The intertidal zone, that area of the coast that lies between the high tide and low tide marks, is a very difficult place for animals to live. Not only must they deal with crashing waves, any creature inhabiting this zone must be able to withstand being submerged for part of the day and exposed to air for the rest. Most, like mussels and barnacles, are able to seal their shells shut during low tide and simply wait for the water to return.
Unfortunately, they also have to deal with predators. Raccoons and sea gulls come to the intertidal zone at low tide searching for vulnerable prey, while crabs and fish move in with the incoming flow of water to do the same. And then there is the oyster drill.
Oyster drills make up a family of carnivorous snails that live mainly in this intertidal zone. Most are small. The most common species in Southern New England is Urosalphinx cinerea, or the Atlantic oyster drill. This snail is just a little over an inch in length, but is one of the most effective coastal hunters this side of the sea star.
Oyster drills feed mainly on bivalve mollusks, but are equally adept at penetrating the defenses of barnacles, periwinkles, and when times get tough, even others of its own species. Like all snails, drills have a mouth that contains a structure called a radula. This is a ribbon-like organ that is studded with hundreds of microscopic teeth. Herbivorous snails use the radula to scrape algae from rocks while carnivorous drills use it to make a small hole through the shell of its prey.
When a drill finds a victim, say a small mussel (a favorite for this species since the shells are so thin) it grips onto the shell tightly with its foot and begins the drilling process. This involves not only scraping with the radula, but also an application of calcium-dissolving sulfuric acid to help soften the shell and make the drilling easier. This acidic compound is secreted by the drill’s eversible gland.
The entire drilling process, depending on the thickness of the prey’s shell, takes about eight hours to complete. This involves alternating the mechanical and chemical processes, usually about one minute of drilling with a half hour of acid production.
Update: Just to be clear, this hole,
although identical in shape to one
made by oyster drills, is actually
from a moon snail. Holes made by moon
snails are much larger than drill holes.
Once the hole is completed the drill inserts its long, tubular proboscis into the perfectly round hole and releases digestive enzymes which basically begin digesting the animal within its own shell. The proboscis then sucks the liquifying meat up. Often the scent produced as one oyster drill breaches the mussel’s defenses attracts other drills to the food source. I’ve seen mussel shells with half a dozen individual drill holes.
A snail’s radula is not a big structure. Therefor, the limiting factor in what type of prey an oyster drill can eat is determined by the thickness of the potential prey’s shell. If the shell is thicker than the radula is long it will be safe from predation by this snail. For this reason the oyster drill’s name is a bit misleading. While they may easily feed on young oysters, an adult’s shell is too thick for the drill to penetrate. I’ve kept adult oysters with oyster drills before without anyone getting hurt. Although the oysters will end up with many partially drilled holes in their shells.
One other little problem with the name is that oysters and oyster drills normally don’t even share the same habitat. Oysters prefer to live in brackish water which has a salinity that is too low for an oyster drill to survive in. Although it should be noted that any oyster spat that tries to settle in higher salinity waters will not survive long around drills and sea stars. In fact it is believed that, evolutionarily, the oyster’s tolerance to low salinities may have been driven by these two predators.
About a hundred years ago oysters were introduced to both England and California by aquaculturists. Unfortunately the Atlantic oyster drill came along for the ride and are now found on both sides of the Atlantic as well as the Eastern Pacific, causing problems for the populations of other bivalve species native to those coasts. Another case of the harm done when introducing exotic animals where they don’t belong. The Olympia Oyster, found in Washington and Oregon, is especially hard hit by this non-native snail.
Oyster drills are unusual among univalves in that they produce young that do not go through a planktonic stage. Eggs are laid on the underside of rocks or shells (sometimes its latest victim’s) and protected in yellow vase-shaped egg capsules that are glued to the substrate. Each capsule may contain two dozen young and a single female may lay as many as a dozen of these capsules at a time. When the embryos develop one of them will eat through the top of the capsule and then all the miniature snails come marching out single file.
Oyster drill egg capsules.
There are two other species of oyster drills found in the same range as Urosalphinx. These are the nearly identical Thick-lipped Drill (Eupleura caudata) and the slightly larger Dogwinkle (Nucella Emarginata).
The thick-lipped drill is a much less common species but is often found mixed in with Urosalphinx colonies. The main difference is a slight ridge found on both sides of the shell and a slightly longer aperture canal. This canal is worth exploring more because it is important in finding prey. Below you can see a picture of this short extension of the shell.
Note the tiny hole in this shell. This
thick-lipped oyster drill was a victim
of cannibalization.
Carnivorous snails find their food by smell using a short "nose" called a siphon, which sticks out of the front of the aperture (shell opening). When searching for food you can see this siphon swinging back and forth as it senses the water currents for signs of prey. Once the scent is locked on the snail will follow the smell against the current to its source. Having this little organ sticking out of the shell is risky since it would be easy for a passing fish to take a bite out of it. By having this canal in the shell the siphon can stick out to search for food and still be protected. You’ll notice that herbivorous snails, like the periwinkle below, have no aperture canal.
Finally, we have the third type of drill, called the dogwinkle. Again it is found in the intertidal zone but this species differs in that it has a much stouter shell. Unlike the bland greyish color of the other two drills, this species varies greatly and may be white, yellow, purple or a combination of all three. The color depends not just on genetics but also what each individual has been feeding on. White and yellow-shelled individuals have been feeding on barnacles while purple-shelled ones have fed mainly on mussels. The color obviously changes over time as its prey selection changes.
The fact that this snail may sometimes be tinged with purple (an unusual color throughout the animal kingdom) may seem surprising until you realize it is closely related to a group of Mediterranean snails called Murexes. Murexes, which are also types of drilling snails, have a mucous-secreting organ within their bodies called a hypobranchial gland. The product of this gland was used in ancient times to produce a dye known as Tyrian purple. This dye was extremely expensive (reportedly worth its weight in silver) and was used as a sign of wealth by the ruling class of the Roman Empire.
Murex snail.
Fun Fact: The story of this purple dye states that it was discovered by Hercules when his dog’s mouth was stained after chewing on murex snails.
Other diaries in this series can be found here.