"The gastropods exhibit nearly every possible modification of sexuality."
R. Tucker Abbott American Seashells (1954)
The marine snails with limpet-like shells in the genera Crepidula, Crucibulum and Calyptraea make up the family Calpytraeidae. They are all protandric hermaphrodites: young snails are males that may grow larger and change into females.
And that isn't the only unconformity the calpytraeids are known for. They also spend their lives in tall stacks with the females on the bottom followed by piles of males on top of them patiently waiting for their turn to mate and then change into females to mate again. How is that for kinky sex?
"Girls will be boys and boys will be girls.The picture above shows the intertidal snail Ilyanassa obsoleta from Chincoteague with a couple of Crepidula on its back. Presumably the larger Crepidula on the bottom was a female, while the one on top a male. The drawing below is a simplified rendition of the sex life of Crepidula.
It's a mixed up, muddled up, shook up world, except for Lola."
From Mark D. Bertness, Atlantic Shorelines-Natural History and Ecology, 2007.
That the calpytraeids are gastropods, that is, snails, is hard to tell without a close scrutiny. Their shells have all but lost the characteristics spires of ordinary snail shells. It would not be possible for ordinary gastropods to pile up on top of each other for more than 2 or 3 snails before they turn into a rather unstable tower. So I suspect the flattened shapes of the shells of the calpytraeids must have coevolved with their sexual habits.