We humans are accustomed to thinking of sexual function as being both fixed and segregated into bodies that we designate as either Female or Male. And while we, as a species, generally do things this way, in the larger animal kingdom sexual function doesn't always follow these rules. Many animals are monoecious, or hermaphroditic, having both male and female sex organs in the same body. Not only that, but lots of animals change from one sex to the other. As in so many aspects of biology, the way humans do things may be thought of by us as "normal," but it isn't the most interesting way.
Take, for example, the slipper shell Crepidula adunca. This is a small limpet-like creature that lives on the shell of a larger snail. Around here the usual host is a turban snail, either Tegula funebralis or T. brunnea.
There are several species in the genus Crepidula, including C. fornicata, which lives on the Atlantic coast of North America. The species epithet gives an inkling of how reproduction occurs in at least these two species of the genus.
Sometimes C. adunca is found in stacks. I've never seen a stack taller than three individuals, but C. fornicata occurs in stacks of about six. The animal at the bottom of the stack is always the largest, and a given turban snail can play host to more than one stack at a time.
As you might guess, it isn't mere happenstance that these stacks of C. adunca occur. It turns out that this unusual living arrangement is key to both sexual function and eventual reproduction in this species. The individual on the bottom of the stack (i.e., the oldest) is always a female; those at the top of the stack (i.e., the youngest) are males. However, every stack begins with a single individual, and the default sex in newly settled C. adunca is male. An experiment conducted at Friday Harbor in Washington State1 showed the change from male to female began when the snails reached a size of 7 mm, and all animals larger than 10 mm were female. Animals that begin life as male and transform into females are described as protandrous hermaphrodites. How common is this phenomenon? Not uncommon among fishes, actually. Clownfishes in the genus Amphiprion are protandrous. Remember how in the beginning of the moving Finding Nemo, Nemo's mom dies? Well, in real life Nemo's dad would have become his new mom!
In any case, all C. adunca begin adult life as males. If they live long enough to reach about 7 mm in length, they might get to become females. Crepidula adunca's unusual living arrangement also facilitates reproduction. Unlike most limpet-like gastropods, C. adunca isn't a broadcast spawner. Rather, it copulates, as hinted at by the species epithet of its congener C. fornicata. A female slipper shell with a male on her back has a convenient source of sperm with which to fertilize her eggs: the male reaches into her mantle cavity and transfers sperm to her. Given the constraint of copulation, a female cannot mate until she carries at least one male on her back, and a male cannot reproduce unless he settles atop a female. Once the eggs have been fertilized, they develop within the mother's mantle cavity until she pushes them out as little miniatures of herself.
Cool little animals, aren't they? They remind us not to think of ourselves as The Way Things Are Done. We have a lot to learn from creatures that are not like us, and it's stories like these that ensure I will never lose my appreciation and love for the marine invertebrates.
1 Collin, R. 2000. Sex Change, Reproduction, and Development of Crepidula adunca and Crepidula lingulata (Gastropoda: Calyptraeidae). The Veliger 43(l):24-33.