There are hundreds of species of barnacle and they do some very peculiar things (from our perspective). The larvae swim freely, but when they are ready to settle, they glue their heads to a rock or mollusk shell, using glue produced at the base of the antennae. They then grow a set of hard plates around the body and spend the rest of their lives head-down, waving their modified legs through the water to capture food particles. Leg length and stoutness can be greater for barnacles living in rough waters than for those living in still waters, facilitating leg movement in moving water.
If that’s not sufficiently weird, consider some of their reproductive habits. Most barnacles are simultaneous hermaphrodites, meaning that an individual is both male and female at the same time. Sometimes they may fertilize their own eggs, but more commonly, an ordinary acorn barnacle mates with its neighbors. When an individual acts as a male, its very long penis, up to eight times its body length, reaches out to search for neighbors and then insert sperm into a space within the neighbors’ protective casing. The mating act is reported to be very quick, just a few seconds long. An individual may mate many times as a male and also as a female. I wonder how paternity of the eggs is determined…by competition among the sperm of different males or by some sort of female choice? The eggs and early larvae are brooded inside the mother’s casing.
Barnacles that live in calm waters where controlling a long, slim, flexible penis (a) is not a problem may be so well endowed, but those living in tougher conditions on wave-pounded shores have shorter and thicker penises (b), allowing better control of sperm placement. At the end of each breeding season, the old penis is discarded and a new one grows for the next season.
Gooseneck barnacles (c) can turn 180 degrees on their ‘necks’, but they can’t reach each other physically in the way ordinary barnacles do, because their penises are too short. Instead, they cast sticky masses of sperm toward their neighbors, which catch the sperm in a way yet to be described.
• Mary F. Willson is a retired professor of ecology.
