IDENTIFYING FRESHWATER MOLLUSCS
Brian Eversham

Version 2.2

April 2013

The main types of molluscs in fresh water
Three main types of freshwater molluscs can be easily distinguished: snails, limpets
and bivalves. The two following keys should enable you to identify nearly all snails andlimpets to species level. For a few species, you may need also to consult published
keys.

A

Shell in one part, coiled

Snails: Key A

B

Shell in one part, conical and not coiled, always less than 10mm, and usuallyless than 6mm long
Limpets: Key B

C

Shell in 2 parts, hinged at the lower edge, usually held tightly together whenhandledBivalves (Cockles and mussels)
See note at end

A warning: land snail or pond snail?
Land snails often fall or are washed into the water, and are frequently
found in rivers, especially in flood debris. If the snail is still alive, look at
the tentacles: all but two uncommon species of British land snails have
two pairs of tentacles, with eyes at the tip of the longer, upper pair;pondsnails have only one pair of tentacles, and their eyes are either atthe base of the tentacle, or on a small swelling a short distance from thebase of the tentacle. These features are obviously not available on empty
shells, and these keys make no allowance for land snails; but if your snail
looks nothing like the species here, it may well be a drowned land snail.

Land snail (Discus)eye

Pond snail (Lymnaea)eye

KEY A: quick key to the commoner species of pond snails
1

2

Shell flat-coiled, like a Catherine-wheel or a slice of Swiss roll2

Shell with a turret or spire, either tall or globular

13

Shell tiny, with a chalky or horny lid (poke mouth of shell gently with a bristle
or grass stalk - feels hard).
Flat valve-snail Valvata cristata
Often larger, no lid (feels squishy when gently prodded)
Ram’s-horn snails (family Planorbidae)

3

Note: empty shells often lose their lids. A few are keyed out when empty,
but others will be difficult to key, but easy to recognise when familiar withthem. Shells which have lids usually have a straighter-edged and betterfinished mouth, often with a ridge against which the lid fits snugly.

Lid in mouthof shell(Bithynia
tentaculata)

Ram’s-hornsnails3

Large: adults over 20mm diameter and 10mm thick, juveniles at least 4mm
thick, with a flared mouth and spiral ridges, sometimes with a row of hairs orbristles on each ridge.
Great Ram’s-horn Planorbarius corneus
Smaller; rarely over 17mm diameter, and never more than 4mm thick evenwhen adult. If with strong spiral ridges, no more than 7m x 1.8mm. Neverwith rows of hairs or bristles.
4

4

Keel

Adults over 10mm diameter; a keel or ridge, in the form of a raised line, runs
round edge of shell; rather large, deep species (adults up to 17mm diameter,and 2.5-3.5mm thick)Planorbis species
5
Smaller when adult; edge of shell either smoothly and evenly rounded, or
angled. The most angled edge can produce a keel, but never in the form of araised line; shell smaller, often thinner and more delicate
65Planorbis carinatus

Anisus vortex

RoundedAngledKeel offset to one side of edge, so visible only from one side as the curve of
the shell wall hides it from the other side; shell rather more tightly coiled. Up
to 17mm x 3.5mm.
Common Ram’s-horn Planorbis planorbis
Keel almost in middle of rim, so visible from above and below; shell spirals
widen rather more rapidly. Up to 17mm x 3.5mm.
Keeled Ram’s-horn Planorbis carinatus
Planorbis planorbis (upper)P. carinatus (lower)

6

Shell very tightly coiled, at least 6 whorls in an adult diameter of 10mm or
less, mouth no barely wider than previous whorl.
7
Shell less tightly coiled and more flared toward mouth

9
Anisus vortex (tightly coiled, left),
Gyraulus albus, less tightly coiled, right.

7

Shell small and thick, very tightly coiled, with a deep, conspicuous umbilicus(pit) at one side, inside which the inner spirals are visible. Often blackish andopaque. Mouth of shell taller than wide, curved as it wraps around theprevious whorl. Edge of shell evenly rounded with no hint of a keel. Adults
6mm x 2mm, with 7-8 whorls.
Twisted Ram’s-horn Bathyomphalus contortus

Bathyomphalus contortus

Mouth

Shell thinner and flatter, often rather translucent brown. Slightly concave on
one side, but no obvious umbilicus. Mouth round or obliquely oval. Oftenwith a strong angled keel round the edge. If shell has 5 or more whorls, it isless than 1.5mm thick.
8
8

