INTRODUCTION TO CRENOBIONT & STYGOBIONT
SHELLED MOLLUSCA OF CONTINENTAL FRANCE

 
– Updated march 2024 –
37 identified species, 30 possible (« (?) ») or under study, and 4 extras

Image by Félix Potuit: Source du Planey (Haute-Saône, France) – Public domain.

INDEX
Presentation
Plates

PRESENTATION

Overview

These pages are intended to present (as many as possible) species of shelled molluscs that live in the sources and in the cave rivers of the french mainland.

Almost all the genera that live occasionaly (stygophile) or exclusively (stygobiont) in underground rivers, or in their springs (crenobiont, thus stygoxen), belong to the Littorinimorpha. In France, less than a hundred of stygobiont species have been described; some of them can be very variable, some are endemic to a single place, some can be found in more than one drainage basin. Each year, new species are discovered.

The systematics of these groups present some difficulties. Depending on the authors, most of the gastropods that are found in the springs or in the cave-rivers belong to one family (Hydrobiidæ), two families (Moitessieriidæ, Hydrobiidæ), or three families (Moitessieriidæ, Hydrobiidæ and Amnicolidæ). Regarding the Ancylus fluviatilis described by Müller, which can be found in the twilight zone of some emergences, it is placed sometimes in the family Ancylidæ, sometimes in Planorbidæ. For the species in the genera Bythinella or Islamia, they are almost impossible to identify by their shells alone, except for a few cases.

Finally, there are innumerable records of Sphaeriidae (Bivalvia) in the springs as well as in the most remote troughs of mountain pasture; some of these animals have even been found alive in cave-rivers of high altitude (for example at Saint-Vincent river, in La Verna Cavern, western Pyrenees).

The author collecting some sediment at La Verna.

 
Habitat and feeding

Crenobiont gastropods are used to feeding on detritic film and micro algae from rocks and leaves.
Crenobiont bivalves are normal filter feeders.

Regarding the stygobiont, their habitats are, in France, of 3 types:

  • Underflows of streams and rivers;
  • Phreatic zones linked to a river;
  • Karstic aquifers.

It has been noticed that no stygobiont gastropod can be found in caves at an altitude higher than about 1000m in the Pyrenees, and possibly less in the Alps. It seems that the reason must be sought on the side of the influence of glaciers during the last ice-age. A freshwater mollusc needs some running water to live, it’s almost a tautology; crystallized water is not a option. Hence the total absence of extant species in cave-rivers over a certain altitude, and above a certain latitude, which correspond to the limits beyond which it was impossible to find a shelter, with free running water, against the past global coolings. The Sphaeriidae of Saint-Vincent river is an exception, which can be explained by a method of recent colonization via air: bird gastrointestinal transportation.

Stygian worlds offer very limited opportunities to get food. The latter comes only from the surface, where it falls in pits and crevices or is carried along through the soil by rain. Decaying over long periods of time, it is swept away by the streams and ends up diluted in the waters of the underground rivers, where the mud shrouds it. Enriched by these nutrients, the mud accumulates then in the lower parts of the siphons where, eventually, the molluscs gather to feed on it. Occasionally, a rising flood expels this small people dowstream, up to the springs.

An emergence at low flow. Be careful not to disturb the bats.


 
 

MANY THANKS TO
Alain Bertrand (France) and Hans Bœters (Germany) for their gentle help. This is much appreciated.
 

PLATES

Click on the pics to open the pages.

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GASTROPODA 

Alzoniella Giusti & Bodon, 1984

Crenobiont, often stygobiont. Previously placed in the genus Belgrandiella Wagner, 1928. Shell small, whitish-translucent, conical to cylindrical-conical, with a narrow umbilicus. Three to four body whorls, quite convex, the last one excepted. The apex, rounded, is less obtuse than in the genus Bythinella.
 

       

Ancylus O. F. Müller, 1773

Shell limpet-shaped, suboval; apex posteriorly placed, somewhat curved; cutaneous respiration. Occasionally encountered in the twilight zone of karst springs.
 

           

Belgrandia Wagner, 1928

Randomly scattered lenghtened bumps along the body whorls. These bumps cannot be considered as some thickenings of the shell, but as actual bulges. The whorls are more rounded than in Alzoniella. No spiral sculpture. Occasionally encountered in the twilight zone of karst springs.
 

           

Belgrandiella Bourguignat, 1870

The differences with Belgrandia are mostly anatomical. Occasionally encountered in the twilight zone of karst springs.
 

         

Bythinella Moquin-Tandon, 1856

Shell smooth, ovoid to conical, with an obtuse apex. The shapes can vary considerably within a same population, and the variation-ranges often overlap between species, making this group very difficult to identify without anatomical studies. The genus can be stygoxen, stygophile, and sometimes stygobiont. Distribution: almost everywhere, as soon as there is a spring.
 

     
     
     
     
         

Bythiospeum Bourguignat, 1882

Stygobiont. Conical to cylindrical shell. Sculpture made up of radial growth marks and, sometimes, obsolete spiral threads. Cf. J. R. Bourguignat: Bythiospeum ou Description d’un nouveau genre de mollusques aveugles, Poissy 1882. Distribution: from the Rhin in the north, west to the Côte d’Or, south to Hérault, Var and Durance.
 

