Ask the Aquarium: What Causes Round Holes in Sea Shells?
Q. I find a lot of shells on the beach with perfect round holes in them. What causes this?
A. The animals that once lived in these shells were most likely the victims of predation -- and the predator was most likely a relative. Nature, after all, isn't sentimental.
Although holes in some shells are from punctures, or wear and tear by the sea, the neat, precisely drilled holes you describe are definitely the work of a hunter.
The majority of shells found on beaches are former homes of mollusks. There are some 50,000 living species of mollusks. The two largest classes are bivalves and univalves. Bivalves, such as clams, mussels, oysters and scallops, have two hinged shells. Whelks, conchs, various snails and other univalves have just one shell. Bivalves and univalves often prey on each other.
In general, the two-hinge shelled animals, the bivalves, lead a quiet, sedentary life, filtering and feeding on the abundance of drifting microscopic plants and animals. Scallops, however, are an exception: These bivalves can move by creating jets of water that propel them along. Interestingly, they are considered the only migratory bivalve.
Univalves, on the other hand, are much more mobile and active. Univalves are gastropods, literally meaning "stomach foot," which is what these animals use to get around. This mobility allows them to search for hapless bivalves to devour.
An example of a marauding univalve is the attractive moon snail, also known as shark's eye. The moon snail is an avid predator. Once it locates its prey -- whether it's a clam, scallop, or another moon snail -- it begins slowly drilling and scraping its victim's hard shell with a file-like rasping tongue called a radula.
This is aided by the secretion of hydrochloric acid which helps soften the shell. When the moon snail breaks through, it inserts its mouth parts and begins feasting on its still-living dinner. Depending on the thickness of the victim's shell, it may take more than a day to drill the hole, but what's time to a snail?
Other snails take the boring radula further. Cone snails have turned this instrument into a venom-tipped harpoon, which they stab with a lightning-fast thrust, almost instantly paralyzing their prey.
Whelks use another approach. Hunting clams and other bivalves just beneath the sand's surface, whelks grip their catch tightly with their extendible body parts. Then, using the sharp edge of their shell like a knife blade, they pry open their victim's hinged shell and begin feasting. Empty clam shells with chipped edges indicate a lost battle with a whelk. Whelks in turn are fed upon by an even bigger snail -- the giant horse conch.
Lovely Scotch bonnets feed on sand dollars and sea urchins by secreting sulfuric acid to dissolve the hard outer layer, then boring through to feed on the insides. Bonnets themselves are fed on by stone crabs, which can crush their shells.
The lettered olive prefers to dine on plentiful coquina clams found at the surf's edge. The olive envelops the small clam with its body and secretes a paralyzing mucous. Some murex snails also use this technique.
Other murex species drill through their prey's shell, and still others apply strong, sustained pressure which eventually opens up even the most stubborn oyster.
It's a tough world beneath the sea. Between univalves and bivalves, it's a never-ending battle of predator and prey.
The state operates three public aquariums; one in Pine Knoll Shores, another at Fort Fisher and a third on Roanoke Island. The aquariums are administered by the North Carolina Department of Environment and Natural Resources, and are designed to inspire appreciation and conservation of North Carolina's aquatic environment. For more information about the Aquariums, log onto www.ncaquariums.com, or call 800-832-FISH.
Sherry White works for the public affairs office of the N.C. Aquariums.
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