Saturday 3 September 2016

Sphendone insolita: A new species of Partulid Snail from Palau.

Partulids are a small group of Land Snails found only on Pacific Islands, where they have colonised many small islands other larger groups have never reached. The family is currently divided into three genera, Eua, which is found in Tonga and Samoa, Samoana, which is found in Polynesia and the Mariana Islands and Partula, which is found from Palau to the Society Islands. The group currently contains about 130 described species, but this probably does not reflect the full diversity of the group, as the Snails are arboreal (tree dwelling) and found in remote and widely scattered islands. As with other island-dwelling Snails, Partulids are considered to be extremely vulnerable to extinction, and poor sampling probably means that many species become extinct without ever being documented.

In a paper published in the journal ZooKeys on 1 September 2016, John Slapcinsky of the Florida Museum of Natural History at the University of Florida and Fred Kraus of the Department of Ecology and Evolutionary Biology at the University of Michigan review the Partulid Snails of Palau and describe a new species.

Palau is a group of about 250 islands that form the western part of the Caroline Island group in Micronesia. The islands are currently home to three described species of Partulid Snails, all of which are placed in the genus Partula, however when Slapcinsky and Kraus reviews these species they concluded that this designation is wrong. These species were originally described in 1909 by Henry Augustus Pilsbry under the generic name Palaopartula, so Spalcinsky and Kraus resurrect this genus and return these species to it.

In addition Slapcinsky and Kraus describe a new species from southern Ngeruktabel Island and nearby Mecherchar, which they deem sufficiently different from all other Partulids to merit being placed in an entirely new genus. This species is named Sphendone insolita, where 'Sphendone' means a sling missile in Greek, a reference to the bullet-shape of the shells of the Snails, and 'insolita' means 'unusual' in Latin, in reference to the shape and ecological habits of the Snail, both of which are unusual for a Partulid. The species is described from 77 shells, measuring approximately 18–23 mm in height and 13–15 mm in width, each with 4.5–5.1 whorls.

Sphendone insolita. Scale bar is 5 mm. Slapcinsky & Kraus (2016).

The species is unusual in a Partulid in that instead of living in trees the species is ground-dwelling, being found beneath limestone rocks in well-developed limestone. The two islands on which the species is found are separated by a channel only 25 m deep, and were probably connected during the last glaciation, when sea levels were significantly lower. Since several other nearby islands would also have been part of the same landmass, is possibly that populations of the Snail also exist on these islands.

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