Wednesday, August 24, 2016

A Genus at a glance

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

In my work for the Delaware Museum of Natural History I have finished a side project that I began in April and returned to the inventory of the freshwater snails. I came across a genus that has very few species, and our holdings include just one lot each of the three most populous, a total of seven shells.

The genus Paramelania was originally named as a subgenus of Tiphobia by Edgar A. Smith in 1881. Although Paramelania has since been elevated to be a genus, Smith is the acknowledged author of the genus description, based on his work with the type species, Paramelania damoni (E.A. Smith, 1881). For this species the author reference is in parentheses because the genus name was changed by the elevation. Smith's original designation was Tiphobia (Paramelania) damoni. Our lot is shown on the left here, and in the following photo in a bit more detail.

A word about assigning scientific names. At the time Smith was working on African freshwater snails, the genus Tiphobia was considered a large genus with several subgenera. Smith had become familiar with exemplars of many of them. When he described the species damoni in 1881 he considered it distinct enough that he created a new subgenus for it within the genus Tiphobia. In 1898 John E. S. Moore described iridescens, and assigned it to the same subgenus. I have not determined which of two Boettger men (Caesar R. or his uncle Oskar) named Paramelania paucicostata, nor whether the elevation of Paramelania had already occurred. The author was probably C. R. Boettger, who spent time in Africa in 1914. Oskar was primarily a herpetologist, though he did describe several mollusk species.

The genus Paramelania, the genus Tiphobia from which it was extracted, and at least fifteen other genera are endemic to Lake Tanganyika in Africa. Lake Tanganyika is the largest of the African Great Lakes, being the second, by volume, in the world (Lake Baikal in Russia is the largest). At 676 km (420 mi) long and on average 50 km (31 mi) wide, and an average 570 m (1,870 ft) in depth, it is a huge lake, spanning more than five degrees of latitude. Imagine a long, very deep, skinny lake spanning the entire eastern boundary of Nevada! Lake Superior is three times as wide, only 80% as long, and not nearly as deep; its volume is less than 2/3 that of Lake Tanganyika, which hosts numerous habitats for numerous species.

This lot and the next were purchased by the museum from New Jersey shell collector Clarence L. Richardson in 1973, during the early days of the museum.

Here we can see that P. damoni has a rather heavy shell with modest ornamentation. Lake Tanganyika abounds with cichlids and other small- to medium-sized fish, and some of them eat snails. Snails that live in shallow water among the vegetation there need to be heavy-shelled to resist the bite of the fish. These two shells give only a hint of the variation found in this species. The aperture, in particular, frequently has a bit of a "wing" at the top, which we see in the next species.

P. iridescens is shinier and has a higher spire, but is otherwise nearly as heavily built as P. damoni. I have not read any ecology studies of these snails, so I can only speculate that this species inhabits slightly deeper water. The ornamentation is less, but the closely-spaced ribs add strength without adding excessive weight. Even for a small snail living in water, the weight of the shell is an important matter.

These two species are the only ones considered "approved" by the author(s) of the Wikipedia article Paramelania. However, the aggregating site Discover Life lists seven species in the genus, including our third exemplar:

P. paucicostata was named by Boettger for having "small ribs". These snails are very similar to P. damoni, although about 2/3 the length and 1/3 the weight. They probably inhabit either shallower waters, or a region more southerly than P. damoni, and the two species may never come into contact.

This lot was a gift of John H. Alexander, a few years before the Richardson purchase. These two men are among the big names on the earliest collections that laid the foundation for the 240,000-lot research collection at the museum.

We have few examples of an entire genus that fits in a shoe box, though a number of genera do contain but a single species. If we were to obtain specimens of the other four species that the contributors to Discover Life list in the genus Paramelania, our holdings would still fit in a shoe box!

No comments: