There are few creatures in the sea with as appropriate a common name as slipper shells. There are little reinforcement shelves on the forward section of the shells that, when the snails are alive, help support their delicate soft, digestive parts. But after they have died, the leftover empty shells that wash ashore look so much like miniature slippers that it is hard for even adult beach-goers not to seek out matching pairs.

As a whole, slipper shells are among the most abundant of shallow water snails in much of the world. Altogether, there are dozens of species within the genus Crepidula. None of these ever get very big - an extremely large one may reach 2 inches in length - and they are not extraordinarily colored.

Superficially, slipper shells resemble true limpets (hence, they are sometimes called "slipper limpets"), and they do often live in similar habitats. But they are not considered to be closely related to limpets. In fact, slipper shells are classified in the same order as cowries and conchs, and not along with limpets at all.

Slipper shells have some interesting "hang-ups" (besides the shelves within their shells). They are capable of moving about, but usually don't. They are filter feeders, so they can remain attached to one spot virtually all of their adult lives, letting the currents and waves bring their food to them.

Well, for some species, though, that "one spot" they are attached too isn't always in the same location in the ocean. While some species stick their slippered feet directly to rocks or pilings, many are especially fond of the shells of other snails. And they can be extremely picky as to which snails they choose. Central California's hooked slipper shell (Crepidula adunca) is commonly found on the black turban (Tegula funebralis), while Norris' slipper shell (C. norrisarium) of more southern waters prefers to live on Norris' top snail (Norrissia norrissii). Even if the original molluscan occupants of the larger shells are no longer around and they have been commandeered by hermit crabs, the particular slipper shells will stay on board.

Often times, too, en masse. Slipper shells are known to pile up, one on top another, to form stacks of slippers 4, 5 even 8 or 10 tall! These high-rises are not due, however, to any lack of living space, but are for the continuance of living slipper shells.

Slipper shells are protandrous hermaphrodites, meaning they begin life as males, then later change to females. After a period of time of being brooded within their mothers' mantles, they step out on their own, going through a free-living planktonic stage. But after some months or years, they seek out an appropriate place to spend the rest of their days.

The small young males are naturally attracted to older, bigger females, but often other males have already been attracted to those same females. So the newcomer will simply settle on top of a forming stack of slipper shells. Within these stacks, usually the bottom one or two snails will be female, the top ones males and the ones in between being, well, in between.

The size and shape of each slipper shell is related to what surface they are attached to and where they sit in the stack. White slipper shells (C. perforans) is one of these "flip-flop" style slippers. If it settles down on a rock, it will have a fairly standard look. However, if it chooses a shell that a hermit crab is occupying, this species of slipper will stick to the inside of the shell and have a bent-back look with a smooth outer surface. Yet it also often picks the inside of the hole in a rock that was bored out by a burrowing clam, and if an individual white slipper shell grows up in that environment, it will again be bent back, but have a shaggy, fuzzy look to the outside of its slipper.

About a dozen species of slipper shells inhabit the California coast, including a couple that inadvertently "slipped in" from the East Coast. So finding slipper shells while diving or beach combing is easy. With their variably ways, though, identifying which species is which can be more challenging. And finding a perfectly matched pair is an illusive goal.