Planorbidae Family - ID Tips
Class: Gastropoda; Order: Pulmonata; Family: Planorbidae
|
Vital Stats |
Rating |
Scale |
|
BMWP Score |
3/10 |
1 = Filthy to 10 = pristine |
|
LIFE Flow Group |
4/6 |
1= Rapid velocity (>100cm/sec) to 6= drought resistant |
|
Family Level Difficulty Rating |
2/10 |
1= Easy to 10= very difficult |
|
Species Level Difficulty Rating |
6/10 |
1= Easy to 10= very difficult |
The Planorbidae are a family of aquatic snails. They are frequently found in most aquatic habitats from ponds to rivers, and although they are unlikely to be found in the fastest flowing streams, each species has its own preferred habitat ranges.
Several of the species in this family are classified as rare or endangered:
Red Data Book 1 (Endangered) – Segmentina nitida
Red Data Book 2 (Vulnerable) – Gyraulus acronicus and Anisus vorticulus.
Notable but not RDB status – Menetus dilatatus.
The rest of the species range from Very Common to Regionally Notable.
Identification
Identification to family level for this group is easy. Planorbids are spiral-shaped snails, flattened in one plane. They have no operculum (calcified plate which covers the snail’s body, allowing it to shut the entrance to its shell). The only other family which Planorbidae might be confused with are Valvatidae, which are sometimes flat, but which always have an operculum – although in Valvatidae, this can sometimes be drawn inside the shell and be difficult to see.
The most widely used key for identifying Planorbidae is published by the FBA ‘A Key to the British Fresh and Brackish Water Gastropods’, T.T. Macan (originally 1977, reprinted 1994). There have been numerous unofficial amendments and updates circulated since this book was published, which are very helpful in providing extra features for identification. An extremely helpful online key can be found on the website of the Conchological Society of Great Britain & Ireland: http://www.conchsoc.org/aids_to_id/fwidbase.php, although this key does rely on the specimens being ‘adult’ in size, which can be difficult to determine if you’re a beginner and aren’t sure what species you are looking at. For smaller ‘subadult’ specimens this key will not be as helpful.
There are 8 genera in the family Planorbidae, most of which used to be classified in the genus Planorbis, but they have been reallocated to separate genera. Therefore care should be exercised in referring to the currently accepted genus and species names, to avoid confusion.
Anisus spp.
The genus Anisus has 4 species. Anisus species each have several tightly coiled whorls, which are quite narrow in comparison to their body size. The main differences in the four species are seen ‘end-on’ so that the shape of the aperture (shell opening) is visible.
Anisus vortex is the most commonly encountered species, and is found in moderately flowing rivers and streams. Its shell is slightly flattened along the lower edge, so that the aperture shape is asymmetrically pointed. The pictures in Macan are very helpful in determining this species.
Figure 1. Anisus vortex, side view
Anisus leucostomais less commonly observed, due to its being slightly more rare (designated ‘Local’), and also due to its preference for marshes and waterbodies which frequently dry up. This means that taxomists who routinely analyse samples from rivers and streams rarely encounter it, but as a result there is the possibility that it is under-recorded due to unfamiliarity. This species can be distinguished from A. vortex by the shape of the aperture, which is quite rounded, almost circular.
Figure 2 – Anisus leucostoma
Figure 3 – Anisus leucostoma, side view
Anisus spirorbiswas until recently regarded as a variant of A. leucostoma, but recently has been acknowledged as a separate species. The distinction is that A. spirorbis has wider whorls, whereas A. leucostoma has narrower ones. In practice, it is extremely difficult to tell these two species apart without access to a variety of reference specimens- as the existence of two distinct species is still under debate.
Anisus vorticulus is very rarely encountered, and confined to marsh drains in Norfolk and Suffolk. It might be confused with A. vortex, as the aperture is pointed; however, the point is more symmetrical than that of A. vortex.
Gyraulus spp.
There are four species in the genus Gyraulus. The whorls on these are more broad than in Anisus, with fewer whorls in total.
Gyraulus crista is the most distinctive of the genus, as its shell is usually traversed by spiny ridges.
Figure 4 - Gyraulus crista
However, caution should be applied as some specimens of this species have ridges which are barely noticeable, and so these may be misidentified. This fact is not mentioned in the key by Macan nor the Conchological Society of Great Britain & Ireland’s website.