Three choices
Shell very thin and flat, often translucent brown. Edge of shell with a strong
angled keel round the edge; mouth therefore wider than tall and drawn out
into a blunt point. Tightly coiled. Adults 10mm x 1.5mm, with 6-7 whorls.
Whirlpool Ram’s-horn Anisus vortex
Shell fairly thin and flat, often dark and less translucent. Edge of shellsmoothly rounded, mouth almost circular, often with a white band inside.
Tightly coiled. Adults 8mm x 1.5mm, with 6-7 whorls.
White-lipped Ram’s-horn Anisus leucostoma

Anisus vortex (left), A. leucostoma (right)Shell fairly thin and flat, often dark and less translucent. Edge of shellsmoothly rounded, mouth almost circular, often with a white band inside.
Less tightly coiled. Adults 5.5mm x 1.5mm, with 4-5 whorls.
Anisus spirorbis
Note: there is a debate about whether A. spirorbis is a good species, separate
from A. leucostoma, and whether it occurs in Britain. Although regarded as
distinct species in mainland Europe, there is a suggestion that populations
change shape between years, depending perhaps on temperature and growthrate. More research is needed.
9

Anisus leucostoma (left), A. spirorbis (right)

Mouth offset from last whorl, so it lies below the rest of the coil. Very small,shell thin and fragile. 0.5-2mm x 2-4mm.
10
Mouth of shell almost in line with previous whorl, lying in the same plane as
the rest of the shell. Often larger and more solidly built.
11

10

Shell often (not always!) with strong ridges running across the whorls. Mouth
rather narrowly elliptical, its long axis along the flat of the shell. Surface ofshell dull. An extremely delicate, fragile shell. Up to 1mm x 3m, with 3-4
whorls.
Nautilus Ram’s-horn Gyraulus crista
Shell smooth, never with strong ridges. Mouth almost perfectly circular, inlife with a round lid (operculum). Surface of shell glossy. Up to 1.25 x 4mm
with 5 whorls.
Flat Valve Snail Valvata cristata

11

Gyraulus crista

Looks like a typical ram’s-horn snail: shell flat-sided with rounded edges.
Each whorl overlapping previous one only slightly, so most of each whorl canbe seen from either side, and the two sides look similar. Adults up to 7mm x
2mm with up to 4.5 whorls.
12
Shell convex on both sides, lens-shaped. Each whorl broadly overlapping theprevious one, so very little of the inner whorls is visible from one side, and thetwo sides are different. Last whorl and mouth very large, making up almost
half the diameter of shell. Shell yellowish or grey-white, glossy. Adults only
1.2mm x 4m, with up to 4 whorls. Flat Ram’s-horn Hippeutis complanata(If the shell is the shape of Hippeutis, but translucent yellow-brown, up to
7mm diameter, and 3-4 spoke-like ridges are visible inside the shell, it may be
the very rare Shining Ram’s-horn, Segmentina nitida, which is thought to be
extinct in BCN.)

12Shell with strong spiral grooves running round it (check with hand lens or
microscope!) as well as the usual radial growth ridges; lip of mouth of shellmeets previous whorl at an obtuse wide angle; a strong, thick rather chalky
shell. Up to 7mm. x 1.8mm with up to 8 whorls.
White Ram’s-horn Gyraulus albus
Shell with no spiral grooves; mouth of shell meets previous whorl almost at aright-angle; shell a rather translucent, glossy amber colour; up to 6mm x2mm,
with up to 6 whorls.
Smooth Ram’s-horn Gyraulus laevis

13

Gyraulus albus

Shell right-handed (dextral): if spire points upward and mouth faces you,mouth is on right.
14
Shell left-handed

25
Lymnaea balthica (on left: right-handed shell)
Physella acuta (on right: left-handed shell)

14Shell with a horny or chalky lid (operculum) - poke mouth of shell gently witha bristle or grass stalk - feels hard).
15
No lid - feels squishy when prodded.

