The Parts of a Gastropod Shell
by Peggy Williams
When you try to identify shells from the shell books, it's important to understand the terminology that describes the shell. Here are some of the terms:
If you hold the shell with the aperture towards you and down, the animal will be standing on its head! At the top of the shell in the position is the apex, consisting (if it's not broken off) of the protoconch. The protoconch is the larval shell, consisting of 2-4 whorls (turns of the shell) and it can tell you a lot about the animal's lifestyle.
If the protoconch is especially large and bulbous (fat) for the size of the shell, that animal probably lays eggs that crawl directly out of the egg case when they hatch. Other mollusks have a swimming larval stage which can last from a few hours to up to a year, depending on the species. Needless to say, these animals need lightweight shells to maintain their position in the water until they are ready to settle to the bottom and start growing, so the protoconch will be small and quite often broken off. Direct-developing eggs (that crawl out of the egg case) need to be larger and stronger to survive on the bottom with all its predators, so the protoconch is heavier.
The sculpture (raised lines) of the protoconch often helps to identify the shell, especially in cases where there might be several choices of species.
Below the protoconch are the adult whorls. They generally increase rapidly in size (according to a measureable formula) and are usually 5-9 in number. Of course, each whorl covers up the "lower" part of the preceding whorl, so the final or body whorl is the only one you can see in its entirety.
Usually you can see the division between the whorls, called the suture. Often the shell has a little shelf at the "top" of the whorl called the shoulder. If there are spines on the shell they are usually at the shoulder, and there may be additional ones on the body whorl.
The opening through which the animal protrudes is the aperture, or mouth, with an outer lip and an inner columella. Often the lip is thickened, which helps to protect the shell from being broken by crab claws. If the thickness is pronounced, it is
called a varix, and some animals dissolve the varix when they grow more shell and some don't. The latter will have thickened vertical places on the early whorls called previous varices (plural of varix). One group, the Distorsios, have not only a thick varix but many teeth on both the outer lip and the columella which makes the opening especially small. This family does not dissolve the varix or teeth when growing, so it has to grow over that old shell material, distorting the shell.
Some molluscan families have a final, adult whorl which looks different from the previous whorls. A good example is the Strombus family, the "conchs". All of the immature whorls have a thin lip, without a varix or any other thickening. However, when they approach adulthood, they add a wide, flaring lip to the final whorl. This adds weight to the shell and protection for the animal as it moves below the shell.
At the "lower" end of the aperture is the opening through which the animal protrudes its siphon, which is the breathing, eating, and sensing organ. It crawls along with the siphon extended ahead of the shell to sniff out its environment and prey. To protect the siphon as much as possible it builds the siphonal canal, a partial tube which just fits the siphon. Generally, rock-dwelling mollusks have a long siphonal canal to better protect the siphon while sand-burrowing mollusks can extend the siphon far beyond the siphonal canal, above the surface of the sand when they are buried.
Raised or incised lines, spines, and knobs help strengthen the shell without making it too heavy. Each species has a unique series of sculpture features - that's how we tell the different species apart without DNA! Raised or scratched lines can be spirals (around the whorl) or ribs (across the whorl). If there are both spirals and ribs they may make little squares in the surface of the shell - this is called cancellate sculpture - and sometimes knobs are formed at the intersections.
Smooth shells are usually sand-burrowers, the better to bury, or cowries, which hide below rocks and cover the shell with part of their bodies.