     
         

Fissuria Bœters, 1981

Minute shell, valvatiform shape. Stygobiont. Distribution: underflow of Durance river. Possible spots in Provence, Alpes-maritimes, Gard.
 

           

Henrigirardia Bœters & Falkner, 2003

Stygobiont. Close to the genus Moitessieria. Strongly conical shell, covered with tessellations. Sometimes arched, the spiral of the body whorls widens whith age, giving the shell a large umbilicus. The distribution of this genus is restricted to some spots along the Hérault river eastern bank.
 

           

Islamia Radoman, 1973

Stygobiont. Minute shell, flattened spire.
The genus is extremely confusing: an Islamia is always difficult to identify without anatomical studies. Distribution: everywhere in France, as soon as there is some karstic aquifer, Parisian Basin and Ardennes excepted.
 

     
     
       

Mercuria Bœters, 1971

In running waters of many kinds. Some species are also crenobiont. « Shell ovate-conical with pointed apex, rounded whorls, deep suture and narrowly perforate umbilicus; milky coloured, especially at the base. » – Bœters & Falkner: “The genus Mercuria Boeters, 1971 in France (Gastropoda: Caenogastropoda: Hydrobiidae). West-European Hydrobiidae, Part 13”, Zoosystema vol. 39(2), Paris 2017, p.229.
 

          

Moitessieria Bourguignat, 1863

Stygobiont. Cylindrical to conico-cylindrical shell; microsculpture made of spirally arranged tessellations, often quadrangular. Distribution: from the Pyrenees in the south to the Dordogne, Aveyron, Ardèche in the north, east to the Alpes de Haute-Provence.
 

     
     
         

Palacanthilhiopsis Bernasconi, 1988

Stygobiont. Large spike on inner face of operculum, wide umbilicus, radial striae. Distribution: Ardèche and Gard.
 

           

Paladilhia Bourguignat, 1865

Stygobiont. Subsutural pleurotomoid split inserted in an angular furrow that gives the peristome an auriform shape. Distribution: Gard, Ardèche, Hérault.
 

         

Palaospeum Bœters, 1999

Stygobiont. Lengthened shell, conical, large, with an almost smooth surface; outer lip and growth marks slightly opisthocline. Distribution: western Pyrenees.
 

           

Potamopyrgus Stimpson, 1865

« Shell ovate-conic, imperforate; apex acute; […] outer whorl nearly two-thirds the length of the shell; aperture ovate, outer lip acute. Operculum corneous, subspiral. » – W. Stimpson: “Diagnoses of newly discovered genera of gasteropods, belonging to the sub-fam. Hydrobiinae of the family Rissoidae”, American Journal of Conchology vol. 1(1), Philadelphia 1865, p.53. The genus is occasionally stygophile (P. troglodytes).
 

           

Sorholia Bœters & Falkner, 2009

Stygobiont. Differs from Moitessieria by the stretched, corkscrew-like spire. Operculum with peg.
Distribution: Adour drainage basin, in the western Pyrenees.
 

           

Spiralix Bœters, 1972

Stygobiont. The conchological differences with the close genus Moitessieria are based primarily on the spiral sculpture, which is, in Spiralix, weak or even absent. Distribution: upper Seine and its tributaries of northern Burgundy, Saône and Rhône drainage basins. But does Spiralix really exist?
 

       

Theodoxus Montfort, 1810

Fluviatile, lacustrian, crenobiont. The genus inhabits freshwater and mesohaline environments. « Shell free, univalve, with a regular spire, flattened; no umbilicus; aperture entire, round, widened, perpendicular to the horizon; outer lip sharp; no tooth at this lip nor at the columella. […] The Theodoxes being devoid of teeth, neither at the columella nor at the outer lip, could not be incorporated with the Nerites, or in the genus Clithon; this is the reason why we established for them a peculiar genus. » – P. D. de Montfort: Conchyliologie systématique vol.II, Paris 1810, p.551. In some species such as fluviatilis, the shell can vary considerably in shape, colours and pattern.
 

           

BIVALVIA 

Euglesa Jenyns, 1832

Fluviatile, lacustrian, crenobiont and even sometimes stygobiont. Suspension and sediment feeder. All the members of the family SPHAERIIDAE Deshayes share the same kind of hinge, with one cardinal in the right valve, and two in the left one (Sterki, 1911). Shell minute to small, equivalve, inequilateral, with commarginal growth marks; valves mediocrely convex; beaks opisthogyrate; « hinge plate narrow, cardinal teeth arched or bent; ligament enclosed » – lucidcentral.org. The genus is found worldwide.
 

           

Sphaerium Scopoli, 1777

Fluviatile, lacustrian, possibly crenobiont. Suspension and sediment feeder. Shell minute to small, equivalve, inequilateral, globose, with commarginal growth marks; beaks opisthogyrate, somewhat projecting; hinge plate narrow, cardinal teeth arched; ligament enclosed or visible. The genus is found worldwide.
 

           

NEITHER SPRING NOR STYX RELATED BUT DEEP CAVE-DWELLER ALL THE SAME:
Zospeum Bourguignat, 1856

Some pulmonata of the family CARYCHIIDÆ Jeffreys, 1830. This group is sometimes included as subfamily into ELLOBIIDÆ Pfeiffer, 1854. Distribution in France: central and western Pyrenees. The genus lives in some caves on humid walls, floors and concretions.