Figure 5 - Gyraulus crista (ridges unclear)
The species which these ridgeless specimens may be confused with are Segmentina nitida and Hippeutis companatus. The way to tell Gyraulus crista from these other species is by looking at the shape of the aperture- G. crista’s aperture is oval shaped and does not overlap the body segments, whereas these other two species have a ‘squashed crescent’ shaped aperture, where the last whorl is wrapped around the previous.
Gyraulus albus is the most common species in this genus. It is found in most freshwater habitats. This species tends to have a whitish colouration. It is easy to identify by looking at the texture of the shell’s surface, which should have spiral lines wound around the spiral of the shell.
Figure 6 – Gyraulus albus
These spiral lines may be less obvious on some specimens, but a helpful way to make them more visible is to dry the shell completely and then check again- on some specimens, spiral striae have become visible on drying, which were completely invisible whilst the snail was in water.
Gyraulus laevis is fairly uncommon, and found in ditches and ponds. It is quite similar to G. albus, but lacks the spiral striae. The colouration is more of a pale horn colour compared to G. albus’s whitish shade, although this is sometimes difficult to judge.
Figure 7: G. laevis
Gyraulus acronicus is more rare, and is classified as Vulnerable. This species can be distinguished from G. albus and G. laevis as it has a slight keel around the edge of the shell, which is particularly obvious on the aperture. The other two species have circular apertures, whereas G. acronicus is slightly pointed.
Bathyomphalus contortus is a relatively common species which is found in a wide variety of habitats. It is identified by the unusual shape to its whorls, which wrap around each other tightly, and as a result of this the upper surface appears to have more whorls than the lower surface, and the whorls appear taller than broad if the aperture is viewed from the front.
Figure 8 - Bathyomphalus contortus
Figure 9 - Bathyomphalus contortus, upper side
Figure 10- Bathyomphalus contortus, lower side
When identifying specimens using a key, this species might be confused with immature specimens of some other species, which also have an aperture taller than broad. Most notably, Planorbarius corneus has an unusual shape when immature, but this can be distinguished from B. contortus as the whorls are much larger and fewer. Additionally, very immature specimens of Planorbis carinatus and Planorbis planorbis may appear to have an aperture which is taller than broad, but these do not have whorls which wrap around each other the same way as B. contortus.
Planorbarius corneus is the largest species of planorbid, and cannot be mistaken when it is mature. The size can be up to 36mm. It is very common in ponds.
Figure 11 – Planorbarius corneus
When it is small, the shape of the shell is very unusual, with the last whorl, and sometimes the whole shell, being taller than broad.
Figure 12 – Planorbarius corneus (juvenile)
Planorbis carinatus and Planorbis planorbis
These two species are very similar. They are the largest planorbids except for Planorbarius corneus, and usually have a slight keel round the edge. They are both very common, with P. carinatus more frequently found in flowing water and P. planorbis more commonly in slow flowing or stationary water. They can be distinguished from each other by the position of the keel, which is fairly central on the shell in P. carinatus, but more ventrally placed on P. planorbis. Additionally, the width of the last whorl when compared to the overall width of the shell, is proportionally broader in P. carinatus, when compared to P. planorbis.
Figures 13 and 14 – Planorbis planorbis
Figures 15 and 16 – Planorbis carinatus
Hippeutis complanatus and Segmentina nitida
These two species are very similar, but S. nitida is classified as Endangered and is very seldom observed, even during surveys deliberately dedicated to locating it; whereas H. nitida is relatively common. These two species can be distinguished from the other planorbids by the shape of the whorls, which overlap the rest of the shell and therefore create an aperture of a very distinctive pointed shape. The shell is lens-shaped, flattening towards the edges. The main difference between these two species is that S. nitida has internal septa, partially dividing the shell into chambers. These are visible as lines cutting across the whorls at regularly spaced intervals. Taxonomists should beware that H. complanatus may sometimes appear to have such lines if the shell has been damaged during growth and repaired itself, which happens fairly frequently as the shell is very thin and fragile in this species. If in doubt, it is advisable to seek advice from an expert, as the incorrect recording ofS. nitida could have serious implications given its endangered status.
Figure 17 – Hippeutis complanatus
Figure 18 – Hippeutis complanatus
References:
FBA publication ‘A Key to the British Fresh and Brackish Water Gastropods’, T.T. Macan (originally 1977, reprinted 1994)
Website of the Conchological Society of Great Britain & Ireland: http://www.conchsoc.org/aids_to_id/fwidbase.php
