Projects > Gourneyras 2004 (July)

"Navacelles" Bythinella, new data

Latest update: December 5, 2004

By Vincent Prié

Introduction

The southern France karst represents an important reservoir of water. This environment, though still poorly known, hosts a fauna showing a high proportion of endemic species and is also a reservoir of biodiversity (MALARD et al. 1996).

Subterranean fauna is traditionally classified as stygoxene or trogloxene (i.e. temporary inhabitants), stygophile or troglophiles (i.e. with strong hypogean affinities) and stygobite or troglobite (i.e. obligatory hypogean) (GIBERT & DEHARVENC, 2002). The prefix "stygo" stands for aquatic animals and "troglo" for aerial ones. Subterranean fauna represents an important part of the world biodiversity. Culver and Holsinger (1992) proposed a global total of 50 000 to 100 000 obligate subterranean species (stygobites and troglobites). Stygofauna is even less known due to the difficulties to access the subterranean aquatic ecosystems (which represent 97% of all continental unfrozen freshwaters, while rivers and lakes only represent 2%!). The endemism is high as any catchments area can work as an isolated ecosystem and thus its fauna fosters evolutionary drifts in isolated populations, which then evolve into species specific to each river basin.

As in surface ecosystems, arthropods (e.g. shrimps of the genus Niphargus) represent most of the stygobic fauna. Mollusks are the second group in diversity, with a total of about 50 species described in France only.

Three strictly stygobite mollusks are known from the Vis canyon and adjacent aquifers: Moitessieria rolandiana, widely spread in southern France; Moitessieria cf. locardi, a form living in the Serrane networks only (and probably a endemic new specie…); Arganiella exilis, endemic to a small region; and a form of Bythinella specie that I am studying particularly and which was the purpose of the samples in Gourneyras.

The first shells have been found two years ago, and live specimen were collected in another cave nearby last year only. This form is related to the Bythinella genus, a wide group of springs dwelling species, with a few known occurrences of strictly stygobite species (e.g. Bythinella padiraci, endemic to the cave of Padirac and the related aquifers). Although peculiar features (including loss of pigmentation, lack of real eyes, morphology and shell characteristics) seem to show a genetic isolation of this morphospecies from the terrestrial ones in the area (Bythinella cebennensis & Bythinella eurystoma), we are waiting for some genetic studies to determine if it can be considered as a new specie or as a stygobite form of the previous ones. In the text, we refer to this form as the "Navacelles" Bythinella.

 

Material and methods

Sampling

The E.K.P.P. team sampled in Gourneyras Saturday the 11th of July 2004. Three samples were made at about 10 to 15 meters deep:

First, two 5 l. buckets have been partially filled up with sand and gravels by Ralph and Chaled. Another 10 l. bucket have been sampled at about the same depth by the same two divers.

Treatment

The samples have been soaked in water for a week in order to allow living specimen to surface. The living specimen have been sought by sight and picked up with forceps.
The samples are then dried out and the sand is poured into water again. The floating elements (including shells filled with air) are collected with a net.

Results

One of the 5 liter samples contained a juvenile Moitessieria rolandiana, my very first one although the specie has been collected before in other places. This specimen is conserved in 90 ° alcohol for genetic studies.

The 10 liter bucket contained a live specimen of Byhtinella related to the Navacelles form.

Discussion - conclusion

The first goal for such samples is to get live specimen, as nearly half of the known species of stygobite molluscs have never been found alive, being described only on the basis of shells found in the sand collected from springs.

The second goal is to find shells of dead specimen that accumulate in the same conditions as the sand collected. We call this gathering of shells a "thanatocenose". Some of these can be very rich and gather a lot of species, including juveniles. The shells witness the fauna present in the up waters and the morphology of the population(s) can be studied.

None of the samples was rich in shells, which means that the sand collected probably doesn't accumulate in the same condition than the shells. However, the previous samples in the same place have not been very rich either, and soft sand seems difficult to find. This is possibly due to the regular and powerful wash out of the cave that floods the small particles away.

The Moitessieria rolandiana is the first to be collected in the area and will be sequenced for genetic analysis and comparison with other forms of Moitessieria species.

The Bythinella found alive seems related to the "Navacelles" form for the shell characteristics, but with a strong pigmentation, a feature that has never been observed before. It could be an intermediate specimen between the terrestrial and stigobite population. If intermediate individual exists, the form is not genetically isolated and the "Navacelles" Bythinella cannot be considered as a distinct specie.

However it is an interesting opportunity to understand the mechanism of colonization of subterranean ecosystems by a previously terrestrial specie.

Many thanks to the EKPP project for sampling in Gourneyras.

About the Author

Vincent Prié works for the "Grand Site de Navacelles" on the environment of the Vis canyon (Natura 2000 program). He prepares a thesis "Systematic and evolution of the Moitessieridae (Mollusca, Gastropoda) of the North Montpelliérain region" at the Ecole Pratique de Hautes Etudes of Paris (Sorbonne) under the scientific direction of the Musée National d'Histoires Naturelles (Paris).

Bibliography

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CULVER, D.C. & HOLSINGER J.R., 1992. "How many species of troglobite are there? National speleological society bulletin 54 : 79-80".
GIBERT, J & DEHARVENC, L, 2002. "Subterranean ecosystems: A truncated functional biodiversity Bioscience June 2002, vol 52, n° 6".
MALLARD, F., GIBERT, J. & LAURENT, R., 1997. "L'aquifère de la source du Lez: un réservoir d'eau … et de biodiversité. Karstologia n° 30 - 2/ 1997, 49-54".
PRIE, V., 2002. "Contribution à la connaissance malacologique des causses méridionaux et des gorges de la Vis (gard et Hérault, France) Documents Malacologiques, 3: 25-33".
PRIE V., 2003. "Note sur les mollusques du bassin du Lez (Hérault, France) - Documents malacologiques vol 4: 19-32".