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Love on the Rocks

Updated: Jul 19, 2019

The unusual world of the Acorn Barnacle


Acorn Barnacles, Balanus glandula, near Southey Point

To some people, they are just an obstacle between land and water. They cut your feet and scratch your kayak. But to me, barnacles are amazing creatures. While leading field trips for Oak and Orca School in Victoria, I would often get kids to take a guess at the barnacles closest relative. The typical answer would be clams. Wrong, but they stand in good company. Carl Linnaeus, the Swedish zoologist who created our system of biological names, made the same mistake. In 1758, he classified barnacles as molluscs. It took

Barnacle Larva, 1/2 mm long, searching for a home https://journals.plos.org/plosone/article/figure?id=10.1371/journal.pone.0158957.g001

about 100 years, for biologists like John Vaughan Thompson and Charles Darwin to show that they are actually crustaceans. Their closest relatives are critters like crabs, shrimp and lobster. They start their lives as microscopic free swimming creatures, often brooded within the shell of a parent. They go through two larval stages before setting out to find a rock to glue their heads to. Once they find the right place, they use cement glands on their antennae and settle-in to build a home.



Orchre Sea Star near Walker Hook, photo by Caroline Topham

So, where is that perfect rock? Acorn Barnacles live in a narrow band between 'can't eat' and 'get eaten'. Settling too low on the beach exposes them to predators such as Ochre Sea Stars and carnivorous snails. But going too high means less time spent eating, not to mention they run the risk of drying out on hot days. Typically, the Acorn Barnacle stays out of water for six to twenty hours at a time, but they can survive for as long as 24. During that time they can lose almost half their body water. When your body is configured in such a strange fashion, getting food can also be a challenge. They do this by eating with their feet, not sort of thing you would do at formal dinner. When the tide is high or a wave hits them, barnacles reach out with net-like feet and scoop plankton. Because they are surrounded by a hard shell, growing is no joke either. The shell

is composed of plates. The plate edges are visible in the picture on the top right. Growth zones between plates allow the them to be pushed out and enlarged. If they are lucky enough to choose the right spot, they can live up to 10 years and grow to a diameter of 22mm (7/8th of an inch).






Well, eating and growing aren't the only things they do. There is the tricky issue of reproduction. This is made more difficult because acorn barnacles use internal fertilization. So how do you, ah, when your head is glued to a rock. Well it seems that barnacles have it figured out in two ways. They are hermaphroditic, so each barnacle can have its own brood.

The other way has to do with size. You know the expression "It's not how big it is..." Well, for barnacles size does matter. In fact, relative to body weight, they are the envy of every male in the animal kingdom. If you want to see what this looks like, you can watch the video on the left but you might want to cover your kids eyes.



Now I find myself on a rocky beach. I want to go to the water but I don't want to cut my feet, so what do I do....? I feel guilty with every crunch. But that isn't the only thing we do to them. In some places they are ground up and used for fertilizer. In Chile, a larger cousin of the Acorn Barnacle is considered a delicacy. A more distant relative, the Gooseneck Barnacle is eaten Spain and Portugal.

Gooseneck Barnacles, Photo by Kerstin Wrba on Unsplash

They also have another problem. When those microscopic larvae activate their shell glands for the first time, they can't always tell the difference between a rock and a boat. So we have ways of dealing with this. Sometimes we scrape or power wash them off our boats. More commonly we use anti-fouling paint. The paint comes in various types, all designed to provide a dose of toxic chemicals such as copper oxide and tributyltin to poison them when they try to attach. This stuff get into the environment, too. Although the copper oxide seems mostly harmless, the jury is still out on the tributyltin.


So the next time you see Acorn Barnacles, stop and consider their difficult life. And while you're there, look for me. I'll probably be walking barefoot.



Links to some of my sources

https://www.britannica.com/animal/cirripede

https://inverts.wallawalla.edu/Arthropoda/Crustacea/Maxillopoda/Cirripedia/Balanus_glandula.html

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4159999/

https://www.atlasobscura.com/foods/picoroco-barnacles

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