Succineoidea
The extant Succineoidea comprise two families, Succineidae and Athoracophoridae. The earliest known Succineidae are from the Palaeocene of Europe, with additional fossils occurring in Tertiary deposits in Africa, Europe, and North America. The family is now cosmopolitan, but with greatest diversity in the islands of the Pacific, in the Indian subcontinent, and the Americas.
Succineidae are generally small animals with a thin, high-spired shell whose dimensions and shape are generally of little diagnostic value below the family level, except for several notable genera.
In several parts of the world a trend to shell reduction and slug form has occurred. In Succinea (Hyalimax) Adams & Adams and Omalonyx (Neohyalimax) Simroth the shell is reduced to the extent that it is internalised beneath the mantle shield.
The life strategy of Succineidae is often incorrectly generalised as amphibious. It is true that many species of Succineidae occur in marshes, swamps, and at lake margins; and on moist soil or on emergent vegetation in freshwater systems. However, these species are strictly air-breathers and do not actively enter the water for feeding or reproduction, although several species are known to passively disperse attached to the surface film of water.
Only species associated with continuously wet rock surfaces near waterfalls, such as Lithotis rupicola Blanford of Western Ghats, India, and Succinea bernardii Récluz of Tahiti, can be truly be regarded as amphibious.
The family is also well represented on the ground and in trees both in moist environments, such as rainforests and marshes, and in dry environments, such as sand dunes, savannah woodlands, and seasonally dry stream banks.
Succineid snails are able to persist through seasonally dry periods in an aestivating state, with the animal retracted into the shell with its aperture sealed and attached to the substrate by a sheet of dried mucus, the epiphragm. In the semi-slug and slug forms, this type of aestivation is generally not possible and these taxa are for the most part confined to permanently moist sites. Nonetheless, there are some succineid slugs that are well adapted to seasonally dry environments and do indeed aestivate in soil when conditions are not favourable for activity.
No comprehensive monograph of the Succineidae is available, although the foundations for such have been established by numerous anatomical studies published over the past 75 years. Generally two subfamilies and 12 genera are recognised.
Athoracophoridae are confined to the south-west Pacific region, with the subfamily Aneitinae occurring from New Guinea through eastern Australia to New Caledonia and Vanuatu, and the Athoracophorinae in New Zealand and the subantarctic. Athoracophoridae are comprised entirely of slug forms. Their dorsal integument has grooves, with conical tubercles in the intervening panels – both most strongly developed in Athoracophorinae.
Various subgroups of Athoracophoridae have been the subjects of systematic and anatomical treatments, but these do not adequately document the evolutionary diversity within the family. There are extensive radiations, particularly in New Caledonia and New Zealand, which have previously been overlooked. The family has no fossil record; Athoracophoridae probably arose as a polyploid mutation of a succineid-like ancestor.
Suggested reading:
Barker GM 2001. Gastropods on land: phylogeny, diversity and adaptive morphology. In: Barker GM ed. Biology of terrestrial molluscs. Wallingford, UK, CABI International. Pp .1–146.
Patterson CM 1971. Taxonomic studies of the land snail family Succineidae. Malacological Review 4: 131–202.
Rundell RJ, Holland BS, Cowie RH 2004. Molecular phylogeny and biogeography of the endemic Hawaiian Succineidae (Gastropoda: Pulmonata). Molecular Phylogenetics and Evolution 31: 246–255.
Wade CM, Mordan PB, Naggs F 2006. Evolutionary relationships among the pulmonate land snails and slugs (Pulmonata, Stylommatophora). Biological Journal of the Linnean Society 87: 593–610.