What? Barnacles Are Arthropods?!

Thanks to @greg_nunes for posting these cool barnacles on Unsplash!

Barnacles are not only members of the phylum Arthropoda, but they also join the ranks of crabs, lobsters and mulch lobsters as crustaceans. I’m just as flabbergasted as you are, but if I’m being totally honest, I don’t know what a barnacle would be if it wasn’t a zit crab. A sticky rock with a beak?

For most of my life, they’re just been very convenient rock-grippies.

Let me explain – I like to go barefoot. Like, a lot. My shoeless record is at least 72 hours, so it’s no surprise that I don’t wear shoes all too often in the summer. I didn’t wear shoes a lot as a kid either, which is how I ended up climbing some barnacle-encrusted rocks at the beach my family vacations at without shoes on. This was a terrible idea. You should never walk on barnacles, or oysters, or even gravel barefoot unless you want to slice up your feet. Through naïve trial and error, I found out that the small, closely-packed barnacles on these specific beach rocks provide great traction for climbing without punching through the skin of my feet, but I would not recommend climbing on barnacles in general. Or at all, really. It’s never a good idea, I just got lucky this time.

That said, I enjoy walking on these barnacles because they help keep me from slipping off those wet, algae-coated rocks when I walk across them to look for crabs. The rocks extend about a quarter of a mile into the ocean and create lots of little tidepools near the shore where blue crabs, minnows and other sea creatures like to hang out. I don’t know why the rocks are there, but I’m certainly not going to complain about their existence. Those tidepools are sick.

Not only do I have a soft spot for the rock-grippy barnacles that keeping me from slipping and falling between the rocks, but I love watching pretty much any barnacles I happen to come across if they’re underwater. I know, I know: “ArthroBlogger, barnacles don’t move – they just sit there. That’s worse than watching paint dry.” Au contraire – barnacles do move, but you can only tell when they’re eating.

Barnacles are filter feeders, which means they feed on tiny food particles floating around in the water. They capture these particles with hair-like appendages called cirri, which are basically their legs. This is what I’m talking about when I say I like to watch barnacles: if you happen to be somewhere where you can get a good look at some barnacles underwater, pay close attention: you’ll be able to spot the cirri swishing back and forth underwater like feathers in the current. Their eating methods may be strange, but barnacles are super beneficial to their ecosystem because they tend to filter out bad particles and lower the turbidity of the water as they eat.

There is a period of time in every barnacle’s life that does include a lot of movement besides eating: childhood. Barnacle larvae swim around as plankton themselves for a little while before gluing themselves head-first onto a solid substrate to form their famous calcareous shell. They tend to settle down around lots of other barnacles, because the presence of barnacles nearby generally implies that there’s lots of barnacle food nearby. Plus, clustering in a big group reduces an individual barnacle’s odds of being singled out and eaten by a whelk or starfish.

Barnacles are such a diverse group of arthropods that they have their own subclass of Crustacea called Cirripedia. There are about 1000 different cirripeds that mostly behave as we would expect them to by sticking to random substrate or larger animals. Most animals could care less about barnacles sticking to them, but there are a few parasitic species they’d rather avoid.

Let me introduce you to the infamous rhizocephalan barnacles. It’s a crime that I learned more about these parasitic barnacles than mulch lobsters in my biology class, but at least I get to share my knowledge with you now. Rhizocephalan barnacles are basically living reproductive systems that imbed themselves inside crabs, tricking the oblivious crustaceans into thinking that the zombie barnacle eggs are their own. I’m calling rhizocephalans “zombie barnacles” because as they grow inside the crabs, they mess with their minds and change the crabs’ appearance and behavior, much like how a zombie bite would dramatically alter a person’s appearance and behavior. The infected crabs take on feminine features and stop growing altogether, diverting the energy normally used for molting and getting bigger to caring for their parasite’s offspring.

Despite their sneaky schemes, we shouldn’t let zombie barnacles ruin the good barnacle name. We must remember the noble barnacles that dot the pillars of our piers and the sides of our docks. We must remember the valiant barnacles that provide traction on those awesome beach rocks. We must remember the cute little barnacles swishing their cirri to clean our waterways.

And we must remember that barnacles are arthropods and not whale zits, no matter what that tv show about an anthropomorphic sponge wants us to think.

One Comment on “What? Barnacles Are Arthropods?!

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