American Scientist

To the Editors:

In “The Rarest Snail in the World” (November–December), Pat Shipman describes her search for the Caymanian land snail Cerion nana (sometimes known as C. nanus). Although the species’ restricted range surely qualifies it for red listing, thousands of land-snail species are known from fewer specimens than C. nana and many have similar risk of extinction. C. nana isn’t even the rarest snail in the Cayman Islands. Geomelania alemon, for example, has only one published collecting report, resulting from the Oxford University Biological Expedition of 1938. Last time I looked, there was a warehouse on G. alemon’s type locality on Grand Cayman.

At least eight museums (not just four) hold specimens of C. nana. In the context of my institution’s collection, C. nana was not historically rare: It’s represented by 29 specimens, well above our median of 7 specimens per land-snail species. When Maynard named C. nana, he reported examining 2,000 individuals, and he later sold specimens for 10 cents apiece. Although high population density does not assure a species’ well-being, it does make it more likely that a subsequent decline will be noticed.

I appreciate Shipman’s article for highlighting the many aspects of rarity and the difficulty of determining the conservation status of species. Unfortunately, the current rarity and critically endangered status of C. nana is not unique. Rather, it reflects, and I hope will serve to publicize, the precarious state of the world’s terrestrial mollusks, which have among the highest modern extinction rates documented for any group of organisms.

Gary Rosenberg, President
American Malacological Society

Dr. Shipman responds:

The phrase “rarest snail in the world” was used by Mike Hounsome to describe C. nanus (or C. nana for those who prefer it) in his chapter on terrestrial invertebrates in The Cayman Islands: Natural History and Biogeography (1994). Not being a malacologist, I wouldn’t presume to make such an assessment myself, but it made a great title. Sadly, the type locality for C. nanus has also been built upon and cleared. I’m not sure if the number of specimens in museums is the best criterion for rarity; maybe the number of living specimens is a better measure. I hope the content of my article was at least as thought provoking as the title, which seems to have bothered quite a few people.