COMPARATIVE POPULATION BIOLOGY OF FISH DIGENES - THE CASE OF 3 HELICOMETRA (TREMATODA, OPECOELIDAE) MESOPARASITES OF MARINE TELEOSTS IN A MEDITERRANEAN LAGOON

In the Etang de Thau (Herault, France), three species of Helicometra p arasitize in their adult stage the digestive tract of the black goby G obius niger, the grass goby Zosterisessor ophiocephalus, the grey wras se Symphodus cinereus, and the common eel Anguilla anguilla (Teleostei ). These fish act as definitive hosts in the biological cycle of these platyhelminths. Also H. gobii exists in G. niger, Z. ophiocephalus, S . cinereus and A. anguilla; H. fasciata is found in G. niger; and H. p ulchella is only found in S. cinereus. In the case of the two gobies a nd the eel, these three congeneric digenes sometimes coexist in the sa me infracommunity. Spatio-temporal analysis of these different populat ions has enabled: 1) detection of seasonal variations in their structu re, 2) identification of the relationships between the nature of their specificity and demographic strategy, and 3) the global absence of st atistical dependence between the intensities of different species in t he same infracommunity to be show. Implications arising from previous research concerning the demographic behaviour of these trematodes in t heir first intermediate host (mollusc compartment), were taken into ac count when interpreting structures observed in the definitive hosts. T he problems of coexistence between these species, which are taxonomica lly very close in the same parasite community, and the under-lying spe ciation mechanisms are discussed in the light of population structurat ion.