Tuesday, October 04, 2016

Two semi-related snails

kw: species summaries, natural history, natural science, museums, research, photographs

At the current stage of my inventory of the freshwater snails in the collection of the Delaware Museum of Natural History, I came across two species that  stretch the limits of the genus in which they have been classified. Indeed, as I shall discuss, they stretched those limits to the breaking point.

The first is Melanoides pantherina (von dem Busch, 1859). The genus Melanoides was created when a very large genus Melania was divided into several genera, and the family Melanoidea was broken into Thiara (to which these belong), Pleuroceridae, and several smaller families of genera.

"Melanoides" means "like Melania", and "Melania" means "dark". Snails in this genus are colored dark brown to nearly black.

This species was named "pantherina" because juvenile specimens are spotted. Faint spots in the smaller whorls of the specimen shown at right are still visible. At first, I thought the spots caused by their habit of laying their eggs on themselves were the reason for the name, but that habit is found in several related species.

Also note on the older label from Richardson's collection, that English and American conchologists tended to spell the author's name "Bush" instead of Busch. This arose during about a century of German-English conflict that culminated in two world wars.

The second species is presently named Melanoides Scabra (Müller), at least at Delaware, but many experts have renamed it Thiara scabra (Müller), and we plan to follow suit. The genus Thiara was created to gather the "Thumb snails", many of which really are about the size and shape of someone's thumb.

This species is a little smaller, but its lower spire indicates its affinity with other Thiara species. These species are also lighter in color than the typical specimen of Melania or Melanoides.

Both these species, and indeed, a great many of their relatives also, are river snails. Their shell shape and presence or absence of spines are clues to the energy of the rivers they inhabit, and the kind of predators they face. Larger, thick-shelled ones, for example, live in faster water and have little to fear from snail-eating fish, but are more likely to fall prey to boring predators such as smaller predatory snails.

The family to which they all belong, Thiaridae, is found throughout east Asia and the islands in the western Pacific and the Indian Ocean. They are found mainly in rivers and a few are found in brackish estuaries.

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