Allodiscus fallax

Allodiscus fallax
Holotype

Nationally Endangered (NZ TCS)[1]
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Order: Stylommatophora
Family: Charopidae
Subfamily: Charopinae
Genus: Allodiscus
Species:
A. fallax
Binomial name
Allodiscus fallax

Allodiscus fallax is a species of land snail belonging to the family Charopidae.[2] The species is endemic to the Karikari Peninsula of Northland, New Zealand, and is known to occur exclusively native coastal forest remnants in an area of only approximately 10 ha (25 acres) in area.

Description

In the original description, Powell described the species as follows:

Shell of similar size and shape to dimorphus but with a darker and more clear-cut tessellated pattern, as well as more numerous axials, 75 to 80 on the penultimate, compared with 45 to 48 in dimorphus. Interstices of radials with 8 to 10 secondary radial threads compared with 10-12 in dimorphus. Very dense and extremely fine spiral threads, only on the latter part of the protoconch, the first post-nuclear whorl and around the closed umbilicus. Whorls 5, including a depressed protoconch of almost two whorls, faintly malleated, with occasional axial growth lines and exceedingly fine dense spirals over the second whorl.[3]

The holotype of the species measures 4.75 mm (0.187 in) in height and 7.25 mm (0.285 in) in diameter,[3] with individuals having shells that reach up to 8.4 mm (0.33 in) in diameter.[4] The protoconch has 1.25 convex whorls, and is translucent yellow-brown. The teleoconch has up to 3.2 broadly convex whorls, and is translucent with a pale buff and irregular red-brown marks. Juveniles have narrowly umbilicate shells.[4]

The species is similar to Allodiscus austrodimorphus, but can be distinguished due to the spiral sculpture (reticulating secondary radials and fenestrating primary radials).[5] A. fallax can be distinguished from most other members of Allodiscus due to having 75-80 primary radials on the penultimate.[5] While Powell suggested that A. fallax can be distinguished morphologically from A. dimorphus, both species have varying colours and colour patterns, and can only be conclusively distinguished due to geographic location and genetic differences.[4]

Taxonomy

The species was first described by A. W. B. Powell in 1952.[3] The holotype was collected from Oruru Bay near Knuckle Point on the Rangiāwhia Peninsula, Northland, New Zealand on 29 January 1950, and is held by the Auckland War Memorial Museum.[6][7]

Powell suggests that A. fallax may have developed as a distinct species when the Karikari Peninsula was an island separated from New Zealand,[3] which occurred during the Pliocene.[4]

Distribution and habitat

The species is endemic to New Zealand, found on the Karikari Peninsula, Northland,[8] in an area approximately 10 ha (25 acres) in size.[4] The holotype was found under leaf mould in coastal scrub on steep cliff face,[3] in a native coastal forest remnants.[4]

Conservation status

Under the New Zealand Threat Classification System, A. fallax is a Nationally Endangered species,[9] due to the species only being known from a single location.[10]

  • Side view of A. fallax holotype
  • Underside view of A. fallax holotype

References

  1. ^ "Allodiscus fallax Powell, 1952". New Zealand Threat Classification System. Retrieved 8 December 2025.
  2. ^ Allodiscus fallax A. W. B. Powell, 1952. Retrieved through: World Register of Marine Species on 8 December 2025.
  3. ^ a b c d e Powell, A. W. B. (1952). "Four New Species of New Zealand Land Snails and the systematic position of Gerontia cordelia Hutton". Records of the Auckland Institute and Museum. 4: 163–168. ISSN 0067-0464. JSTOR 42906060. Wikidata Q58676731.
  4. ^ a b c d e f Marshall, Bruce A.; Barker, Gary M. (2008). "A revision of the New Zealand landsnails referred to Allodiscus Pilsbry, 1892 and Pseudallodiscus Climo, 1971, with the introduction of three new genera (Mollusca: Gastropoda: Charopidae)". Tuhinga: Records of the Museum of New Zealand Te Papa Tongarewa. 19. Te Papa: 57–167. doi:10.3897/TUHINGA.19.E34170. ISSN 1173-4337. Wikidata Q106839774.
  5. ^ a b Powell, A.W.B. (1979). New Zealand Mollusca: Marine, Land and Freshwater Shells. Auckland: Collins. p. 319. ISBN 0002169061.
  6. ^ Blom, Wilma M. (2025). "Annotated Catalogue of Fossil and Extant Molluscan Types in the Auckland War Memorial Museum". Bulletin of the Auckland Museum. 22. doi:10.32912/BULLETIN/22. ISSN 1176-3213. OCLC 1550165130. Wikidata Q135397912.
  7. ^ "Allodiscus fallax". Collections Online. Auckland War Memorial Museum. Retrieved 8 December 2025.
  8. ^ "Allodiscus fallax Powell, 1952". NZ Mollusca. Retrieved 8 December 2025.
  9. ^ Mahlfeld, K.; Brook, F. J.; Roscoe, D. J.; Hitchmough, R. A.; Stringer, I. A.N. (21 June 2012). "The conservation status of New Zealand terrestrial Gastropoda excluding Powelliphanta". New Zealand Entomologist. 35 (2): 103–109. doi:10.1080/00779962.2012.686313. ISSN 0077-9962. Wikidata Q54573982.
  10. ^ Hitchmough, Rod; Bull, Leigh; Cromarty, Pam (January 2007). New Zealand Threat Classification System lists 2005 (PDF) (Report). New Zealand Department of Conservation. ISBN 978-0-478-14128-3.
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