Ariunculus

Ariunculus is a genus of air-breathing land slugs in the family Arionidae, the roundback slugs. Sometimes it has been considered as a subgenus of Arion,^[2][3][4] and sometimes the subgenus Ichnusarion has been raised to generic rank.^[5]

Ariunculus
A live individual of Ariunculus speziae, coin diameter is 16.25 mm
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Order:	Stylommatophora
Family:	Arionidae
Genus:	Ariunculus Lessona, 1881[1]
Type species
Ariunculus speziae Lessona, 1881
Diversity
2 species, each in its own subgenus

Etymology

The stem of Arion plus the suffix -unculus, which is a diminutive in Latin. Hence "little Arion". Pollonera^[6] justifies why "Ariunculus" is appropriate rather than "Arionculus" (cf. "homo" and "homunculus").

The subgenus name Ichnusarion is also based on "Arion", to which has been added "Ichnusa", a Latin name of Sardinia, where the single species of this subgenus is endemic.

Species

Only two species are currently recognised:^[7]

subgenus Ariuncululus

• Ariunculus speziae Lessona, 1881^[1] – type species of the genus; Italian Alps^[8] with one record in Switzerland^[9]

subgenus Ichnusarion Pollonera, 1890^[6]

• Ariunculus isselii Lessona & Pollonera, 1882^[10] – Sardinia

Other species have been discounted as distinct members of the genus:^[7]

• A. mortilleti and A. camerani are considered synonyms of A. speziae (all three were originally described in the same article,^[1] but differ only in size and coloration, not genitalia);^[8]
• A. moreleti is considered a species of Letourneuxia;^[11]
• A. pallaryi matches Letourneuxia numidica in its description;^[7]
• A. austriacus has been synonymised with Arion subfuscus;^[12]
• A. tricolor and A. nigratus are thought to have been misidentified juveniles of a large Arion;^[11]
• A. ischii was apparently a misprint for A. isselii.^[7]

Diagnostic characters

Genital characters that have been proposed as diagnostic of the genus are:

• the genital pore is very anterior, anterior to the pneumostome;^[10]
• the atrium is large;^[10]
• the duct of the bursa copulatrix opens near the end of the epiphallus;^[10]
• the oviduct is long with two bends;^[13][14]
• the epiphallus blends imperceptibly with the vas deferens;^[10]

At least in one species of Ariunculus, this last character reflects that sperm is transferred naked rather than in a spermatophore formed in the epiphallus; this is a fundamental contrast with Arion and Geomalacus.^[7]

Ariunculus differs from Geomalacus, and is similar to Arion, in that the caudal gland is prominent and the mantle contains only calcareous granules rather than a shell plate.^[6]