Ionic aspects of the physiology and biology of Macrobrachium rosenbergii (De Man) 1879.

Abstract

The giant Malaysian freshwater prawn, Macrobrachium rosenbergii spends its juvenile and adult life primarily in freshwater. The larval stages of this species require brackishwater for their survival and development. Adult females of Macrobrachium rosenbergii migrate into brackishwater to spawn, and some populations of this prawn live entirely in brackishwater. Other Macrobrachium species have larval stages that require brackishwater for development. Some Macrobrachium species do not require brackishwater for larval survival and have abbreviated larval development. The ,ionic requirements for successful larval development and metamorphosis were studied using a formulated artificial seawater. It was found that trace element impurities, either in the salts used in the artificial seawater, or contained within the artemia fed to the larvae, were sufficient to allow normal larval development. The omission of bromide from the artificial seawater was found to cause total mortality to early stage larvae. Further work attempted to discover the minimum threshold concentration of bromide required by the larvae. The manner by which Macrobrachium rosenbergii adults regulate the ionic composition of their haemolymph when exposed to freshwater and brackishwater of varying salinity was investigated. The effect of the moult cycle on divalent cation regulation is studied. It was found that in varying salinity adult Macrobrachium showed a strong regulation of its haemolymph osmotic pressure, Na, Cl, Mg, Ca, K and Sr concentrations. The haemolymph Ca and Br concentration increased with increasing salinity, while the haernolymph Cu concentration decreased. The high concentration of bromide and strontium in the haemolymph of prawns held in freshwater was regarded as an indication that they might be essential to the adult prawn. The implication of their roles was in the process of cuticle hardening. A close relationship between haemolymph strontium and magnesium concentrations was revealed, although the significance of this is uncertain. The effect of the moult cycle on haemolymph divalent cation regulation revealed that Ca and Cu decreased after the ecdysis. Haemolymph magnesium concentrations were elevated during pre- and postmoult. Strontium increased markedly prior to ecdysis, this was further evidence as to a potential role in the calcification process. The haemolymph bromide concentration was reduced during pre- and postmoult. This was attributed to, either its incorporation into the cuticle, or increased ionic fluxes during the pre- and postmoult period. The calcification of the larval stages of Macrobrachium rosenbergii was investigated. The larvae are calcified in a similar manner to the postlarvae. Concentrations of strontium and bromine in the postlarvae decrease when they are transferred to freshwater. Analysis of larval exuviae revealed high concentrations of bromine (=1000ppm) confirming its role in the sclerotisation of the larval cuticle. A close relationship was found between the strontium and magnesium concentrations of the larvae and postlarvae held in seawater, confirming this discovery in the adult haemolymph. Such a relationshipwas not found between these ions and calcium. Salinity did not affect the function of Macrobrachium rosenbergii haemocyanin significantly. This was attributed to the relatively stable internal ionic environment that this prawn is able to maintain over a wide range of salinities. Temperature had a pronounced effect on the haemocyanin oxygen affinity. The oxygen transporting characteristics of Macrobrachium rosenbergii haemocyanin were similar to those found for marine and brackishwater crustaceans. The P02 of Macrobrachium rosenbergii haemolymph was found to be substantially lower than the P02's recorded for marine and bracklshwatar species. This was attributed to a reduced perfusion of the gills by haemolymph. The reason for this was supposed to be a means by which this prawn reduces its permeability, and hence loss of ions, when in freshwater.