Here is way to much info if you want it. Also described is how to tell the difference between the two.
For many aquarists the appearance in their systems of tiny white to tan snails, much smaller than a rice grain, is a cause for concern. This concern is definitely warranted as some of these species of tiny snails are in large group of very small snails called the Pyramidellidae. Pyramidellids, also called pyrams, are shelled opisthobranch snails that are typically ectoparasitic on some other animal. The appearance of pyrams generally foretells either a series of losses for the aquarist or some very hard work spent eradicating the pests. These little snails will seek out their hosts, either larger snails, tridacnid clams, or even feather duster worms. The snails will attach to the host where they can conveniently reach the soft tissues and then proceed to dine on the blood of their host. One or two small parasitic snails on large clam cause an insignificant amount of damage and does not constitute a serious problem for a clam or snail. In the closed confines of the reef tank, however, one or two of these pests can soon reproduce many hundreds of offspring, and the snail populations can reach sufficiently high numbers to kill their hosts. These pests have to be manually removed, and with diligence they may be eradicated from a system. The eradication effort may take time, effort, and a lot of work.
The major confounding factor is the common presence in many reef tanks of some other small snails quite similar in appearance to the parasitic pyrams. These small animals, in the gastropod group called the Rissoidae are quite beneficial; they feed on microalgae or small detritus particles. While it is necessary for a hobbyist to remove the problem snails, it is much to the same hobbyist’s benefit to leave the rissoids alone. Additionally, if the system is populated with rissoids, it is no cause for concern. Consequently, hobbyists can save themselves a lot of time and trouble if they can differentiate between these two types of animals.
Conditions and Definitions:
A pyramidellid is defined, for the purposes of this article, as a snail in the huge taxonomic group referred to as the family Pyramidellidae; (1, 2, 3, 4, 5, 6, 7). Pyramidellids are members of one of several lineages of shelled opisthobranchs, a group that also includes sea slugs and nudibranchs. Pyrams are rather odd little beasties, and that oddity is reflected in their shells. All snails pass through a larval stage referred to as “a veliger.” Veligers have a small coiled shell consisting of one to two whorls and a large ciliated structure called a “velum” used both in locomotion and to capture prey. The veliger or larval shell is often relatively plain and simple; however, when the animal has completed its larval existence, the shell shape often changes. As the larval shell remains connected any subsequent shell produced by the animal, those shell changes can be easily seen, provided the larval shell has not been eroded or broken off. One characteristic that sets pyrams apart from most other snails, including the rissoids is that when they pass from the larval stage into the juvenile stage, they reverse the direction of the coiling of the shell. This results in the larval shell being turned at a right angle to all subsequent shell whorls. The fancy term for such a change in the shell orientation is heterostrophism, and consequently, pyrams are said to have heterostrophic shells. Pyrams also lack the operculum found in many, but not all, other shelled snails. This operculum is a “trapdoor” that plugs the opening, or aperture, of the shell. Finally, their shell aperture is ovoid and on its inside edge, the calcareous shell material forms a “fold” of unknown function.
Rissoids (Rissoidea in the larger sense, and the subfamilies of the Family Rissoidae, the Subfamily Rissoinae and the Subfamily Rissoininae in the more precise sense) are similar to the pyrams in general size and color. Rissoids differ from the pyrams in that they lack the heterostrophic shell coiling, they possess a proteinaceous operculum and the inner edge of their shell apertures are smooth, and lacking any hint of a fold in the calcareous shell material. Any one of these three characters will always separate Pyrams from Rissoids. Both groups are in a state of either taxonomic flux (Rissoids) or sheer, utter, chaos (Pyrams). Consequently, identifying individuals in either group to a lower taxonomic level, such as species or even genus, will likely be very difficult or impossible. Nevertheless, with a bit of work, a good magnifying implement, and some luck, it is easy to distinguish between animals from the two groups.
