What is a barnacle?
Barnacles closed up during low tide using plates made of calcium. Some of the small white and gray shells are empty. (Falmouth, Massachusetts)
The barnacle is one of the less familiar crustaceans, distantly related to crabs, lobsters, shrimps, and other animals. We usually see them exposed on rocks on the beach during low tide, closed up to prevent themselves from drying out.
But what do they actually look like?
Barnacle anatomy, notably showing the location of its legs at the top of the shell and cement gland at the bottom. © 2007 U of Washington
Most barnacles are hermaphrodites, so every individual has an ovary and a penis. The penis can be up to 8 times the length of its body, making it the longest penis relative to body size of any animal! Why? Because they can't move! (Well, some can…)
After starting life as tiny animals, floating along with other plankton, they molt their skeleton a few times (as crustaceans and other animals do) and explore a good surface to settle on. Once they settle on a satisfactory spot, they secrete a cement that permanently attaches them to the rock, a boat, or even another animal, like a whale!
The compound that barnacles use to attach to a surface is one of the strongest natural glues. If the barnacle is attached to man-made surfaces, it can even corrode stainless steel!
What are those long things up top?
Barnacles have feathery legs called cirri that are used to catch food. If you're lucky enough to see barnacles open underwater, these appendages will be all you get to see. Luckily, we see them often in aquariums when they're provided with planktonic food to catch.
Five giant acorn barnacles (Balanus nubilus) reach out their dozen purple-colored, feathery legs to catch tiny planktonic shrimp (Artemia) in an aquarium. The appendages come in pairs and, after catching food, retreat into the shell to deposit the food inside the barnacle's mouth.
Check out this molt!
A barnacle's molted exoskeleton (/cuticle/outer shell) sits on a dark table, dried. It grew a slightly larger one underneath before shedding this one.
Hairs on the legs help the barnacles catch plankton! The scientific group that contains barnacles is named after the shape of their legs—Cirripedia, "curl-footed" from Latin. (And the study of barnacles is called cirripedology!)
You can find barnacles eating at the beach!
Can you see the legs of these small acorn barnacles at the beach? They were feeding just under the water, and undoubtedly catching lots of plankton getting tossed around in the waves.
Seeing barnacles feeding is such a treat! Did you know they looked like that? They're very odd little animals, but they've been quite successful… I hope you have a new appreciation for this unique group of animals!
Various small acorn barnacle species on Cape Cod. Some barnacle species are swarming, meaning they prefer to settle next to others!
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