Barnacle penises are amazing. That’s not a direct quote from Charles Darwin, but it might as well have been; the naturalist was obsessed with the little crustaceans’ very large members. Now scientists say he missed out on one of the best barnacle facts: Many species can change the shape and size of their penises to suit their environment. A report of the findings was published in the journal Integrative & Comparative Biology.

Relative to its body size, the barnacle has the largest penis in the animal kingdom. This is, of course, not just some hilarious joke by Mother Nature—the length is necessary in order for these sessile (non-moving) animals to reach one another. Barnacles grow in colonies clamped to rocks, piers, boats, and sometimes large animals like whales. They feed by extending little feathery sensory appendages called cirri into the water to catch plankton.

In their resting state, barnacles are simultaneous hermaphrodites, meaning that they have both male and female parts. In many species, mating season begins when one barnacle’s egg glands begin to swell, rendering it “functionally female.” That barnacle releases chemicals into the water that tell its neighbors that it’s time to get busy. In response, they become “functional males,” unfurling their long, flexible penises into the water like party horns, questing for the right opening.

For obvious reasons, these bizarre sexual adaptations fascinated Darwin. The naturalist devoted seven years of his life to studying barnacles, in the process compiling descriptions of the “probosciform” (elephant trunk–like) penises of hundreds of species. (We are not making this up.)

A small sample of Darwin's barnacle collection. Image credit: FunkMonk via Wikimedia Commons // CC BY 3.0

Darwin was determined to learn all he possibly could about the animals, their anatomy, and their behavior. When he got wind that a friend of a friend had been lucky enough to see barnacle sex in action, he wrote a pretty intense letter, peppering his contact with questions. An excerpt:

Was the probosciformed penis inserted into more than one individual? For about how long [and how many] times was it inserted? Was it inserted deeply & at which end of valves? Especially did the recipient individual continue during the times exerting its cirri? Did it keep its opercula valves widely open for the reception of the organ? I am anxious to know whether this recipient was a willing agent or adulterer. or whether it was a case of rape by act. If the recipient was in full vigour, I think it wd be impossible to insert anything without its consent. Were the specimens under water at times? Who was the observer that I might check his authority.

We told you he was obsessed. But he was far from the only one. Darwin’s love of (and thorough notes on) the lumpy little animals left a scientific legacy that continues to this day. Marine biologists J. Matthew Hoch and Daniel T. Schenk of Nova Southeastern University and Quest University’s Christopher J. Neufeld have spent years studying barnacles. They collected their research—and the findings of others—in the recent review paper, and they came to some very interesting conclusions.

Barnacle reproductive organs. Image credit: J. Matthew Hoch, Nova Southeastern University

“We know that barnacles are robust in their ability to overcome environmental challenges and to mate successfully,” they write. Sometimes they do that by stretching their penises, making them thicker, or changing their shape altogether. “In the most extreme circumstances,” they say, “it degenerates and is shed during the first post-mating molt and is re-grown for the next mating season."

These animals are “so robust” in their sexual prowess, the authors say, that in one case a barnacle with two penises was still able to mate successfully.

“As barnacle biologists,” they write, “we understand and identify with Darwin’s excitement about barnacle reproduction and look forward to new knowledge generated through the modern-day comparative approach that has yielded so many new insights.”

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