Effect of water conductivity on survivorship, reproduction, growth and shell toughness of Biomphalaria peregrina (Gastropoda, Planorbidae)

MARTÍN, PABLO

Santiago de Chile

Congreso; IV International Congress of Medical and Applied Malacology,; 1996

International Society for Medical and Applied Malacology

Biomphalaria peregrina is one of the most widely distributed Planorbids in the  Neotropical Region. It has been demonstrated experimentally that it is a potential host of Schistosoma mansoni. It commonly behaves as fugitive before other members of the genus with greater degree of susceptibility to this parasite (B. glabrata, B. tenagophila y B. straminea), thus, the factors which determine its spatial distribution could be important in the transmission of schistosomiasis in the region. In a previous field study in the Napostá Grande stream (Buenos Aires, Argentina) it was de­termined that B. peregrina showed a maximum abundance under intermediate condi­tions of electrolytic content of the water, and was absent only in the higher parts of the course (where slopes are the steepest and conductivity the lowest). The aim of this work was to determine experimentally the effect of the degree of mineraliza­tion of the water on survival, growth and reproduction of B. peregrina.  Aquaria with five different levels of relative conductivity of water were used, covering all the range found in the stream. Food and water were renewed weekly. The number of survivors, egg-masses, eggs per mass and eroded shells counted, and the  size of the snails under each treat­ment measured. Viability and other parameters of egg-masses and new-born snails were evalu­ated on two occasions. Ash dry weight of shells was determined at the end of the essay (50th week). Survivorship, growth, fecundity and viability were not affected by conductivity. Nevertheless the intrinsic rate of natural increase showed a negative relation with this factor because of an alteration in the temporal pattern of oviposition. On the contrary, integrity and toughness of the shell showed a positive relation with the degree of mineralization of the water. The long term response of B. peregrina to the electrolytic content of water un­der laboratory conditions does not explain completely the distribution pattern ob­served in the stream, in particular its absence in the higher course. Nevertheless, this part of the stream is characterized by its severe hydrodynamic and substrate conditions; under which the weakness of the shell would become a major problem, enhancing the snails´ vulnerability to this adverse environment.