The referential Clausiliid

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The gathering of biological species into genera (plural of genus), and of genera into families, can be a difficult matter. This is particularly true of animals that are variable in their expression, including mollusks, and gastropods (snails) in particular. The history of discovery proceeds to-and-fro: Species after species will be collected and described, and at first, many similar creatures will all be described under a certain genus. A later researcher may then distinguish a common set of features for certain of those species, and a different set of features for others, leading to setting them aside as a new genus. A similar but more arduous process is needed to discern family membership…usually.

In the case of a unique family of terrestrial snails, the Clausiliidae, one and only one distinguishing feature is needed to determine whether a newly discovered species belongs: the presence of a clausilium. This requires a little explanation.

You may know that many marine snails have a kind of "door" that they can shut behind them when they retreat into their shells. It protects them from many predators and also keeps them from drying out when they are exposed to the air for too long. These two pictures show different species of whelk; the operculum for each is visible. First we see a Lightning Whelk all pulled inside after a wave threw it up onto the beach. Its operculum is the brown oval thing in the aperture.

The second photo shows a different species of whelk crawling on the sand. The animal's foot, at the lower left, is white with dark spots, and the operculum is also brownish, attached atop the end of the foot. Many, many families of marine gastropods have opercula (the plural of operculum). Twenty families of freshwater gastropods also have opercula, as do a small number of non-pulmonate (that is, gilled) land snails. Only one prominent family and two very minor, though related, families of pulmonate land snails have opercula. A pulmonate snail has a lung rather than gills, or in addition to gills, and can thus spend prolonged amounts of time out of the water.

A clausilium is not an operculum. This structure is not kept external to the shell, as an operculum is, but is internal, as seen in these semi-transparent shells. Rather than being attached to the animal's foot, it is attached by a muscular structure to the columella, the central column around which the whorls of the shell are wrapped. When a Clausiliid snail retreats into its shell it pulls its body inside, then deploys the clausilium to block the aperture. So the clausilium's purpose is the same as that of the operculum, but is a distinct evolutionary development.

In addition, whereas an operculum is sometimes thin and even translucent, it is usually robust and may be a thick shell in its own right with a spiral structure. By contrast, a clausilium, although always calcareous, is thin and rather fragile. Apparently, members of the Clausiliidae, being small and having a small aperture, do not encounter the determined predators that attack many marine snails.

Today in my work on the current project at the Delaware Museum of Natural History I began to take inventory of a little more than 1,000 lots of the family Clausiliidae. The type genus of the family is Clausilia, described by Jacques P.R. Draparnaud in 1805. He based his description of the genus on the species Clausilia dubia, but a later evaluation by physician and malacologist Louis C.G. Pfeiffer led to a re-description of the genus as referred to Clausilia scalaris Pfeiffer, 1850. Ironically, this species has been renamed and then made synonymous with another species, and is now called Muticaria macrostoma (Cantraine, 1835; Cantraine originally called the species Clausilia macrostoma).

I found that DMNH has just one lot containing shells of this species, which is endemic to the island of Malta. The collector, whose material was obtained by Ralph Jackson in the 1950's and later donated to the museum, notes that this species is "very rare". Yet he somehow obtained six shells.

This photo shows the shells close to natural size. It is hard to see in this image that the shells are well-decorated with ridges or striations. Clicking on the image to see it larger is a little better, so I also took a closeup through a low power microscope.

The species name "macrostoma" means "large mouth". Clausiliids all have small apertures, and the aperture of this species is only "large" by contrast to many other species in the family, and that primarily because of the wide lip around the "mouth".

All the shells in this lot have the first one or two or three whorls broken off. That fact and the ridges indicate that these little animals live in a rather high-energy environment. I became curious about the locality so I looked it up, and found that Mistra is on a protected bay. Malta is not very large, being 17 miles long and 8 miles wide (27x13 km), but it has room for at least five places named "It-Torri", which means "red tower". The nearest to Mistra is about three miles to its northwest. This is a much windier locale than Mistra, so perhaps such environmental stresses shaped the species.

Few clausiliids are so heavily decorated, and, indeed, most are smooth or nearly smooth. So it is ironic for this to be, in effect, the "poster child of the Clausiliidae."