| | The families of non-marine molluscs of Britain and Ireland (slugs, snails and mussels) |
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.
Representation in Britain and Ireland. Aplexa (1), Physa (1 native, plus 2 or 3 introduced: “Bladder snails”).
Illustrations. • Aplexa hypnorum, Physa acuta, P. heterostropha, P. fontinalis (Ellis). PHYSIDAE. 1–3, Physa sp., referred by Ellis to P. heterostropha Say. 4, Physa sp., referred by Ellis to P. acuta Draparnaud. 5, Physa fontinalis (L.), "Common Bladder Snail", 8–12 mm. 6, Aplexa hypnorum (L.), "Moss Bladder Snail, 9–13 mm. From Ellis (1926),. • Aplexa hypnorum and (live) Physa fontinalis (Reeve). PHYSIDAE. 1, Physa fontinalis (L.), "Common Bladder Snail". 2, Aplexa hypnorum (L.), "Moss Bladder Snail". From Reeve (1863), with approximate scales added. • Aplexa hypnorum and Physa fontinalis, with Planorbidae (Adams). PLANORBIDAE. 1, Segmentina nitida (Müller), "Shiny Ram's-horn". 2, Hippeutis complanatus (Linn.), "Flat Ram's-horn". 3, Gyraulus crista (Linn.), "Nautilus Ram's-horn". 4, Gyraulus albus (Müller), "White Ram's-horn". 5, cf. Gyraulus laevis (Alder), "Smooth Ram's-horn" (referred by Adams to Planorbis parvus Say.). 6, Anisus leucostoma (Millet), "Button Ram's-horn, White-lipped Ram's-horn". 7, Anisus vortex (Linn.), "Whirlpool Ram's-horn". 8, Planorbis carinatus Müller, "Keeled Ram's-horn". 9, Planorbis planorbis (Linn.), "Margined Ram's-horn". 10, Planorbarius corneus (Linn.), "Giant Ram's-horn". 11, Bathyomphalus contortus (Linn.), "Twisted Ram's-horn". 12, Menetus dilatatus (Gould), "Trumpet Ram's-horn". PHYSIDAE. 13, Aplexa hypnorum (Linn.), "Moss Bladder Snail"; 14, Physa fontinalis (Linn.), "Common Bladder Snail". From Adams (1896). • Physa acuta (L.W.). Physa acuta. Tropical aquarium, Leek, Staffs., May 1948.
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, 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. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2005 onwards. The families of non-marine molluscs of Britain and Ireland (slugs, snails and mussels). Version: 5th August 2019. delta-intkey.com’.
