Bernhard Rensch discovers new fauna on Nusa Penida (1938)
Nusa Penida lies on a very narrow ridge between Lombok and Bali. Between Lombok and Nusa there is a stretch of sea-floor of 220m below sea level in an otherwise much deeper Lombok Straight (at a depth of more than 600m), whereas the much narrower ridge between Nusa Penida and Bali lies at 210 metres below sea level. It was to be expected that Penida's fauna was similar to Bali's fauna.
This, however, is not the case. As Prof. E. Streseman points out in 1938 (a report based on the findings of Baron Viktor von Plessen, who visited Nusa Penida in February/March 1938 - see elsewhere on this website) the majority of bird species here have more in common with Lombok than with Bali.
Images above left: Type (left) and a Paratype of the Asperitas (inquinata) penidae; above right: Type (left) and 2 Paratypes of the Sasakina plesseni
Rensch mentions the differences between the Oriental fauna region of Bali (the eastern-most extreme tip of what now is called the Wallace Line) and the Indo-australasian transitional fauna region on Lombok. He states that out of the 20 then known bigger snail species on Lombok, only 6 are also found on Bali. Rensch discusses three larger species of land snails, which Von Plessen had collected and conserved in alcohol for him carefully during his trip, of which two at the time were new: Sasakina plesseni sp.n., Asperitas (inquinata) penidae nov. and Amphidromus perversus (L.). All of these snails are described in detail in the article: the exterior form, colour patterns and genitalia.
Penida land snails
Images below, left: The snail Asperitas waandersiana, endemic to East Java and Bali, is represented on Nusa Penida by this distinctive form. (By A.J.Whitten); right: Asperitas trochus penidae (personal correspondance with Cédric Audibert, June 2014; image source: here)
The newly discovered Sasakina plesseni belongs to a genus typical for the Lesser Sunda Islands and is related to the Sasakina perinsignis (Sm.) from Lombok. The Asperitas (inquinata) penidae was hitherto known from the island of Sumbawa, Selayar (Sulawesi) to Banda and Seram, but the Nusa Penida species is definitely the largest of the bunch. The third species Amphidromus perversus could be found in a large region from Malacca to Sumbawa, Selayar and Sulawesi. The 'perversus' could have reached Nusa Penida either from the East or from the West. Rensch thinks that the latter is more likely, since this species at the time had not yet been identified in Lombok.
In conclusion, Rensch states that the bird and snail distribution on Nusa Penida leads him to believe that the narrow (land) ridge between Bali and Nusa Penida disappeared before the broader stretch of land between Lombok and Nusa Penida was engulfed by the sea during the Pleistocene due to rising seas levels. He then suggests that the Penida snails point in the direction of Bali's easterly neighbour and have a relationship at species level (?) with the snails in Lombok rather than Bali. "In any case, it is striking that all the three species of snails collected on Nusa Penida merely judging by their formidable size occupy a unique place within this relationship group (Verwandtschaftsgruppe)", he adds.
- Rensch, Bernhard - Neue Landschnecken von der Insel Penida [Von Bernhard Rensch, Münster (Westf.), Mit 4 Abbildungen., Eingeg. 16 August 1938], in: Zoologischer Anzeiger, 123,Band, VAG 1938, Akademische Verlagsgesellschaft M.B,H. in Leipzig, Nr.1/2, July 1938, p.302-307
Terrestrial and Freshwater Molluscs: Endemic species
Image right (Whitten 1996:p.293): Sasakina plesseni, a species of snail known only from Nusa Penida (by A.J. Whitten, 1996)
To date, 65 species (31%) of land and freshwater molluscs appear to endemic to Java alone, 11 to java and Bali, and 10 to Bali (including Nusa Penida). Land snails favour moist habitats and avoid sunlight, either by being nocturnal or by living in shady places, because they face desiccation in hot and dry situations. Most snail species appear to be found in the regions of highest rainfall, where they also occur in their greatest abundance, but dry, vegetated areas can have surprisingly rich fauna, and this is particularly true of the remaining forested areas of Nusa Penida. Such dry areas have attracted relatively few collectors. The other major factor influencing snail distribution is the presence of lime (calcium carbonate), without which the shells cannot be made. It is not surprising then, that many species and individuals are found in limestone areas, but volcanic deposits are not without lime, and so snails are by no means restricted to limestone areas.
- Whitten, Tony (et al.) - The Ecology of Java & Bali, The Ecology of Indonesia Series, Volume II, Periplus Editions, 1996, pp.286-287, 293-294