Relationship between some environmental parameters and trypanorhynch cestode loads in banana prawns (Penaeus merguiensis de Man)

Abstract

The population of P. merguiensis in the Norman River was sampled at weekly intervals for 6 months during the 1978-79 summer. The size-frequency distribution of one recruitment cohort to that river was progressively studied. and individual prawns examined for parasites as they grew from post-larvae to adolescence. The juvenile banana prawns were essentially unparasitized by pierocerci of Parachristianella monomegacantha Kruse until they reached 13 mm carapace length. Thereafter. the larger adolescent prawns accumulated trypanorhynch parasites in proportion to their increases in size. This was attributed to a change in feeding habits at 13 mm carapace length. Previous studies had shown that, at least on spring tides, juvenile banana prawns fed in the lower littoral zone at low tide. and adolescent prawns were more conspicuous at high tide when waters entered the mangrove areas. These studies, coupled with the above data, suggested that the main infective area for the prawns was mangroves in the upper littoral zone. When marine salinities increased from 32 to 37‰ prevalence of Paiachristianella monomegacantha also rose. Estuarine salinities of 12-32‰, had no appreciable effect on parasitism, with the level being low and constant throughout this range. This result was possibly due to differential survival of infective stages in the estuary. As a corollary to this. the percentage of infected Penaeus merguiensis dropped from a maximum of 84% at the mouth to 26% at Normanton, 80 km upstream. When considering the use of parasites as biological tags for the estuarine phase of catadromous penaeid prawns, the size of the prawns and salinity where they are captured have substantial effects on parasite loads.