Abstract
This paper describes the life cycle of Pisidium henslowanum living in Lake Piediluco (southwestern Umbria, Italy). Morphological and histological analysis of samples taken monthly over a period of two years (February 1993 - February 1995) allowed us to follow the annual changes in the gonad tissues, and to ascertain that the species is fertile for most of the year with two peak periods, which occur in July and November, when there is massive gamete release. A short period of non-fertilization takes place in January and February when few spermatozoa occur and no ova are produced. Through the monitoring of the ontogenetic stages of P. henslowanum in Lake Piediluco, it was possible to determine the average life span of two distinct generations per year to be 12-14 months for the main one (winter generation) and 14-16 months for the subsidiary one (summer generation). The population is semelparous and bivoltine, with hatchings in late spring and late autumn. Self-fertilization was documented in many specimens, since a large number of free spermatozoa and ova were observed within the female follicles.
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