Pravonitor insularum ( Iredale, 1937 )

Hyman, Isabel T. & Köhler, Frank, 2022, Homoplasy in shells discombobulated the taxonomy: revision of the larger helicarionid land snails of northern Queensland, Australia (Stylommatophora: Helicarionidae), Journal of Natural History 56 (41 - 44), pp. 1727-1799 : 1773-1775

Pravonitor insularum ( Iredale, 1937)

( Figures 4 , 14 (a))

Nitor (Pravonitor) kreffti insularum Iredale, 1937: 3 .

Material examined

Holotype. AM C.29647, Mer Is., Torres Strait, Far North Qld , Australia.

Paratypes. AM C . 170795 156 specimens; AM C .104171 1 specimen.

Other material. See Table 1 .

Diagnosis

External morphology. Shell ( Figure 14 (a)) medium-sized (SW 11.3–13.6 mm), pale golden with a pale orange-brown supraperipheral band, 5.2–5.5 whorls, discoidal with a low conical spire, whorls rounded; umbilicus very narrowly open. Protoconch and teleoconch sculptured with very fine spiral grooves.

Remarks

The first name to be applied to Torres Strait Pravonitor species was Helix (Thalassia) annulus Brazier, 1876 from New Guinea ( Brazier 1876). This name was restricted to New Guinean populations by Iredale (1937), and the new subspecies Nitor (Pravonitor) kreffti insularum was introduced for Torres Strait Island populations (type locality Mer Island). The new subspecies was originally described as differing from P. krefftii (type locality Cape York) in its slightly smaller size, more depressed spire and the presence of a coloured anteperipheral band ( Iredale 1937).

The two subspecies were synonymised by Smith (1992) based on the original descriptions and an examination of types. This was upheld by Hyman and Ponder (2010), who stated that shell size and the presence of a coloured band is highly variable and found no clear difference between the shells of mainland and island forms, and illustrated specimens from Gerbar Island under the name Pravonitor kreffti . Stanisic (2008) recorded P. kreffti from Mua, Badu, Iama, Erub, Mer and Waibene Islands as well as Cape York, based on the collections of the Australian Museum and Queensland Museum.

However, in the current study we found that specimens from Iama Island and Gerbar Island (situated midway between Australia and New Guinea) differed considerably in their genital morphology and mitochondrial DNA to specimens from Mua, Badu, Waiben and Muralug Islands, which are much closer to Australia. Specimens from Cape York matched the latter group. Comparison of type material (where present) and original descriptions confirmed that these two species corresponded to Pravonitor annulus and Pravonitor kreffti .

The available material from Mer Island was too old for successful DNA extraction and sequencing, and all preserved specimens were subadult. Dissection demonstrated that the specimens belong to the genus Pravonitor , but did not provide sufficient information for species assignment. Shell measurements showed that these shells were significantly flatter than those of all other Torres Strait Island groups, as suggested by Iredale (1937). However, the available shells were also slightly lower in whorl count than those of other island populations (see Table 4 ), possibly inflating the perceived difference, since in land snails shell growth is allometric and relative height generally increases with whorl count ( Goodfriend 1986). Examination of the shells also confirmed the presence of an anteperipheral band, also present in Pravonitor annulus but not P. kreffti .

In the absence of anatomical and molecular data, the only evidence that P. insularum is distinct from the other Torres Strait species lies in the significantly flatter shell and the presence of an anteperipheral band. There is considerable evidence to show that shell shape is often related to habitat preference rather than shared ancestry (e.g. Köhler et al. 2008; Stankowski 2011; Criscione and Köhler 2013). However, Mer Island is also more distant than the other islands, and has a different origin, having been formed from volcanic activity rather than being part of the continental shelf. Given that different species exist on the mainland/Inner islands/Near Western islands and on the Central islands/Top Western islands/New Guinea, it appears mostly likely that the isolated Mer Island also harbours a unique species.

Pravonitor insularum is currently placed in synonymy with P. kreffti , but is more similar to and lies geographically closer to P. annulus . Herein we remove P. insularum from synonymy with P. kreffti ; however, we do not feel there is sufficient evidence to place it in synonymy with P. annulus , and therefore we retain this name for the Mer Island populations alone.

The Eastern group of islands also contains Dauar Island, Waier Island, Erub Island and Ugar (Stephens) Island. Four shells from Erub Island were measured and were found to differ significantly from the Mer Island shells in both size and shape (SW, SH, SW/SH: p <.0001); however, they did not differ significantly from P. annulus or P. kreffti . It is possible that the Eastern Islands may contain more than one species.

Figures

Figure 4. Distribution map. Grey symbols: Geminitor. Blue symbols: Pravonitor. Red symbols: Elatonitor. Biogeographic barriers are marked in black; A = Normanby Basin, B = Black Mountain Corridor, and C = Atherton Tableland.

Figure 14. Shells of Pravonitor. a) Pravonitor insularum, AM C.170795 (paratype). b) Pravonitor annulus AM C.425415,Gabba Is. c) Pravonitor aquilonia, QM MO48285 (holotype). d) Pravonitor ferrugineus, QM MO12752 (holotype). e). Pravonitor kreffti, AM C.1352, Thursday Is. f) Pravonitor kreffti, AM C.425474, Simpson’s Bay, Utingu. g) Pravonitor monteithi, QM MO48587 (holotype). h) Pravonitor septentrionalis, AM C.425422. i) Pravonitor stuarti, QM MO80213 (paratype). Scale bar: 5 mm.