Effects of salinity on growth, hematological parameters, gill microstructure and transcriptome of fat greenling Hexagrammos otakii

The fat greenling, Hexagrammos otakii, is an emerging aquaculture species and is being evaluated for production in recirculating aquaculture systems (RAS). To better understand the tolerance of H. otakii to varying environmental salinity in actual aquaculture operations, the effects of salinity on survival, growth, hematological parameters, gill microstructure and gill transcriptome were investigated. Juvenile H. otakii (22.0 +/- 1.8 g; 11.2 +/- 0.7 cm) were reared at different salinities, 6.7 +/- 0.1 PSU (7 PSU), 12.3 +/- 0.1 PSU (12 PSU), 18.2 +/- 0.1 PSU (18 PSU), 23.9 +/- 0.6 PSU (24 PSU), and 28.9 +/- 0.8 PSU (29 PSU) in triplicated RASs for five weeks, and the impacts of salinity on these endpoints were evaluated. Fish in the 7 PSU group exhibited significantly lower survival, final weight, feed conversion ratio, and specific growth rate than fish in other treatments. While no statistically significant differences were found among mean white blood cell counts, red blood cell counts and hemoglobin concentrations, mean corpuscular volume was significantly lower in the 7 PSU group than in other groups. At the histological level, there were smaller numbers and smaller volume of mitochondria rich cells in the 7 PSU group, and the numbers of pavement cells and mucous cells were significantly higher than in other groups. Transcriptome sequencing for a sample of individuals from the 7 PSU, 18 PSU and 29 PSU groups indicated that gene expression in the gills of H. otakii differed significantly among groups. Gene ontology (GO) enrichment showed differential expression between the 7 PSU and 29 PSU groups for seven genes with activities as neurotransmitter transmembrane transporters and neurotransmitter: sodium symporters. Comparing to the 18 PSU group, 16 genes with activities in gas or oxygen transport were down-regulated in the 7 PSU group. No such differences in expression of gas transport genes was found between the 18 PSU and 29 PSU groups. Overall, our exploratory transcriptome analysis suggested that fish in the low-salinity 7 PSU group exhibited high gene expression in the osmoregulation and gas transport pathways. Our results at the histological and gene expression levels suggest why rearing juvenile H. otakii in salinities of 12-29 PSU is optimal for commercial aquaculture production.

Automated summary

The fat greenling species Hexagrammos otakii is being evaluated for aquaculture production in RAS systems. A study on the effects of salinity on survival, growth, blood parameters, gill structure, and gene expression in H. otakii revealed that low salinity can impact fish health and growth, causing changes in blood parameters and gill structure. Overall, maintaining salinities between 12-29 PSU is optimal for commercial aquaculture production of juvenile H. otakii.