How They Do "It"

by Peggy Williams

Mollusks, as every living organism, need to reproduce themselves to maintain their species. They accomplish this in a variety of ways.

Most of the gastropods have regular male and female sexual organs, one at a time. In these cases, the male and female get together and the male fertilizes the female in the way we're used to. However, there are some twists to this in the mollusk world.

Occasionally, on a cold winter day when the tide is far out, you may be wading across a sand bar and encounter a large Lightning Whelk (Busycotypis contrarium) with one, two, three, or even four smaller ones close by or even on top of the larger shell. The big one is a female and the smaller ones males. After mating, the female will bury in the sand and begin extruding egg sacs containing her progeny. You can even watch the string of egg sacs slowly billowing out of the sand. These eggs will grow in their safe haven until they are large enough to crawl and then break out of the sac to begin moving through the sand.

But what happens to those small male whelks when they get bigger? They change sex! Yep, the females are the big 'uns!

The "Slipper Shells" (Crepidula fornicata) so common around Florida and up to New England are often found, when live, in stacks of up to seven individuals, with the largest on the bottom. Why is this? You guessed it - the female is the big one and all the rest are males. (When the large female dies, the next animal changes sex from male to female so there's always a female in the stack.) When it's time to spawn, they all lift up their shells and the female releases eggs into the water while the males release sperm. The two get together in the water column and the eggs are fertilized.

For awhile, these eggs are part of the plankton that drifts freely or swims feebly in the water column. They become "veligers" - a larval stage of their lives during which they have swimming lobes that enable them to steer, somewhat, in their wanderings. When their shell has grown too heavy to continue floating, they fall to the bottom and, if they find a place that's just right for their species, settle down to grow up.

Many mollusks achieve a large distribution area by this method, and when the species is depleted in one area, it may renew itself when new veligers arrive from another place. This is the case with the "Queen Conch" (Strombus gigas), which is now protected in Florida. Though once very scarce in the Florida Keys, now there are areas where you cannot help stepping on huge conchs whose parents were recruited from someplace in the Caribbean. The Triton family (Ranellidae) and their relatives have a very long veliger stage - Cymatium parthenopeum, the Hairy Triton, lives as long as a full year among the plankton, so that the species is found throughout the world wherever the water temperature is suitable.

You can tell the difference between shells that have a veliger stage and those that come out of the egg sac crawling on the bottom if the earliest whorls of the shell (the "protoconch") is available: the crawlers' first whorl is bigger, sometimes even huge and bulbous, while the swimmers' are small and lightweight, sometimes translucent.

Most bivalves behave like the slipper shells and release eggs and sperm into the water at the same time. Somehow, the individuals of a species that are near one another get a signal to spawn at the same time, or this method wouldn't work.

There is at least one bivalve species, however, that broods its young. A Cardita species from South Africa has a special shell pouch on the inside of its shell which shelters the eggs until they are ready to hatch. Similarly, some Volutes nurture their young inside their shells and some people have been privileged to witness the smaller shells crawling free of their mother. I have also found tiny shells inside freshwater snails.


Cowries and octopus will guard their eggs and keep them clean and safe from predators. Female octopus are so careful of their eggs that they become exhausted and die after the eggs have finally hatched. Cowries lay their eggs in a mass that just fits under the shell and its mantle and crouch on the eggs until they hatch. Usually, the male and female are both in attendance, though I don't know if they both brood the eggs. If you are in cowry territory and turn a rock to find two cowries together, neither moving away from the sunlight you have exposed them to, they are probably guarding eggs. Conscientious collectors leave these cowries alone to perpetuate the species!

The males of some cephalopods, such as squids and cuttlefish, have an extra tentacle which is really a kind of penis. They insert this tentacle into the body cavity of a female and deposit a package of sperm, which the female keeps until she is ready to spawn.

Finally, some mollusks, including the shell-less "Raggedy Sea Hare" (Aplysia species) we have in Florida, are hermaphroditic. This means they are both male and female at once. They gather at certain times of the year in large groups and fertilize each other, then lay huge masses of egg strands resembling green spaghetti.

I learned some of this from reading books, but other information came from observations of living mollusks. I have seen octopus and cowries on their eggs, collected the brood-pouch bivalve, watched over a dozen sea hares at night in Sarasota Bay mating and depositing eggs, and observed whelks both mating and laying eggs. Curiosity led me to ask and read more about these phenomenon, and this has enriched further my field experiences. So get out in the water, watch and learn you'll love it.

This article is copyrighted 2006 by Peggy Williams. It may be reproduced for Shell Club publications, providing it is attributed to Peggy Williams, found on the website, www.Shelltrips.com.