Molecular and whole animal responses of grass shrimp, Palaemonetes pugio, exposed to chronic hypoxia

Hypoxic conditions in estuaries are one of the major factors responsible for the declines in habitat quality. Previous studies examining effects of hypoxia on crustacea have focused on individual/population-level, physiological or molecular responses but have not considered more than one type of response in the same study. The objective of this study was to examine responses of grass shrimp, Palaemonetes pugio, to moderate (2.5 ppm DO) and severe (1.5 ppm DO) chronic hypoxia at both the molecular and organismal levels. At the molecular level we measured hypoxia-induced alterations in gene expression using custom cDNA macroarrays containing 78 clones from a hypoxiaresponsive suppression subtractive hybridization cDNA library. Grass shrimp exposed to moderate hypoxia show minimal changes in gene expression. The response after short-term (3 d) exposure to severe hypoxia was up-regulation of genes involved in oxygen uptake/transport and energy production, such as hemocyanin and ATP synthases. The major response by day 7 was an increase of transcription of genes in the mitochondrial genome (16S rRNA, cytochrome b, cytochrorne c oxidase I and III), and up-regulation of genes encoding proteins involved in iron metabolism. By day 14 a dramatic reversal was seen, with a significant down-regulation of both mitochondrial and Fe-metabolism genes. Validation of the macroarray results with q-PCR showed similar up- or down-regulation at multiple time points for 9 genes. At the organismal level, our studies showed condition factor of grass shrimp exposed to severe chronic hypoxia was lower than normoxic controls during the first 7 days of the experiment, but there were no differences after that time point, or in grass shrimp exposed to moderate hypoxia. Surprisingly, chronic hypoxia appeared to enhance grass shrimp reproduction; females exposed to moderate hypoxia. had higher fecundities and a greater percentage produced first, second and third broods than normoxic shrimp. The hypoxic shrimp took longer to produce their first brood than the normoxic controls, although starved larvae from hypoxia-exposed mothers lived longer than normoxic control larvae. Shrimp exposed to severe hypoxia also had higher fecundity than nonnoxic controls, although embryos from hypoxia-exposed mothers took longer to hatch than normoxic control embryos. ne gene expression and reproductive results suggest that expression levels of genes encoding proteins involved in oxygen and electron transport, energy, and iron metabolism may be useful molecular indicators of both short term (< 7 d) and moderate (14 d) exposure to severe hypoxia, and that chronic hypoxia, may have population-level impacts on grass shrimp. (c) 2006 Elsevier B.V. All fights reserved.