The following is edited from a post I published in 2008.
It's the dead of winter and you must start to think about finding a lover and getting your gametes in the water in time for the Spring algal bloom! But wait a second, you're a permanent fixture on a rock. Can’t move. What is a young, love-struck sessile hermaphrodite to do? Well, if you are hung like a barnacle you don’t really have to move that far.
Barnacles are known as one of the more 'hung' of the invertebrate world with penises reaching up to 10 times their body size. Not all barnacles are equally endowed though. To make the situation more complicated, barnacles are simultaneous hermaphrodites - they carry both male and female reproductive parts at the same time. Famous evolutionary biologist and theorist Eric Charnov proposed an extension of his Sex Allocation Theory to simultaneous hermaphrodites. It is this theory that makes predictions for how much resources should be allocated in being male or female based upon how competitive your local environment is.
How much does a penis cost?
Barnacles tend to flock to one another, their larvae exhibiting what researchers refer to as gregarious (="live in a flock", not unlike Gregorian monks) settlement. This results in dense mats of barnacles upon barnacles on rocky shores. For the sessile, once you choose your "build site", there is no chance to move house! Since barnacles are unable to move, being packed like sardines must be a good thing, right? One would think the closer your neighbors are, the less effort you need to "reach out" to them.
Biologist Matt Hoch (2008) tested the effects of overcrowding and wave exposure on the penis morphology of the acorn barnacle, Semibalanus balanoides. As males from many species might tell you, bigger is not necessarily always better. Hoch explains:
“Variation in barnacle penis traits may be important when comparing sex allocation of barnacles for several reasons. As the number of individuals in the mating group increases, mate competition intensifies, which is predicted to lead to greater relative allocation to male function. The penis itself represents a significant investment into male function, in terms of construction, maintenance and performance costs. For example, the presence of the penis, located between the feeding cirri on the terminal body segment, may reduce feeding efficiency. As penises grow larger, interference with feeding is expected to increase.”
For most of the year, the barnacle’s penis is rather languid. The vernal season for this little fellow is September to October where it rapidly grows until ready to spring into action come November. Unfortunately, the excitement of it all is short lived and the penis is cast off with the next moult upon mating. Because the growth and decay of the penis happens so quickly, Hoch hypothesized that it must be costly to maintain this single-use appendage. If the closest neighbor is far away, it will need to invest more energy into maleness, resulting in a longer penis. Such a trade-off might occur at the expense of the femaleness necessary to make and nurture the barnacle babies until they are spawned.
Hoch compared barnacles found in a crowd, when their shells were touching their neighbors (see photo above for example), to those in uncrowded situations. Studying barnacles in scenic Shinnecock Bay in Long Island, NY, Hoch found that crowded barnacles had significantly shorter penises than its uncrowded brethren. Yet, the thickness at the base remained unchanged and there was no difference in the relationship between penis length and body volume between the living arrangements. Additionally, as the distance between neighbor’s increased, there were fewer fertilized egg masses.
Take a lickin’ and keep on kickin’
The rocky intertidal is no place for a wuss. Tides and wave forces make it an intensely dynamic environment, not to mention stress from heat and drying out. One needs a thick skin in order to take the constant, rhythmic pressure of wave exposure so Hoch studied his phenomenon at two sites. The first was exposed to a higher frequency of waves, facing the Atlantic (exposed). The second faced into the bay where it was more tranquil (protected). Perhaps unsurprisingly, fertilization was more successful in the protected area. Additionally, the disparity in fertilization success between protected and exposed areas grows as the distance between neighbors increase.
So, exposure to waves appears to hinder fertilization success, but why? Is it because waves break off the barnacle's penis, or sweep it away from its intended target? While there is no difference in penis length between exposed and protected areas, the basal diameter was thicker in exposed barnacles. The penis seems to adapt to take a pounding by buffing up at the base. Yet, its not enough to fertilize at the same rate as in protected areas.
Hoch provides an interesting hypothesis that, at least in barnacles, the physical environment plays a strong role in molding morphology. Such characteristics are said to be plastic. In order to understand how evolution of novel morphology arises, the degree of plasticity in the trait should be assessed. Reproductive morphology is typically considered to be under a high degree of selection, because it is directly associated with producing the next generation, among other reasons.
What is the take home message here if you are a barnacle? If a barnacle’s goal is to be the "John Holmes" of sea world, live in uncrowded areas in wave-exposed environments. If the barnacle’s goal is to fertilize as many eggs as possible, live in close quarters at areas protected from wave exposure. I’m not quite sure which goal barnacle’s are seeking. They are cunning creatures with a knack for keeping their cirri to themselves.
HOCH, J. (2008). Variation in penis morphology and mating ability in the acorn barnacle, Semibalanus balanoides. Journal of Experimental Marine Biology and Ecology, 359(2), 126-130. DOI: 10.1016/j.jembe.2008.03.002