Distribution patterns of Temnocephalids commensal with Crustacea and Mollusca from Argentina

An analysis and a comparison of abundance, prevalence and distribution patt erns for three temnocephalid species are presented. Three temnocephalid pop ulations from Rio de la Plata river, Republica Argentina, were studied: Tem nocephala digitata Monticelli, 1902 associated with Palaemonetes argentinus Nobilli, 1901 (Crustacea Caridea), T. axenos Monticelli, 1899 associated w ith Aegla uruguayana Schmitt, 1942, and A. platensis Schmitt, 1942 (Crustac ea Anomura) and T. iheringi Haswell, 1893 associated with Pomacea canalicul ata (Lamarck, 1822) (Mollusca Gastropoda). These host species provide diffe rent environments and consequently different opportunities for the developm ent of their symbionts. Increasing gradients of abundance and prevalence, a nd a decrease in the aggregation index (k) were observed. The distribution pattern of the three symbiotic species is aggregated. The original distribu tions were compared with the parameters of the negative binomial. The obser ved overdispersion may be due to heterogeneity of the host population and r eproduction of the commensal within the host.