Salinity is one of the most important abiotic factors affecting growth, metabolism, immunity and survival of aquatic species in farming environments. As a euryhaline species, the black tiger shrimp (Penaeus monodon) can tolerate a wide range of salinity levels and is farmed between brackish to marine water conditions. The current study tested the effects of six different salinity levels (0‰, 2.5‰, 5‰, 10‰, 20‰ and 30‰) on the selected physiological, biochemical and genetic markers (individual changes in the expression pattern of selected candidate genes) in the black tiger shrimp. Experimental salinity levels significantly affected growth and survival performance (p < 0.05); the highest levels of growth and survival performance were observed at the control (20‰) salinity. Salinity reductions significantly increased free fatty acid (FFA), but reduced free amino acid (FAA) levels. Lower salinity treatments (0–10‰) significantly reduced hemolymph osmolality levels while 30‰ significantly increased osmolality levels. The five different salinity treatments increased the expression of osmoregulatory and hemolymph regulatory genes by 1.2–8-fold. In contrast, 1.2–1.6-fold lower expression levels were observed at the five salinity treatments for growth (alpha amylase) and immunity (toll-like receptor) genes. O₂ consumption, glucose and serotonin levels, and expression of osmoregulatory genes showed rapid increase initially with salinity change, followed by reducing trend and stable patterns from the 5th day to the end. Hemocyte counts, expression of growth and immunity related genes showed initial decreasing trends, followed by an increasing trend and finally stability from 20th day to the end. Results indicate the farming potential of P. monodon at low salinity environments (possibly at freshwater) by proper acclimation prior to stocking with minimal effects on production performance.

Elucidating the molecular basis of adaptation to different environmental conditions is important because adaptive ability of a species can shape its distribution, influence speciation, and also drive a variety of evolutionary processes. For crustaceans, colonization of freshwater habitats has significantly impacted diversity, but the molecular basis of this process is poorly understood. In the current study, we examined three prawn species from the genus Macrobrachium (M. australiense, M. tolmerum, and M. novaehollandiae) to better understand the molecular basis of freshwater adaptation using a comparative transcriptomics approach. Each of these species naturally inhabit environments with different salinity levels; here, we exposed them to the same experimental salinity conditions (0‰ and 15‰), to compare expression patterns of candidate genes that previously have been shown to influence phenotypic traits associated with freshwater adaptation (e.g., genes associated with osmoregulation). Differential gene expression analysis revealed 876, 861, and 925 differentially expressed transcripts under the two salinities for M. australiense, M. tolmerum, and M. novaehollandiae, respectively. Of these, 16 were found to be unannotated novel transcripts and may be taxonomically restricted or orphan genes. Functional enrichment and molecular pathway mapping revealed 13 functionally enriched categories and 11 enriched molecular pathways that were common to the three Macrobrachium species. Pattern of selection analysis revealed 26 genes with signatures of positive selection among pairwise species comparisons. Overall, our results indicate that the same key genes and similar molecular pathways are likely to be involved with freshwater adaptation widely across this decapod group; with nonoverlapping sets of genes showing differential expression (mainly osmoregulatory genes) and signatures of positive selection (genes involved with different life history traits).

Change in environmental salinity level is a major limiting factor for the aquaculture productivity because it imposes severe stress on organisms that in turn retards growth. The orange mud crab (Scylla olivacea) is an important coastal aquaculture species (farming is practised in 10 parts per thousand-20 parts per thousand salinity levels) in Bangladesh. The present study was conducted to investigate the changes in growth, O(2)consumption and mRNA expression levels of five selected genes in the orange mud crab (S. olivacea) exposed to three different experimental salinity levels (0 parts per thousand, 10 parts per thousand and 20 parts per thousand) for three months. Crabs reared at 10 parts per thousand and 20 parts per thousand, showed significantly higher (p < .05) growth performance and expression of growth regulatory genes (Actinand alpha-amylase). The highest levels (p < .05) of O(2)consumption and expression of ion regulatory genes (Na+-K+-ATPase,V-type H+-ATPaseandDiuretic Hormone) were obtained at 0 parts per thousand. Moderate levels of growth and expression of selected candidate genes were observed at 10 parts per thousand treatment while the highest levels of growth and gene expression were obtained at 20 parts per thousand (control salinity). Strong interactions were observed between growth performance and expression of growth genes (R-2 = 0.81-0.91), and rate of O(2)consumption and expression of ion regulatory genes (R-2 = 0.83-0.93), implying that the selected genes are important candidates for growth and ionic balance inS.olivacea. Growth performance was found to be very low at 0 parts per thousand initially, after 30 days crabs showed better growth performance at this salinity level. It is thus inferred that orange mud crab individuals might require 3-5 days for acclimation to salinity stress but it can take at least 30 days for acclimation to regular growth. Results indicate that with proper acclimation, the orange mud crab (Scylla olivacea) can be farmed at low salinity conditions and possibly in freshwater condition.

Understanding the molecular basis of adaptive response to variable environmental conditions is a central goal of evolutionary biology. Here, we sought to identify potential outlier single nucleotide polymorphisms (SNPs) in 3 wild populations of a freshwater prawn (Macrobrachium australiense) that are exposed to differing osmotic niches by using a comparative transcriptomics approach. De novo assembly of approximately 542 million (75 nt) pair end reads collected from 10 individuals revealed 123396 longer contigs/transcripts of variable length, that showed 97.38% transcriptome assembly completeness. Differential gene expression analysis of major osmoregulatory genes revealed that calreticulin, Na⁺/H⁺ exchanger, and V-type (H⁺) ATPase showed the highest expression levels in the Blunder Creek (low ionic) population, while Crustacean cardiovascular peptide (CCP), Na⁺/K⁺-ATPase, Na⁺/K⁺/2Cl⁻ co-transporter (NKCC) and Na⁺/HCO₃⁻ exchanger showed the highest expression levels in the Bulimba Creek (higher ionic) population. In total, 16 gene ontology term categories were functionally enriched among the 3 studied populations. We identified 4144 raw and 835 high quality filtered SNPs in the 3 M. australiense populations, of which 84 SNPs were identified as outliers. Outliers were detected in 4 important osmoregulatory genes that include: calreticulin, Na⁺/H⁺ exchanger, Na⁺/K⁺-ATPase, and V-type-(H⁺)-ATPase. All outliers in the osmoregulatory genes were located in noncoding regulatory regions (untranslated regions) of the gene. We hypothesize that the outlier SNPs identified here in M. australiense populations exposed naturally to different osmotic conditions influence specific gene expression patterns that allow individuals to respond to local environmental conditions.