Relationships between the annual generations of the snail Lymnaea truncatula (Mollusca Gastropoda : Lymnaeidae), altitude, and the type of its habitats in central Morocco

Bimonthly investigations were performed over a year in the valley of Oued A in Chkef (district of Fes, central Morocco) and in that of Oued Zerrouka (d istrict of Ifrane) to study (the annual generations of Lymnaea truncatula i n four habitats differing in their altitude (465 or 1630 m, respectively) a nd their type (banks of river or swampy zones). Two peaks in the density of L. truncatula and in that of egg-deposits were noted : the first took plac e in winter or at the beginning of spring, and the second peak was in autum n. Numerical fluctuations in the height of snails were also found and the l owest mean values were recorded during two periods : the first stretched ov er spring (3 sites) or at the end of August (swampy zone in the valley of O ued Zerrouka), while the second period was at the end of November or at ear ly December. The type of snail habitat had a significant effect on the dens ities of L. truncatula, those of egg-deposits, and the heights of snails, a s they were higher in swampy areas. A significant effect between altitude a nd snail densities was also noted, as snail numbers were lower in the valle y of Oued Zerrouka. Despite some variations in the dates, two annual genera tions were found in the four populations of L. truncatula studied and winte r egg-deposits were more numerous than autumnal ones.