Beautiful Brawny Barnacles

My daughter and I went for a walk on the beach after school yesterday. It was chilly, but calm, with a clear mid-winter sky and dolphins in the surf.

Washed up on the beach was a large tree stump crusted with barnacles whose colours mirrored the evening sky. The size and profusion of barnacles indicate the tree was lodged in the water for a long time before it was hurled onto our beach.

Barnacles are strange creatures. They’re crustaceans—kin to crabs, shrimp, and lobsters—but they don’t look anything like their relatives. Barnacles have given up the ability to move on their own (after a brief mobile larval stage), and instead settle down in places where the water moves around them—intertidal zones, the backs of whales and the bottoms of ships are popular barnacle real estate. Instead of having to wander in search of food, moving water carries lunch to the barnacles, and they filter it out of the water with feathery appendages called cirri.

Staying put in places where waves pound day in and day out isn’t easy. Barnacles produce a ‘glue’ that’s one of the strongest natural adhesives around. With a tensile strength of 35 N/mm2 (5000 psi), it rivals the best commercial epoxies. In a strange quirk of biology, the glue is produced by a gland at the base of the barnacle’s antennae, and so the animal is glued head-first to the substrate. Once glued to its home, the barnacle forms a ring of plates around its body to protect it (this is all that is left of the barnacles pictured here—the animals themselves have died).

There are costs to gluing oneself in place. Unlike other sessile animals like corals who release their eggs and sperm into the water for fertilisation, most barnacles rely on internal fertilisation. To reach their neighbours and manage this feat, the hermaphroditic (both male and female) animals have penises that can be 8 times the length of their bodies. Until recently, researchers thought this was the only way barnacles could reproduce, but in 2013, a study found that at least one species of barnacle (with a particularly short penis) can capture sperm from the water if it’s too far from its neighbours.

Barnacles are of human importance. They encrust ships, leading to increased drag and fuel costs. But their habit of attaching themselves to all sorts of debris can also be used forensically to track marine wildlife like whales and turtles, items from shipwrecks and airplane crashes, and marine debris. Additionally, barnacles are considered culinary delicacies in Spain, Portugal and Chile.

Beautiful and brawny, weird and wonderful, barnacles always make me smile.

Feeling Bad? Consider the Barnacle

DSC_0027 cropThe lowly barnacle is well-known. Most people can point to one and say, “that’s a barnacle.”

Well done, but how many people know exactly what a barnacle is?

“It’s this…thing…that lives on rocks at the beach.”

“Don’t they grow on whales?”

“Ships get covered in them.”

As an entomologist, I know a bit about barnacles, because they are Arthropods, just like insects, spiders, millipedes, crabs, and lots of other creepy crawlies. In fact, they are crustaceans, closely related to crabs and crayfish.

Wait, you say. Don’t Arthropods have ‘jointed legs’—that’s what the word means, after all—but barnacles don’t have legs.

Or do they?

Barnacles are perhaps the strangest of the Arthropods. The free-living larva is a weird, tiny, spiky creature with one eye. It goes through a metamorphosis, like insects do, in which it changes shape dramatically. In its last larval stage, it finds a nice place to spend the rest of its life, presses its forehead against it, and secretes a calcium-rich cement from near the base of its antennae that permanently affixes its head to the spot.

In its adult form, a barnacle grows a protective shell, complete with a clever two-part ‘door’ that it can snap shut to conserve water at low tide, or to protect itself from predators. Its legs grow long and feathery, and act as tentacles to waft particles of food to its mouth.

As you can imagine, adult barnacles don’t have much of a social life. Most species are hermaphroditic, meaning individuals are both male and female. Surprisingly, though, self-fertilisation is rare. Like other arthropods, most barnacles have what’s euphemistically known as ‘internal fertilisation’—that is, the male has a penis, and he deposits his sperm inside the female. How does an animal glued by its head to a rock get together with another to mate? The answer is a very long penis.

So the next time you feel like your life is rough, be thankful you’re not a barnacle.