If you ever needed to know one thing about barnacles, its that they have large penises. Sure, you might be thinking barnacles are so small. But relative to total body size, they have the largest penises. It is a result of living a sessile life, remaining attached to the spot which they settled as larvae. Since they are unable to move to mate, they had to develop a different strategy. Hence, the large penises.
Former Stony Brook University Department of Ecology and Evolution student J. Matthew Hoch (now at the Southeast Environmental Research Center at Florida International University) spent his time here researching barnacle penis morphology. Some of his findings were published in the most recent issue of Marine Biology.
Some of his interesting findings were that both wave action (yes! the motion in the ocean… this makes SO much more sense to me now) and population density can have significant impacts on penis morphology. His study organism, the Atlantic acorn barnacle, Semibalanus balanoides, is known to have a penis with a exoskeleton with “accordion-like folds” that allows it to stretch to many times its relaxed length in order to find a mate. This is useful due to their sessile lifestyle, and makes copulation with their neighbors much easier.
What Hoch found was that population density has an impact on overall penis length. Barnacles which were sparsely populated had more of the folds in the penis, indicative of having a greater fully stretched length. This is presumably an adaptation to low population densities, allowing for the chance of successful mating. Those barnacles in crowded conditions had fewer folds, indicating that they are likely to have less ability to stretch (and also less need to do so).
And now the motion in the ocean part. Barnacles on wave exposed shores grow larger and their penises grow thicker/wider. They aren’t necessarily longer than those that live in protected sites, nor do they have more folds allowing them to stretch greater distances. They just have thicker penises. This is likely a result of the water action. These barnacles have to have thicker penises for more support, making them less likely to break in the wave action and more likely to produce successful mating attempts.
All in all, it was an interesting read, seeing how barnacles adapt reproductive mechanisms to their surroundings. In case you want to learn more about this research, Dr. Hoch did a write up last year fr the Deep Sea News site, and his work has also been highlighted on the NewScientist.
J. Matthew Hoch (2010). Effects of crowding and wave exposure on penis morphology of the acorn barnacle, Semibalanus balanoides Marine Biology, 157, 2783-2789 : 10.1007/s00227-010-1536-z