Many children collect these shells on the beach and thread them onto a string. However, most of them didn’t know how the hole got into the shell! These shells are from bivalves, which are marine snails that instead of having the spiral shell we associate with snails, have two dish like shells called valves. The snail can fully withdraw itself between the two shells and close them tightly to avoid predators.
We don’t see these animals as we walk along the beach because they live buried in the sand usually below the waterline. They are perfectly suited to this environment with a well-developed muscular foot that they use to burrow down into the sand, out of sight of predators. To get food and oxygen they have two siphons which like periscopes, are pushed up through the sand and extend just above the surface. One siphon draws water into the animal bringing in food and oxygen, the other siphon exhales this water and the animal’s wastes.
Bivalves share the sand with many other creatures. One such animal is the moon snail. A carnivorous snail that likes to eat bivalves. The Moon snail moves across the surface of the sand looking for the telltale siphons of a hidden bivalve. Once found, the bivalve retreats into its shell while the moon snail wraps its foot around the bivalve holding it tight, preventing escape. The Moon snail then starts drilling a hole into the bivalve shell. Its tongue, called a radula, is specially designed for this purpose. It is a ribbon of tiny teeth that are rubbed back and forth across the shell. The work of the radula is aided by acidic secretions that soften the shell. Once the hole is complete as shown in the picture below, the Moon snail delivers its digestive enzymes into the bivalve, breaking down the bivalve so that the moon snail can suck it up through the hole.
That all sounds very one-sided in favour of the moon snail! However, if you collect enough bivalve shells you will find some with incomplete holes as in the picture below! This means the bivalve has managed to escape the clutches of the moon snail before it can be eaten. Some shells have grooves across their surface showing how the bivalve slowly wriggled free of the moon snail while the radula continued to carve away at its shell!
This following clip shows some different shells with holes in them. This is just one amazing adaption that snails have to feed on other snails, and if you would like to learn more about marine invertebrates such as snails, and how they adapt to survive, feed and breed and/or joint the Education Team when restrictions ease for a fascinating ramble through the rock pools please contact Education Director Mandy Robertson on education@dolphinresearch.og.au
