Collecting seashells on the beach has long been a favorite activity of many children and adults. Perhaps you have carried them by stringing the clamshells together through the holes in them.
Wait a minute, holes? Why would some clams have holes in their shells?
Actually, the holes represent the predatory work of several varieties of snails, which are also relatives of clams. These predators feed primarily on the abundant clams and their relatives, but also attack limpets and some other types of snails.

Enter the Snails
Well over 500 different types of snail shells can be found on South Carolina beaches; most are considered uncommon. (Approximately 150 types of clams and their relatives may also be found.)
These snails are predatory, and use some amazing "tools" to obtain their meal. These snails can be quite important. Those called oyster drills can kill many oysters when their populations are high, greatly reducing the numbers that can be harvested by humans.
Gastropods (stomach foot animals) are mollusks best represented by snails, conchs, whelks, limpets and slugs. It is easiest to understand the name when you think of that slug crawling along on its belly, with its small head about the only other visible part.
Most gastropods secrete a shell, often coiled or spiraled, to give them protection. Among the other types of mollusks are those with and some without a shell; these include octopuses, squid, nautiloids, chitons and tusk shells.

How Snails Feed
Clams and many other shells have a special protein cover, called the periostracum, which prevents most organisms from entering the hard calcium carbonate part of the shell. It is often a darker color, as seen in the flaky covering on oyster shells.
Clams and oysters represent the Bivalve (two-shelled) group of the Phylum Mollusca. This group of animals with two shells hinged together along their back also includes the edible scallops and common razor clams of muddier beaches.
When boring snails come upon a suitable food item, they first need to extend a special gland out of their foot. This gland produces an acidic secretion that can eat through the special protein cover on most clams.
Following the penetration of the outer protein layer the acid can also help dissolve the calcium in the shell. The snail also uses its "teeth," called the radula, which is everted, or unfolded, out of its mouth. This group of teeth scrapes away some of the protein fibers and calcium which make up the shell.
The process can take a while. The cycle of scraping with the radula for a minute, followed by 30 to 40 minutes of acid secretion, continues as the shell is demineralized. A shell two millimeters thick (approximately the distance across the top of the letter w) can take eight hours to penetrate. Snails can penetrate a shell as thick as five millimeters.
The snail completes its work by extending its proboscis through the hole, using the radula to tear up the tissues and ingesting the particles of meat.

Another predator - sea stars
Another creature that preys on clams is the sea star, also known as the starfish. Sea stars are generally five-armed creatures moving around on an army of small tube-like feet.
While sea stars are also predatory, they are sometimes falsely accused of creating the "drill holes" seen in clams, oysters and limpets. While many sea stars eat bivalves, they never "drill" their prey.
Sea stars that feed on clams place their mouth above the edge of the clam that opens. They extend their arms around the clam, attach their tube feet, and then slowly exert pressure to open the clam up.
The tube feet of sea stars are like suction cups. With a unique water pressure system that exerts force without additional effort once the water has been "pumped out," the sea star can simply wait until the clam tires.
As the two muscles holding the clam shut begin to fatigue and weaken, the clam begins to "gape," prompting the sea star to add more pressure. Soon the clam gapes further, and the sea star turns its stomach inside out and everts it into the clamshell. While its stomach is hanging out of its mouth, it digests the clam and absorbs the nutrients.
Nature doesn't leave clams unprotected - some clams can actually fight back! Once the stomach of the sea star has entered their shell, a few can, with great muscular force, slam their shells shut and cut off the sea star's stomach.
The injured sea star has to forgo eating for a while, but it can actually regenerate its stomach. This process of regeneration is being studied in several laboratories. It provides hope that we can learn how to regenerate important tissues in humans, such as nerves and organs, which could lower the need for transplants.

The end?
Next time you are on the beach, keep an eye out for the many fascinating shells, and those clues to other animals' activities such as the drill holes in clams, oysters, limpets and some snails. And when you pick up a sea star, can you find the end?