**3. Immune genes**

Crustins, belonging to family of antimicrobial peptides (AMPs) are shown to be differentially expressed in response to various immunostimulants and microbes [22]. Different isoforms of crustins have been isolated from a variety of penaeid shrimps [23-25]. The up-regulation of crustin-like AMP in shrimps suggests functional role of AMPs primarily in the shrimp immune response [26]. Under hyperosmotic stress, crustinPm5, crustinPm1, and crustin-like Pm gene levels were found to be up-regulated [27]. In our study, we observed crustin gene was upregulated in all the three shrimp tissues of gills, gut, and muscle analyzed at low salinity stress [20]. Hence, crustins having antimicrobial activity also functionally respond to salinity stress in shrimps.

Another class of AMPs, the penaeidin gene was found to be down-regulated in gill, gut, and up-regulated in antennal gland tissues of shrimp exposed to high salinity stress (55 ppt) conditions [21]. These changes in the gene expression levels of penaeidin may be regulated through variations in hemocyte numbers in salinity-stressed shrimp. Antibacterial proteins such as lysozymes are involved in nonspecific innate immunity in shrimps [28]. Significant up-regulation of lysozyme gene expression occurred in gill (16.25-fold) tissues of shrimp exposed to low salinity stress [20]. In these shrimps, the anti-lipopolysaccharide factor, which is considered to play an important role in shrimp immune response [29], was significantly down-regulated in gill and gut tissues as compared to up-regulation in the muscle tissues of shrimp.

In general, as a result of environmental stress, the expression of immune-related shrimp genes transcripts gets affected [30]. The shrimp exhibits reduction in immune parameters when exposed to salinity stress conditions, leading to decreased resistance against pathogens [31-32]. White shrimp *L. vannamei*, when transferred to low salinity conditions showed significant decrease in hemocyte count, phenoloxidase activity, respiratory burst, and superoxide dismutase activity, which further reduced on challenge with *Vibrio alginolyticus.* Hence, innate immunity in shrimps reduces with combined effect of low salinity and bacterial challenge [32].