15

Viviparus
contectuswith lid(operculum)and brown
spiral lines

21

Shell large when adult (up to 40mm tall), greenish with brown spiral lines
16
Shell never more than 15mm high, pale brown to blackish, not greenish andnever with coloured spiral lines
17

16

Shell rather thin, top of spire quite sharply pointed. With more swollen,bulging whorls, deeper sutures, and usually glossy. Adults up to 35mm x
30mm.
Lister’s River Snail Viviparus contectus
Shell wall thicker, with blunt point, whorls less swollen, sutures shallower,shell rarely glossy. Adults up to 40mm x 30mm.
Common River Snail Viviparus viviparus

Viviparus contectus (left) andV. viviparus (right)

17

Shell flat and very thick and heavy, last whorl making up over half the shell.
Marbled whitish or yellow on a dark green, brown or purplish background.
Mouth and lid a narrow oval or semicircle, lid often yellow or orange. Adults
up to 6mm x 11mm with 2.5 whorls. Confined to fast flowing water.
Freshwater Nerite, Theodoxus fluviatilis
Shell taller, thinner and with more even and more numerous whorls. Nevermarbled pale and dark (though marbling on the body of the snail may show
through the shell). Lid broader and rarely yellow or orange. May be larger,
and always with 3 or more whorls when adult.
18

18

Three choices
Shell narrow, sharply pointed, and shell more than twice as tall as it is wide.
Mouth about a third the total height or less. Mouth and lid oval or pointedoval. Adults small, up to 5.5mm tall and 3mm wide.
Jenkins’s Spire Snail Potamopyrgus antipodarum(Several saltmarsh Hydrobia snails would key out here, too.)
Shell broader, less pointed, roughly twice as tall as wide. Mouth about half
the total height. Mouth and lid oval or pointed-oval. Adults often larger (upto 15mm x 9mm)
Bithynia species 19
Shell small and squat, about as tall as wide, or wider than tall Can be ram’shorn shaped). Mouth and lid very rounded, almost circular. Adults up to 6mm
x 6mm.
Valvata species 20

19

Theodoxus fluviatilis

P. antipodarum(above), B.
tentaculata(above right) andV. piscinalis(right): not toscale

Mouth rounded, with no obvious corner at the top; whorls swollen and suture
deep; apex rounded; small (up to 6.5mm x 4.5mm) with up to 5 whorls.
Leach’s Bithynia Bithynia leachii
Mouth with an angle or corner at top; whorls flatter, sutures shallower; apex
more sharply pointed; often larger, up to 15mm x 9mm with up to 6 whorls.
Common Bithynia Bithynia tentaculata
Bithynia leachii (left) andB. tentaculata (right)

(The very rare riverine Taylor’s Spire Snail, Marstoniopsis subrica, might key
out here. It is tiny (3mm x 2mm), slenderer than Bithynia leachii and plumper
than Potamopyrgus antipodarum,, but with a much blunter spire than either.)20

Three choices
Shell about as tall as it is wide, up to 5mm x 5mm. Spire about as tall as thediameter of the mouth. Umbilicus (the channel which runs up inside the spiralfrom next to the mouth) narrow but deep. (See picture at couplet 18).
Common Valve Snail Valvata piscinalis
Shell smaller, up to 4mm x 2mm, wider than tall, with a flatter spire andproportionately larger mouth. Umbilicus deep and wide. Rare, and usually inrich fen habitats. Can be very similar to young V. piscinalis.
Large-mouthed Valve Snail Valvata macrostoma
Shell small, up to 4mm x 1.25mm, very flat, ram’s-horn-like, with a very low
spire, and mouth offset below the plane of the coils. Umbilicus very wide.
Flat Valve Snail Valvata cristata

21

Mouth very large and flared, at least two-thirds the height of shell; spire small
or very small
22
Mouth rarely more than half shell height, with a prominent conical spire
above it
23

22

Shell thin, usually translucent; mouth large but not enormously flared, lipmeeting shell at less than a right-angle; spire rather blunt to the touch. Adults
up to 20mm x 13mm with up to 5 whorls. Wandering Pondsnail Lymnaea
balthica

Lymnaea auricularia (left) andL. balthica (right)
Succinea putris, an Amber Snail

Shell usually thicker and more opaque; mouth huge, lip joining shell at rightangle or more; spire small but sharply pointed. Adults up to 35mm x 30mm,with up to 5 whorls.
Ear Pondsnail Lymnaea auricularia
Note: these two species are very variable in shell shape, especially the heightof the spire.
(If the shell is a warm amber colour, with a very regular rounded mouth,
check that it is not an Amber Snail (Succineidae), common wetland land snails,
which are sometimes found in the water. As with all land snails, they have
two pairs of tentacles, with eyes at the tips of the upper pair.)
Succinea putris, an Amber Snail23

