Notodiscus

This species was described under the name Helix hookeri by an English conchologist Lovell Augustus Reeve in 1854.^[1] The specific name hookeri is in honor of English botanist Joseph Dalton Hooker, who collected this snail during the Antarctic expedition led by James Clark Ross.^[1] Reeve's type description reads in Latin and in English language as follows:^[1]

Species 1474 (Mus. Brit.)

Helix hookeri. Hel. testá mediocriter umbilicatá, orbuculari-depressá, sordidè olivaceá, subirrigulariter rugoso-striatá; spirá subplanulatá, suturis impressis; anfractibus quatuor, convexis; aperturá lunato-circulari, labro simplici.
Hooker's Helix. Shell moderately umbilicated, orbicularly depressed, dull olive, rather irregularly roughly striated; spire rather flat, with sutures impressed; whorls four, convex; aperture lunar-circular, lip simple.
Hab. Kerguelen's Land; Dr. J. D. Hooker.
A small depressed species collected by Dr. Hooker in the Antarctic Expedition of the Erebus and Terror, peculiarly characterized by the sombre olive-horny coating of

Paludina and Ampullaria.

Henry Augustus Pilsbry classified this species as Helix hookeri in 1887^[4] or within the genus Amphidoxa as Amphidoxa hookeri within the family Endodontidae in 1894.^[5]

Also Alan Solem classified this species within the family Endodontidae in 1968.^[6]

A subspecies Notodiscus hookeri heardensis Dell, 1964^[7] was recognized in Heard Island.^[8]

Notodiscus hookeri has a wide distribution in the sub-Antarctic region.^[2] It is the only native terrestrial gastropod species found in the South Indian Ocean islands and archipelagos, and also in the South Atlantic Province:

South Indian Province:

• Crozet Islands.^[2] For example, Notodiscus hookeri is the only terrestrial snail among about 50 species of native invertebrates in the Crozet Islands.^[2]
• Kerguelen Islands^[2]
• Heard Island^[2]
• Prince Edward Islands^[2]

South Atlantic Province:

• Notodiscus hookeri is limited to South Georgia in the South Atlantic Province.^[2]

The type locality is the Kerguelen Islands.^[1]

The land snail Notodiscus hookeri is not an endangered or a protected species.^[2]

The shell growth does not stop on reaching sexual maturity, but decelerates considerably, with the biggest shells measuring 7.5–7.7 mm in size.^[2]

Large intraspecific variations in shell morphometrics have been reported for this species on Possession Island,^[8] with endemic variants being described as local adaptations to environmentally distinct islands.^[2]

The shape of the shell is depressed. The umbilicus is open.

The width of the adult shell is up to 7.5-7.77 mm.^[2] The weight of the snail of the shell length 6.13 mm is 52.88 mg.^[9]

The micro structure of the shell was analysed by Charrier et al. (2013).^[2] Their study was the first to demonstrate that gastropod shell micro structure responds to environmental heterogeneity, leading to the formation of ecophenotypes.^[2] The adults of Notodiscus hookeri have evolved into two ecophenotypes, which the authors referred as MS (mineral shell) and OS (organic shell):^[2]

• The MS-ecophenotype is characterised by a thick but small mineralised shell.^[2] This ecophenotype is primarily found along the coastline, and may be associated with the presence of exchangeable calcium in the clay minerals of the soils.^[2]
• The OS-ecophenotype is characterised by a thin but large organic shell.^[2] This ecophenotype is primarily found at high altitudes in the mesic and xeric fell-fields, in soils with large particles that lack clay and exchangeable calcium.^[2] Snails of the OS-ecophenotype are characterised by thinner and larger shell sizes compared to snails of the MS-ecophenotype, indicating a trade-off between mineral thickness and shell size.^[2] The OS-ecophenotype has a highly flexible shell.^[2]

Notodiscus hookeri has unique^[2] shell micro-scale structure among gastropods:

• A dense and homogeneous organic layer is loosely attached to the upper periostracum and the inner mineral layer.^[2]
• In the organic layer of the shell, there is prevalence of glycine-rich proteins (glycine, leucine, isoleucine, valine), and an absence of chitin.^[2] Almost all other gastropods with reduced shells have chitin.^[2] The only other known example of the absence of chitin is the internal shell of the slug Ariolimax columbianus.^[10][2]

This land snail is a gregarious species that lives under moist stones, moss and wet vegetation; however, it is also widespread in fell-field areas, which are characterised by very low vegetation cover.^[2] This snail live in relatively simple ecosystems, that is caused by harsh environmental conditions on subantarctic islands.^[9] It is a litter-dwelling species.^[9]

The soil is known to be a nutrient resource for Notodiscus hookeri, since this species has been found to significantly increase calcium release in solutions derived from plant litter.^[2]

Notodiscus hookeri exclusively feeds on lichens such as Orceolina kerguelensis, Usnea taylorii and Pseudocyphellaria crocata.^[9] Notodiscus hookeri appears as a generalist lichen feeder able to consume toxic metabolite-containing lichens.^[9]

Hatchlings have a shell width of < 2.0 mm.^[2] Juveniles have a shell width of about 2.0-4.0 mm.^[2] Adults have a shell width larger than 4.0 mm.^[2]

The biology of this species is poorly known.^[2]

Notodiscus hookeri was depicted on the 2012 €0.60 French Southern and Antarctic Lands postal stamp.^[11]

• List of non-avian fauna of Heard Island and McDonald Islands