 10.1098/rspb.2012.1943