Monday 24 May 2021

Partula lutaensis: A new species of Partulid Tree Snail from Rota Island, in the the Mariana Archipelago.

Partulid Tree Snails are found across the islands of the tropical Pacific, reaching their maximum diversity towards the eastern end of their range, with the genus Partula reaching its highest diversity in the Society Islands, and the genus Samoana reaching maximum diversity in the Samoan Islands, although it is thought that these Snails originated further to the west. The Mariana Archipelago is a crescent-shaped chain of 15 volcanic islands extending roughly 2520 km from Farallon De Pajeros in the north to Guam in the south. These islands were long thought to be home to six species of Partulid Snails. Partula radiolata and Partula salifana were known only from Guam, Partula desolata, described from sub-fossil shells and probably extinct was known only from Rota, Partula langfordi was known only from Aguigan, and may also now be extinct, Samoana fragilis was known from both Guam and Rota, and Partula gibba known from seven islands, from Guam, in the south, to Pagan Island in the north.

In 2017 David Sischo of the US Department of Land and Natural Resources and the Pacific Biosciences Research Center at the University of Hawai‘i at Mānoa, and Michael Hadfield, also of the Pacific Biosciences Research Center at the University of Hawai‘i at Mānoa, published a paper in the Biological Journal of the Linnean Society, which described the results of a generic study of Partula gibba, which concluded that the population of this Snail on Rota Island is in fact genetically distinct, and represents a new, cryptic species which had not previously been identified.

Map of the Mariana Islands. Islands where partulids were sampled are indicated by stars. Sischo & Hadfield (2017).

In a second paper, published in the journal ZooKeys on 17 May 2021, David Sischo and Michael Hadfield formerly describe the new Tree Snail species as Partula lutaensis, which combines 'Luta', the indigenous Chamorro name for the island of Rota, and the suffix '-ensis' implies coming from.

The shell of Partula lutaensis is dextral, moderately thin, ovate-to-conic, and slightly perforate. The umbilicus is open, the whorls are moderately convex, and the suture is adpressed. The aperture is ovate-to-elongate, and slightly oblique. The outer lip is reflexed, thick, and glossy, the parietal lip glossy with light or dark colouration. The colour of embryonic the whorls and post-embryonic whorls is variable from shades of brown, buff, white and yellow with a prominent white subsutural band. The 48 specimens measured had an average height of 15.98 mm, and an average width of 10.64 mm.

(Left) Colour morphs of Partula lutaensis found within a 10 × 10-meter quadrat. (Right) Closeup of a Partula lutaensis with a dark shell. Sischo & Hadfield (2021).

All of the known specimens of Partula lutaensis were collected from two species of Plant, the Devil's Ivy, Epiprenmum aureum, and the Halberd Fern, Tectaria crenata. The species is currently present at fairly high densities on Rota Island, however other species of Partulid Snails in the Mariana Archipelago have gone from relative abundance to near-or-actual extinction in very short periods of time, largely due to the activities of introduced predators; a pattern that has been repeated on islands across the Pacific. Particularly troublesome for Partulid Snails are North American carnivorous Snail species in the genus Euglandina, and the New Guinea Flatworm, Platydemus manokwari. On Rota Island Platydemus manokwari has been observed predating Partula lutaensis, suggesting that this species is at risk. Partula gibba, the species to which these Snails were formerly assigned, is protected by a federal Endangered Species declaration, but this will not automatically transfer to the new species, for which reason Sischo and Hadfield consider it imperative that Partula lutaensis be listed as Endangered as soon as possible.

A Partula lutaensis freshly depredated by Platydemus manokwari observed on Rota. Sischo & Hadfield (2021).

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