Osmoregulation and genetic polymorphism in Palaemonetes spp

Abstract

The osmoregulatory capabilities of the grass shrimps Palaemonetes kadiakensis Rathbun, Palaemonetes pugio Holthuis and Palaemonetes vulgaris (Say) were compared with steady-state and nonsteady-state experiments conducted to measure hemolymph chloride concentration, hemolymph osmolality, standard oxygen consumption and tissue free amino acid concentration. In addition, starch gel electrophoresis was used to compare genetic variability among the Palaemonetes populations. P. kadiakensis could hyper-regulate hemolymph osmolality and chloride concentration in fresh water though the mechanisms for extracellular anisosmotic regulation were not effective above 17('o)/oo. This species tolerated hyperosmotic shock well though extracellular regulation was impaired above 17('o)/oo. Weight-specific oxygen consumption rate was highest at 17('o)/oo and lowest at 24('o)/oo. There was no apparent trend with increasing salinity and adjustment to osmotic shock was slow. Oxygen consumption rate decreased after hyperosmotic shock and increased after hypo-osmotic shock. This species exhibited a well developed capacity for intracellular isosmotic regulation. Glycine was the most abundant amino acid comprising over 50% of the total pool. Total FAA, glycine, glutamate and aspartic acid concentrations increased with increasing salinity. Concentrations of the other non-essential amino acids (proline, arginine, glutamine, alanine and asparagine) were highest at 24('o)/oo. Aspartic acid was the only amino acid that successively increased in percentage of the total pool. Results confirmed that P. pugio and P. vulgaris are euryhaline regulators of hemolymph. The isochloride concentration was 17('o)/oo for both species. Both species adjusted hemolymph concentrations quickly after change in ambient concentration. P. vulgaris maintained hemolymph osmotic and chloride concentrations high than P. pugio. For P. pugio, weight-specific oxygen consumption rate was lowest at the isosmotic salinity (17('o)/oo) and highest at 3('o)/oo and 31('o)/oo. P. vulgaris showed no change in metabolic rate over the spectrum of experimental salinities. Acclimation to hypo-osmotic shock was slow for both euryhaline species. Results of the electrophoretic assay indicated that P. vulgaris, the most homeostatic species, exhibited the lowest degree of genetic polymorphism (H = 0.028; P = 26.3). The intermediate and least homeostatic species were P. pugio (H = 0.032; P = 33.3) and P. kadiakensis (H = 0.063; P = 35.1), respectively. Gdh was a fixed monomorphic locus for all three species, and Got-1 exhibited low variability in the euryhaline species with a single instance of a rare allele.