The families of British non-marine molluscs (slugs, snails and mussels)


L. Watson and M. J. Dallwitz


Morphology. Snails, with a conspicuous, spiral, univalve shell.

The animal with one pair of tentacles only. Eyes at the bases of the tentacles.

The shell inoperculate; rising-spiral; 4–7 whorled; typically sinistral; higher than wide; 8–12 mm high (P. fontinalis), or 10–18 mm high (introduced species); height about 1.5–1.7 x the width (Physa), or 2.7 x the width (Aplexa); with the body whorl predominating and the spire small and short (in Physa), or high-spired and tapered gradually from the body whorl (in Aplexa). The height of the spire about 0.1–0.2 x that of the shell (Physa), or 2.7 x that of the shell (Aplexa). The shell fusiform (in Aplexa), or inverted-pyriform (the Physa spp. being much less elongate). The whorls neither shouldered nor keeled. The aperture with neither teeth nor calluses. The shell thick-lipped to thin-lipped; thin and translucent (glossy); yellowish- or reddish-horn coloured or even red in Aplexa, pale horn-coloured to dark brown or blackish in Physa; plain. Morphological comments. The animals grey to almost black.

General biology, ecology. Freshwater aquatic. Breathing air directly via the lung-like mantle cavity (and via the skin in general). The native Physa fontinalis occurs on aquatic plants in still or running, hard or soft, unpolluted water, while the introduced Physa species are more tolerant of de-oxygenated conditions and pollution. Aplexa hypnorum, which can survive periodic dessiccation, is found on vegetation in swampy pools, roadside ditches and small, grassy ponds.

Hermaphrodite (but individuals acting as either male or female when mating).

Classification. Gastropoda; Pulmonata.

British representation. Aplexa (1), Physa (1 native, plus 2 or 3 introduced: “Bladder snails”).

Illustrations. • Aplexa hypnorum, Physa acuta, P. heterostropha, P. fontinalis (Ellis). • Aplexa hypnorum and (live) Physa fontinalis (Reeve). • Aplexa hypnorum and Physa fontinalis, with Planorbidae (Adams). • Physa acuta (L.W.).

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2005 onwards. The families of British non-marine molluscs (slugs, snails and mussels). Version: 4th January 2012.’.