Why Are There Holes In Some Of Tybee’s Seashells?

A group of shells on a tan surface.

One of the best features about Tybee Island beach combing is the variety of shells that you can find.  From day to day you never know where the best places will be.  Tybee has a large variety of marine species because of its position on the east coast where both northern and southern plants and animals contribute to our species richness.  We also see a wide spectrum of shell colors that is often caused by past environments where a particular shell has been buried or spent time.  For example, our most common bivalve shells, the Ark shells, range from gray, dark red, orange, gold, and white.

As you search through our bivalve or clam-like shells you’ll soon find one with a perfectly round, small hole in it – just right for making a necklace.  You’ll see these holes in Arks, Surf Clams, Lucines and others.  It might surprise you to find out, though, that the animal that lived inside and made that shell did not make that hole.  To find out where that hole came from, we need to look at another mollusk, a gastropod or snail, that we also often find on our beach.

If you know where and how to look, it’s not too difficult to find Moon Snails on Tybee’s beach.  Their round, light-brown shells often wash up along the high tide line.  But sometimes you can find a live one burying through the wet sand near the low tide line.  If you find a live one, it will probably quickly withdraw back into its protective shell.  But if you lay it back onto the wet sand or put it into some seawater, and be patient, it might re-emerge and start gliding across the surface.  You’ll be amazed at how large its body is, and wonder how it can possibly pack all that body back into that small shell!

Moon Snails, also called Shark-eye Snails, are predators, and they eat Arks, Coquinas, Surf Clams and other bivalves that live buried in the sand.  To accomplish this, a Moon Snail burrows through the sand, hunting, until it encounters one of its clam-like prey.  As you can imagine, the clam quickly closes up for protection inside its two shells.  The Moon Snail is not able to pry the two shells apart, but it wants to eat the soft-bodied animal inside.  Within the snail’s mouth is a hard, tongue-like structure called a radula.  It is like a small file or rasp covered with tiny sharp teeth-like structures.  With its radula, the Moon Snail drills a perfectly round, small hole through the bivalve’s shell (and that is the hole you see).  The hole is too small for the snail to crawl through, but it can extend its radula down through the hole to the inside of the bivalve.  The snail will then slash its radula around in there, shredding and chopping the bivalve’s body into “soup.”  The snail can then just suck the contents out, leaving behind a couple of empty shells – one of which has the hole in it!  So the hole wasn’t originally a part of the bivalve’s shell; but instead that hole was pretty much the last thing that happened to that animal.

So while you’re beachcombing at Tybee, and you find that perfect size, shape and color shell with a hole in it for making your necklace; you can thank the bivalve animal that made the shell.  But you also need to thank some predatory snail, like our Moon Snails, for drilling the hole!

Dr. Joe Richardson (Ph.D. Marine Sciences) is a professor emeritus of Marine Science who continues doing research and conducts “Tybee Beach Ecology Trips” for families and groups year-round.  He can be reached through www.TybeeBeachEcology.com or email DrJoe@TybeeBeachEcology.com

You might also enjoy