Growth, reproductive traits and habitat patterns of Unio ravoisieri (Mollusca: Bivalvia) from Ichkeul Lake tributaries (Northern Tunisia)

Abstract

Habitat patterns, growth and reproductive traits of the freshwater mussel Unio ravoisieri were investigated for the first time in two streams (Sejenane and Elmaleh) of the Ichkeul Lake basin (northern Tunisia) over the course of a year. A very similar profile was observed for the two studied populations. The median age class dominated in the age structure, indicating good recruitment and viability. The reproductive period lasted approximately 10 months (from September to June). Females at different gametogenesis stages were found during autumn, winter and spring. Gravid females were sampled during winter and spring, and larval release occurred from early spring to early summer. The number of embryos produced per female reached 84,000 per year. In Elmaleh stream, characterized by high salinity, reproduction of the mussel took place slightly earlier than in the other habitat (Sejenane). However, the variation in reproduction traits between the two populations was not significant (p > 0.05). Similar patterns, but with a difference in the timing of the breeding season, were recorded for Spanish populations. The results highlight a reproductive plasticity of U. ravoisieri and an ability to tolerate harsh abiotic conditions. Nevertheless, salinity and temperature seem to have a stressful effect on different reproductive stages, mainly during the brooding period; negative correlation was found between condition index and the two parameters: Pearson’s r = 0.4–0.8; p < 0.05. To avoid their adverse effects, U. ravoisieri appears to be able to adjust its reproductive cycle to the wet season when these parameters are moderate.