SPATIAL-DISTRIBUTION OF 4 FRESH-WATER GASTROPOD SPECIES IN A DITCH NEAR ORSAY, FRANCE

The distribution (both specific and individual) of four pulmonate gastropods was studied in a ditch near Orsay, France, from April 1987 to August 1988. Physa fontinalis L. and Anisus albus (Müller) were restricted to the part of the ditch which never dries up and positively associated with the hydrophytes Ceratophyllum submersum L. and Callitriche hamulata Küntz. On the other hand, Anisus rotundatus (Poiret), which is drought resistant, was abundant in the shallowest part of the ditch and was positively associated with helophytes while Lymnaea palustris (Müller) was ubiquitous and present throughout the ditch. Plant morphology appears to be a major factor determining snail-plant relationships. Within their respective habitats, each species presented an aggregated distribution; their variances were stabilized by logarithmic transformation for L. palustris, Downing's transformation for P. fontinalis and Taylor's transformation for the two remaining species. © 1990 Kluwer Academic Publishers.