In The Studio: Barnacles

Often mistaken for another group of animals and originally classified as mollusks, barnacles are arthropods, which means they have jointed legs.

By Susan Knight


Welcome to the studio of the great outdoors. I’d like to discuss an animal not typically considered as sexy as some of the larger marine animals, but you may soon agree that barnacles could actually be considered one of the sexiest. There are many different barnacle species that live underwater, in the intertidal zone, or on whales and other animals. Often mistaken for another group of animals and originally classified as mollusks, they are arthropods, which means they have jointed legs. Arthropods also have exoskeletons made of chiton, segmented bodies, bilateral symmetry and an open circulatory system. They are related to insects, spiders, crabs and lobsters.

Let’s look at the intertidal zone, the place between low tide and high tide, where we can easily study barnacles. It’s a place where many divers begin their journey to explore the underwater world, hiking the shoreline to dive sites. The barnacle and other animals that live here are very specialized and sturdy. Because they are sometimes submerged and sometimes exposed, they have special challenges to overcome. They have to deal with extraordinary ranges in temperature, sun exposure, wind, fresh water (rain), intense wave action and land predators. They must also spend enough time underwater in order to have enough time to do important things like eat.

If you look closely, you’ll find that specific species live in clearly separated zones; where they live depends on various factors. The top zone is mostly limited by how well particular animals can withstand the stresses of extended periods out of the water. The bottom zone is determined more by predatory factors. In other words, if you live higher, you will be exposed to some of your sub-tidal predators for a shorter time. Hard shells, super-strong glue or threads to keep you attached to rocks also help keep you protected in such an environment. Barnacles close up and hold a tiny droplet of salt water in their shell to keep hydrated at low tide.

Barnacles actually start life free-floating in the water with plankton. After a time, they find a substrate, such as a rock, and settle down. This is where it gets interesting. They plant their heads upside down on the substrate and attach with super-strong glue and then begin creating a hard shell made of several plates. Living at the top of the intertidal zone, they spend the longest time exposed as the tide recedes. When underwater, they feed on plankton by continuously reaching their feathery jointed legs out of their shell, and then pull those legs in to bring the food down to their mouth.

Reproduction is required for species continuity, so how easy is sex when you’re attached to your home? This is where it gets really interesting. Most barnacles are hermaphrodites and can fertilize themselves if required, although it’s far better for species diversity if another barnacle does the fertilizing. The barnacle’s claim to fame is their fantastically long penis, about 20 times their body length — a penis-to-body ratio record in the animal world. I have seen a barnacle penis (patience pays off) unfold and reach out to creep into another shell for some fertilization action. It felt all around the surrounding area, much like when you’re fumbling for the light switch in the dark. It finally inserted itself into a nearby shell so the tube could deliver barnacle sperm. A successful strategy, it seems, to create more barnacles.

I encourage you to check for low tide in your area sometime and enjoy a compelling barnacle safari. You can also discover many subtidal barnacles on dives. You may be amazed what at occurs before you, if you simply stay in one place and observe.

Remember, on your dives, that relocating animals can be detrimental to their health. The intertidal area is so diverse that moving an animal (“Oh, no, this poor sea star will die up here, I will save it by tossing it into the sea”) can be like plucking someone from Florida in their shorts and T-shirt and dropping them in Alaska. These are totally different environments, and that sea star is there for a reason.

Susan Knight is a dive instructor, tour guide and marine biologist who has spent the last 20 years traveling, teaching and writing about the oceans in the Pacific Northwest, Hawaii and the Caribbean. She has transitioned mostly to freediving and become know for her work with large marine animals such as whales, dolphins, sharks and elusive mermaids. Her studio has always been in the great outdoors. You can see her work and video projects at