On exposed shores, it’s better for barnacles to grow shorter, thicker penises (Image: J. Matt Hoch) In calm waters, barnacles grow longer, flexible penises with greater reach (Image: J. Matt Hoch)

Longer isn’t always better, according to some men, and it seems the same is true for barnacles, too. The hermaphroditic filter-feeders can grow penises up to eight times their body length – they have the longest penis length relative to body size in the animal kingdom – but new research suggests that stouter members are sometimes more effective for mating.


Glued to a rock year-round and unable to self mate, a lengthy penis increases a barnacle’s odds of spreading its seed.

The animals regrow their penises each year, just before their brief mating season, and previous research has shown that water conditions play an important role in shaping the budding penis.

In calm waters, acorn barnacles grow long, flexible members in order to reach as many mates as possible. However, in choppier waters, the barnacles develop more muscular penises with far less reach.

“It’s kind of like toughness versus flexibility,” says J. Matt Hoch, a marine biologist at Stony Brook University in New York, who tested whether barnacle penis plasticity actually affects reproduction.

Fertile waters

To do this, he set up two experimental barnacle beds – one on the wave-exposed Atlantic shore and the other in a protected harbour near his university. A few months before breeding season and before penis growth started, Hoch collected barnacles from a site exposed to moderate waves and moved them into his two experimental love nests.

After their penises sprouted and mating began, Hoch quickly took half of the barnacles from the moderate site and transplanted them into the exposed site, and vice versa. At the end of the mating season, he counted up the number of fertilised eggs.

Predictably, barnacles raised in calm waters that grew thin, flexible penises struggled when forced to mate in choppier waters. They fertilised significantly fewer eggs compared with their lengthy counterparts that stayed in calm waters.

Barnacles with thicker penises, on the other hand, fertilised just as many eggs in the harbour as they did in the open ocean. However, barnacles with thin penises mating in calm waters fertilised the most eggs out of all the groups.

Interestingly, Hoch noticed that in rough waters, barnacles with thin penises suffered fewer injuries and breaks than barnacles with more muscular members.

He reasons that the waves were so rough at times that the barnacles with thin penises didn’t dare come out to look for a mate.

Journal reference: Evolution (DOI: 10.1111/j.1558-5646.2009.00668.x)