Shell often very large (up to 50mm tall); spire long and slender, body whorl
broader, so it juts out, giving a concave shape to shell outline. Up to 50mm x25mm, with up to 7.5 whorls.
Great Pondsnail Lymnaea stagnalis

Lymnaeastagnalis

Smaller (up to 24mm), with spire more straight-sided, and body whorl andmouth less broadened
24

24

Umbilicus open; whorls swollen, sutures deep; shell surface smooth; Adults
up to 12mm x 6mm, with up to 6 whorls.
Dwarf Pondsnail Lymnaea truncatula
Umbilicus closed by lip of mouth; whorls flatter; shell often with radial andspiral ridges, giving a hammered texture; often larger, up to 24mm x 11mm
and with up to 7 whorls.
Marsh Snail Lymnaea fusca
Lymnaea truncatula (left) and
L. fusca/palustris (right), not to scale

Note: the very closely related L. palustris is separable only by dissection of the
reproductive organs. Although L. fusca is much the commoner than L.
palustris over most of Britain, both have been recorded from eastern England.

25

Shell, tall, slender and elegant. Body whorl not especially large, and mouth
about half total shell height. Body of snail black or blue-black, and does notoverlap outside of shell. Shell very glossy. Up to 13mm x 5mm, with up to 7
whorls. Often in ponds and ditches which dry out regularly.
Moss Bladder Snail Aplexa hypnorum
Shell shorter and more rounded. Body whorl swollen, and mouth well overhalf shell height. Body of snail pale brown or greenish, and surrounding shellwith finger-like growths when alive. Shell dull. Up to 18mm x 11mm, with up
to 4 whorls.
26

26

Shell thin, rather delicate and translucent. Apex of shell rounded. Mouth notflared, and not jutting out, usully joining body whorl at a very obtuse angle.
Never more than 12mm x 8mm, and usually less than 10m, with 4 whorls.
Common Bladder Snail Physa fontinalis

Aplexa hypnorum(above), Physa
fontinalis (above
right) andPhysella acuta(right): not toscale

Shell often thicker, stronger and more opaque. Apex pointed. Mouth more
flared, lip of mouth juts out, and usually joins body whorl at closer to a rightangle. Often larger, up to 18mm x 11mm and up to 6 whorls.
American Bladder Snail Physella acuta(It is possible that other introduced Physella species occur in Britain.)

KEY B: Identifying the three species of freshwater limpets
Recognising left, right, front and back: all three limpet species have the apex of theshell pointing backwards, and the two flatter species have the shell wider at the front
and tapering toward the rear.
1

Taller (about twice as long as tall) and more conical. Seen from above,
broadly oval to circular, length less than twice the breadth, symmetrical.
Confined to fast-flowing water; often larger (up to 9mm long, 5mm tall)River Limpet Ancylus fluviatilis

Ancylus fluviatilis, from above(left) and from side (right)

Flatter (usually 3-4 times as long as tall). From above, a long oval or rather
rectangular, and wider at the front than the rear, so not symmetrical. 22Apex of the shell leans to the left. From above, the shell is rather straightsided in the rear 2/3. In side view, the rear slope below the apex is almost
straight and the apex just out like a little shelf. Up to 7mm long.
Lake Limpet Acroloxus lacustris
Apex leans slightly to right. From above, the side are convex. In side view, therear slope below the apex is rather concave, as the blunt apex curves
backwards. Up to 6mm long.
Wautier’s Limpet Ferrissia wautieri

Acroloxus lacustris (above)

Ferrissia wautieri
From above (left) and from side(right)A note on freshwater bivalve molluscs
The keys above do not include the bivalves, cockles and mussels. There are sevenspecies of larger bivalves, the mussels, Anodonta, Dreissena, Margaritifera,
Pseudanodonta and Unio. These have elongate shells, from 25-200mm long when
adult. They are fairly easy to learn to identify. Initially, empty shells are easier to namethan live animals, as some of the best features are on the inside of the shell.
There are 27 or so species of smaller bivalves, the cockles, Sphaerium, Musculium,
Corbicula and Pisidium. These have rounder shells, and are much smaller. Apart froman introduced species reaching 30mm long, the others are from 1.5-22mm long when
adult and are very difficult to identify: if you want to try, it’s best to collect a range ofsamples and compare lots of shells together, rather than trying to key out one or two
individuals.
There is a well illustrated guide to bivalves (Killeen, Aldridge and Oliver, 2004) whichmakes identifying mussels fairly straightforward. Nothing except years of practice will
make identifying Pisidium easy.

