Sometimes it’s difficult to decide whether I should blog about a group of animals, or one specific species. Oftentimes a certain species doesn’t have enough information, but the group as a whole is interesting. Other times there is no specific information for the group, so I have to go down to the species level to make the posts exciting. Today, my decision was easy.

There are over 1,200 species of barnacles, so there was no way I could pick just one of those to write about. Barnacles belong to the infraclass Cirripedia, which is part of Crustacea. So yes, barnacles are related to crabs and lobsters. They dwell mostly in shallow ocean waters, though some species can be found at depths of 600 m.

Some barnacles in the intertidal zone. The big things in this shot are limpets, which compete with barnacles for space.
Image by Wilson44691 at English Wikipedia, Public domain, via Wikimedia Commons

Barnacles don’t look a whole lot like animals. They don’t really move, but inside those white shells there actually is a little creature. It’s quite a simple creature, but it is there. Adult barnacles spend their lives being stationary; they find a suitable surface, attach to it, and then stay there. Some barnacles attach themselves right onto the surface, while others like to sit on top of a stalk. They stick to their chosen spot using glands in their antennae — which means they are basically sitting on their foreheads.

Inside their hard shells (which are like the shells of crabs and other crustaceans), barnacles sit on their backs and stick their limbs in the air. These limbs (called cirri) are covered in ‘feathers’ and are used by barnacles to sweep food into their mouths. Other than their cirri, barnacles are fairly simple. They don’t have a heart, instead using an esophageal sinus to pump blood around, and they don’t have gills, absorbing oxygen through their legs. Barnacles rely mostly on touch, but they do have a single eye, which is thought to only see light and dark.

As is probably wise for animals that spend their lives in one spot, barnacles are hermaphroditic. Self-fertilization is possible in barnacles, but rarely occurs. Instead, barnacles have extremely long penises to help them reach nearby barnacles. In fact, it’s thought that barnacles have the largest penis length to body size ratio in the animal kingdom. Pretty impressive for little guys that just sit around all day.

A close up of a barnacle nauplius.
Image by ottolarink5, CC BY-SA 3.0, via Wikimedia Commons

Barnacle eggs are brooded by the parent that lays them. Once they hatch, they enter the nauplius stage, which is a creepy thing consisting of an eyeball, a head, and a little tail-like segment called a telson. The nauplius grows for the next six months, undergoing five metamorphoses before becoming the next stage: a cyprid. This stage has one single goal: to find a nice home for the rest of the barnacle’s life. Cyprids swim around, using their antennae to investigate potential resting spots. They are quite selective at the beginning of their journey,  but once they start to run out of fuel, they’ll pretty much pick anywhere.

Barnacles are pretty tough — I don’t know if you’ve ever walked on them but it’s quite painful. Those that live in intertidal zones have to survive long periods without water, which they are able to do thanks to their shells, which are impermeable to water. They also have two plates which cover their appendages when the barnacles are not feeding; if you’ve ever watched a wave roll over barnacles you’ll see them opening and closing, like jaws.

I’ve always thought barnacles were fairly interesting, though I really hated standing on them at the beach. Come to think of it, they probably hated that too. Maybe from now on I’ll try and be a bit more careful about these guys.

Cover image by Rhododendrites, CC BY-SA 4.0, via Wikimedia Commons