Shells

in May 2011 - Marine Scene

No doubt as evolutionary adaptations go, the shell must rank as one of the clever ones. The ancestral gastropod is thought to have been a shelled burrower, creeping through ancient sands in search of slower prey. It wouldn’t have been like the elegant spiral wonders we see today, but was probably more like the covering on limpets, which are the more ancient members of the group.

Protection from predation is one of the big challenges of being alive and a shell acts as a mobile fortress for retreat. Which is brilliant, but snails have had to overcome the drawbacks of living in a house with only one exit. The different lineages of snails have varying solutions for breathing, feeding and getting rid of waste. Almost all members of the group use a toothed tongue to rasp at their food. They also have sensory tentacles with eye spots at the base for collecting information about their surroundings.

Limpets breathe and excrete around their shells. Keyhole limpets direct their waste streams away from their bodies out of a fleshy chimney, while perlemoen opt for the multiple chimney approach. The more modern spiral-shelled snails have gills only on the right hand side of their bodies. An inhalant siphon has evolved, so that water takes a one way route through the snail. Predatory cone snails shoot a venomous harpoon through this siphon to catch their prey. Some snail species have such virulent toxins that they can kill humans.