Literature on freshwater molluscs
Boycott, A.E. 1934. The habitats of land Mollusca in Britain. Journal of Ecology, 22, 138.
Boycott, A.E. 1936. The habitats of freshwater Mollusca in Britain. Journal of Animal
Ecology, 5, 116-186.
Two thorough accounts which have never been superseded; still useful.
Ellis, A.E. 1978. British freshwater bivalve Mollusca. Linnean Society Synopses of the
British Fauna, (New
Series) 11. London: Academic Press. I (E)Good keys, excellent descriptions, b&w photos of all species. Although superseded by
Killeen et al. (2004), still useful.
Eversham, B.C. & Jolliffe, A.S. 1992. Freshwater molluscs: their habitats anddistribution. Report of the
Huntingdonshire Fauna & Flora Society, 1991, 44th, 30-42. E
Gloer, P. & Meier-Brook, C. 1994. Süsswassermollusken: ein Bestimmungsschlüssel für
die Bundesrepublik Deutschland. Hamburg: Deutscher Jugendbund fürNaturbeobachtung.
Excellent illustrated keys to freshwater molluscs; text in German.
Janus, H. 1968, reprinted 1979 and later. The young specialist looks at Molluscs.
London: Burke.
Despite title, a thorough and well illustrated guide to land and freshwater snails andbivalves, with effective
keys, descriptions and line drawings. Account of slugs out of date.
Kerney, M.P. 1999. Atlas of the Land & Freshwater Molluscs of Britain and Ireland.
Colchester: Harley Books
Kerney, M.P. & Stubbs, A.E. 1980. The conservation of snails, slugs and freshwater
mussels. 23pp. London:
NCC.
A short account of the need for, and methods of, mollusc conservation.
Killeen, I., Aldridge, D. & Oliver, G. 2004. Freshwater bivalves of Britain and Ireland.
(Field Studies
Council Occasional Publication No. 82)
Extremely well illustrated and clearly presented, making the difficult Pisidium species
at least more

approachable.
Macan, T.T. 1977. A key to the fresh- and brackish-water Gastropods. (FBA Sci. Publ.
13, 4th edition). E,I
Straightforward and well illustrated, and generally reliable, though a few species areomitted and someerrors remain.
Willing, M.J. 1997. Fresh- and brackish-water molluscs: some current conservation
issues. British Wildlife, 8,151-159.

Useful websites
Using a search engine with the genus or species names of freshwater snails will usually
produce a large number of illustrations to compare with a specimen you are trying to
identify. Most such photographs and drawings are correctly named: with pondsnails, I
would estimate about 90-95% of photographs are accurately labelled (with somegroups, like slugs, this falls to 50-60%).
There are a few sites especially relevant to this course. The illustrations used in thiskey, and many more pondsnail photos, will be available online at
http://www.flickr.com/photos/cladoniophile/sets/72157633163393637/
There is an interactive key to British species, using the excellent black-and-whitephotographs in Gloer & Meier-Brook (1994), at:
http://www.conchsoc.org/aids_to_id/fwidbase.php
It operates by clicking on the photograph most like your specimen at each couplet.
Very good photographs and helpful descriptions of the species which occur in Ireland
are available at:
http://www.habitas.org.uk/molluscireland/splist.asp
The most recent checklist of the British non-marine molluscs, with explanations of all
the recent additions to the fauna, and the technical reasons for name changes, may be
downloaded from:http://www.conchsoc.org/resources/Anderson.pdf
A spreadsheet of the names of British molluscs (and indeed, the rest of our fauna) is
available at:
http://www.nhm.ac.uk/research-curation/scientific-resources/biodiversity/ukbiodiversity/uk-species/checklists/NHMSYS0001700910/index.html
If you do not have access to the Atlas of British molluscs (Kerney, 1999), you canretrieve up-to-date maps of any species from the National Biodiversity Network at:
http://data.nbn.org.uk/
Type in the name of any species into the box at the top right, wait till it lists a range oftypes of maps, and click on Grid Map if you want simply a set of dots on an outline
map, or Interactive Map (slower to load) if you want to see records overlaid on azoomable map of Britain. Beware, the NBN Gateway includes some records whichthhave not been thoroughly verified, and sometimes 19 century records appear as‘modern’ dots.