[This diary was originally published to DailyKos on July 21, 2006.]
The Slipper Limpet (Crepidula fornicata) is an odd type of snail that is extremely common on the Atlantic coast. Unlike most snails, this animal lacks the typical spiral shape of its shell and is instead more conical with only the slightest hint of a shell twist. To start off, I'll help get your mind out of the gutter (don't worry, we'll be diving right back in below the fold) and explain that the species name fornicata is Latin for "vaulted", which describes the convex shell shape quite nicely, and the name was given by none other than Carl von Linne (aka Linnaeus) himself.
These snails are sessile, meaning they remain in one place for their entire lives, attached to rocks or other shells by the powerful suction of the foot hidden beneath the shell. (Those of you on the West Coast, think "small abalone"). The larvae are planktonic and as they drift through the water they look for a suitable place to settle down. The preferred spot is on top of another limpet (adult limpets actually secrete a hormone that attracts the larvae to them). As this larva grows into an adult it in turn attracts other larvae, the result being that the animals form a living chain of mollusks that may number in the dozens. If a larva is unable to find another limpet it will settle on a rock and begin its own chain as others join it.
In spite of what I said above, very young slipper limpets can crawl around like other snails. However, as they grow the edges of their shells mold to match the contours of the substrate they've settled on. So once they reach a half-inch or so in size they can't grip any surface other than the one their shell has adapted to. In this way a motile animal has now become sessile. Remove one from its base (the foot is so powerful you will probably kill it if you tried) and the snail can't grip onto anything else and will eventually die or be eaten.
Now, let's see what we have here: A half-dozen one to two-inch long shelled animals all living stacked up on top of each other. Because of the hard shell and their foot strength they are very well protected but, like other sessile marine animals, they have two major problems: Feeding and reproducing, neither of which are particularly easy to do if you can't move.
The feeding part is easily solved because unlike land animals, marine life are surrounded by an extremely rich source of food in the form of drifting plankton. These various plankton-eaters have evolved several different strategies for sifting the tiny plants and animals from the surrounding water (we'll get to that in an upcoming diary). Animals that eat plankton are known as filter feeders, and for now lets just simplify the slipper limpet's special case and say it involves incurrent pumps, excurrent pumps and a sticky, slimy something called a "mucous net". Stay tuned. [Update:That promised diary is here.]
The reproduction problem is a little bit trickier. Most sessile and semi-sessile animals, like clams and mussels, fertilize their eggs by simply spewing the gametes into the water column and hoping for the best. Pretty simple solution, however there is such a large failure rate due to unfertilized eggs, wasted sperm and tons of predation by other filter feeders, that females must produce huge numbers of eggs for their species to survive. A single adult oyster, for example, will shed nearly a half billion eggs in just one season. Producing all these eggs is a tremendous waste of energy that could be devoted to shell production or growth.
Slipper limpets produce much fewer eggs per female and these eggs are protected inside little yellow capsules held underneath the shell until they hatch. It's a great way to preserve the energy those other mollusks waste. The only catch is that fertilizing these eggs requires internal rather than chance external fertilization. Now, how can an animal that doesn't move find another to mate with? Remember those stacks they form? By growing in colonies like this there are always other members of their own species conveniently close by. (As you can see in the photo above, some of them are still pretty far away from each other. Let's just say the males are very well endowed and leave it at that.)
But what happens if by chance all members of the limpet chain are females or all are males? Crepidula has solved this problem by using a little trick known as protandric hermaphroditism. This means that all slipper limpets are born male and turn into females as they age. So in our chain of a half-dozen snails the older bottom members are females and the three top individuals are male. If another snail joins the party the presence of this new male triggers the oldest male to begin transforming into a female. So in any chain of limpets there are always an equal number of each sex (plus one or two in the middle in the process of switching).
But oh my, it gets even better. We now have seven members of our snail chain. Let's say limpet number four, smack in the middle, suddenly dies and the happy family is divided into two separate chains, one made up of three males and the other made up of three females. Since females cannot switch back into males this stack revs up the larvae-attracting hormones to encourage more males to join their group. For the all-male stack it's even simpler. The bottom one immediately begins the transformation into a female. (And this stack will keep dead number four's empty shell as its substrate.)
To complicate things a bit, limpet stacks aren't always linear. Sometimes you'll get mini chains forming off the sides of the main colony. To be honest, I can't say how this effects the individuals that are part of both stacks but you know there's got to be some serious gender issues going on in this case.
Fun Fact: The genus name Crepidula comes from the Latin word for rattle, Crepitaculum, because a stack of limpets resembles the tail of a rattlesnake.
Other diaries in this series can be found here.
Last night's bonus MLS diary was Mighty Aphrodite.