Hydrobiologia 199: 193-199, 1990. © 1990 Kluwer Academic Publishers. Printed in Belgium. 193 The effect of eyestalk ablation on haemolymph osmotic and ionic concentrations during acute salinity exposure in the freshwater shrimp Macrobrachium olfersii (Wiegmann) (Crustacea, Decapoda) John C. McNamara, L.C. Salomao' & Elaine A. Ribeiro Departamento de Fisiologia e Biofisica, Instituto de Cidncias Biomedicas, CP 4365, Universidade de Sao Paulo, Sao Paulo, 05508 SP, Brazil; Departamento de Fisiologia Geral, Instituto de Biocidncias, CP 11461, Universidade de Sao Paulo, Sao Paulo, 05499 Brazil Received 23 November 1988; in revised form 15 March 1989; accepted 25 July 1989 Abstract The effect of bilateral eyestalk ablation on haemolymph osmotic and Na + , Cl-, K , Mg 2+ and Ca 2 + concentrations was investigated in the freshwater shrimp Macrobrachium olfersii during acute exposure (1, 3, 6, 12 and 24 h) to sea-water of 21%o S. Analysis of variance treatment indicated that haemolymph osmotic and Na', Cl-, K , Mg 2+ and Ca 2 + concentrations were affected by exposure time (p < 0.01). However, only [Na + ] was affected by eyestalk ablation (p < 0.01), destalked shrimps exhibiting haemolymph Na+ concentrations approximately 15 to 30% lower than control shrimps. These results are interpreted to indicate that less Na+ enters the haemolymph of eyestalkless shrimps on exposure to seawater of 21%o S, presumably resulting from the absence of an eyestalk-located neurofactor. A factor causing the influx of Na + into the haemolymph is thus proposed to be present in the eyestalk ganglionic system of intact Macrobrachium olfersii in freshwater. Reduction in secretion of the factor may act to reduce Na+ entry in high salt environments. Introduction The capability to regulate the osmotic and ionic concentrations of their body fluids has permitted the penetration of the crustaceans into the fresh- water biotope; the evolution of a selectively permeable integument, together with high affinity ion uptake mechanisms and a well developed excretory process, systems which maintain ele- vated intra- and extracellular fluid concentrations while reducing ion loss to the external environ- ment, have allowed the palaemonid shrimps, particularly those of the genus Macrobrachium, to exploit this habitat successfully. These palae- monids typically maintain a high haemolymph osmotic concentration of around 300 to 400 mOsm/kg water in fresh water (see Denne, 1968; Castille & Lawrence, 1981; Moreira et al., 1983; McNamara, 1987) and produce a urine hypo-osmotic to the haemolymph (Pannikar, 1941; Parry, 1954, 1957; Born, 1968; Denne, 1968). Such mechanisms of osmotic and ionic regula- tion are apparently under neuroendocrine control (see Kamemoto & Tullis, 1972; Tullis & Kamemoto, 1974; Kamemoto, 1976; Kleinholz, 1976; Kamemoto, 1982; Mantel& Farmer, 1983; Mantel, 1985; Kamemoto & Oyama, 1985 for discussion and references). Among the palae- monid shrimps for example, Kamemoto & Tullis