Changes in free amino acids in the hemolymph of giant freshwater prawn Macrobrachium rosenbergii exposed to varying salinities: relationship to osmoregulatory ability

Changes in free amino acids (FAA) in the hemolymph of the giant freshwater prawn, Macrobrachium rosenbergii, were examined in individuals exposed to varying salinities for up to 1 week. In freshwater and under conditions of low salinity, total FAA concentrations were maintained between approximately 0.85 and 1 mM and did not exhibit changes in response to salinity exposure. Under high salinities, total FAA concentrations increased dramatically, reaching up to 2.1 mM depending on treatment. Examination of individual amino acid concentrations revealed that these increases were based on specific changes in glycine, arginine, alanine, proline and lysine. Among these, alanine showed the greatest increases, resulting in levels six-fold higher under high salinity than in freshwater and under low salinity. The other amino acid species showed increases of 2.5-fold compared to original values. These five FAAs in freshwater and under low salinity together occupied approximately 45% of total FAA contents and under high salinity comprised more than 70% of total FAA contents. These results suggest that specific hemolymph FAAs are involved in mediating response to salinity exposure in freshwater prawns.

Determination of amino acid sequence and site of mRNA expression of four vitellins in the giant freshwater prawn, Macrobrachium rosenbergii

Four major yolk proteins, designated as vitellins (Vns) Macr-VnA, B, C, and D, were extracted from mature ovaries of Macrobrachium rosenbergii. These were purified to homogeneity by reversed-phase high performance liquid chromatography (HPLC) employing a unique separation system based on the hydrophobic properties of the Vn molecule. Using standard techniques of protein sequencing, more than 33 N-terminal and 57 internal amino acid residues were determined for each of the four Vns. The cDNA fragments encoding the four Vns were amplified by PCR using degenerate oligonucleotide primers derived from the N-terminal and internal amino acid sequences. These cDNA fragments were cloned, sequenced, and used as probes to examine the transcription of mRNAs encoding the four Vns. Significant accumulations of these mRNAs were observed in female hepatopancreas only, while mRNA expression was not detected in male hepatopancreas or any other female tissue including ovary, subepidermal adipose tissue, gill, and muscle. This is the first occasion in Crustacea in which multiple Vns were demonstrated to be synthesized simultaneously in a single tissue.

Characterization of Na/K-ATPase in Macrobrachium rosenbergii and the effects of changing salinity on enzymatic activity

A ouabain-sensitive Na/K-ATPase kinetic assay system based on the hydrolysis of ATP and the oxidation of NADH was adapted in order to characterize enzymatic activity in gills and examine the effects of changing salinity in Macrobrachium rosenbergii. Maximum inhibition by ouabain occurred at a concentration of 1.4 mM, and the K(m) of the reaction was 0.2 mM. In a first experiment, animals were acclimated to freshwater, 1/3 seawater, 2/3 seawater and full seawater for up to 1 week. Na/K-ATPase activity in front gills was 1. 62+/-0.19 micromol ADP/mg protein per h in freshwater, and was seen to increase slightly in 1/3 seawater (1.88+/-0.19 micromol ADP/mg protein per h) and 2/3 seawater (2.09+/-0.24 micromol ADP/mg protein per h), decreasing slightly in full seawater (1.92+/-0.43 micromol ADP/mg protein per h); however, differences were not significant. Back gills showed slightly higher levels, and a similar pattern of Na/K-ATPase activity. In a second experiment, animals were acclimated to 1/3 seawater and 2/3 seawater, and then transferred to freshwater. However, no changes in activity were seen, indicating that exposure to dilute media did not effect enzymatic activity. Whereas Na/K-ATPase is important in osmoregulatory function in marine euryhaline crustaceans, it may not play a significant role in adaptation in freshwater crustaceans that inhabit a more narrow range of salinities.

Ecdysteroid fluctuations during embryogenesis in the giant freshwater prawn, Macrobrachium rosenbergii

Ecdysteroid levels during the embryogenesis of the giant freshwater prawn, Macrobrachium rosenbergii, were determined by radioimmunoassay and high-performance liquid chromatography. Ecdysteroids consisting of significant amounts of 20-hydroxyecdysone and high-polarity products (HPP) and lesser amounts of ecdysone and low-polarity products (LPP) were detected in mature ovaries and newly laid eggs. All ecdysteroid groups decreased gradually during the nauplius phase. With the formation of the compound eye and the appearance of the carapace and other body-like structures, marking morphogenesis to the zoeal stage, embryos showed the beginning of a continuous and dramatic increase in ecdysteroid concentrations sustained until larval hatchout. Ecdysteroid levels at hatchout were above 20-fold greater than ecdysteroid levels in newly laid eggs. More specifically, HPP and 20-hydroxyecdysone increased concomitantly, with a decrease in 20-hydroxyecdysone only at the end of the embryogenic period, while ecdysone and LPP levels remained low or undetectable. It may be postulated that the presence of ecdysteroids in ovaries and eggs represents a reserve of maternal ecdysteroids which are necessary at the commencement of embryonic development; with the differentiation of embryonic tissue capable of ecdysteroid synthesis, ecdysteroids increase rapidly to play a role in later embryonic development.