by Bob Thomas
Have you ever found a shell on the beach with a perfectly round hole piercing it? I hope you had the good sense to pick it up, lace a string through the hole and wear it as a beach necklace.
I’m sure you’ve wondered why such perfectly round, beveled holes are so commonly found. These are evidence of carnivorous snails on the beach. The beveled hole is almost always near the umbo (that is, at the hinge where the two shells attach) of a bivalve such as a clam. The hole is bored by an Atlantic Moon Snail (or Shark Eye, Polinices duplicatus, Family Naticidae), that cruises just beneath the sand searching for clams. When a moon snail finds its prey, the moon snail attaches to the clam, usually at the umbo. There are two steps needed to reach the meat through the shell. First, the snail uses its accessory boring organ (ABO), which produces hydrochloric acid, enzymes, and other substances to soften the prey’s shell. Once soft, they rasp it away with their radula (actually an elongate, self replicating radular ribbon), thus creating the characteristic beveled hole. Once the hole breaks through the shell, the moon snail sticks its proboscis inside the clam and, using its radula, rasps away the clam’s tissue, basically eating it alive.
Moon snails are also cannibalistic. If they choose to eat another moon snail, they tend to attack the thick portion of the shell toward the back of the spiral so the prey cannot pull back and avoid the radula. Sometimes, the attackee becomes the attacker and eats its predator!
Another predatory snail in coastal Louisiana is the Oyster Drill (Stramonita [Thais] haemastoma). Though the snails themselves live in very salty water, meaning that they tend to be offshore from the beaches, beachcombers will recognize them since their shells are the familiar pointed spiral shells most commonly used by hermit crabs. They, and all other predatory members of the Family Muricidae, use an ABO in conjunction with a radular ribbon when feeding on oysters. Their holes, in contrast to those of moon snails, have straight sides – they are not beveled.
Depending on the species and size of the snail, and thickness of the prey’s shell, it may take hours or even more than a whole day to drill the hole.
Interestingly, studies have revealed mussels in the Mediterranean that can be fed upon, reseal the holes, then withstand more borings. There may be such situations in the Gulf of Mexico, but it has not been reported.