Physiological mechanisms used by fish to cope with salinity stress

Abstract

Salinity represents a critical environmental factor for all aquatic organisms, including fishes. Environments of stable salinity are inhabited by stenohaline fishes having narrow salinity tolerance ranges. Environments of variable salinity are inhabited by euryhaline fishes having wide salinity tolerance ranges. Euryhaline fishes harbor mechanisms that control dynamic changes in osmoregulatory strategy from active salt absorption to salt secretion and from water excretion to water retention. These mechanisms of dynamic control of osmoregulatory strategy include the ability to perceive changes in environmental salinity that perturb body water and salt homeostasis (osmosensing), signaling networks that encode information about the direction and magnitude of salinity change, and epithelial transport and permeability effectors. These mechanisms of euryhalinity likely arose by mosaic evolution involving ancestral and derived protein functions. Most proteins necessary for euryhalinity are also critical for other biological functions and are preserved even in stenohaline fish. Only a few proteins have evolved functions specific to euryhaline fish and they may vary in different fish taxa because of multiple independent phylogenetic origins of euryhalinity in fish. Moreover, proteins involved in combinatorial osmosensing are likely interchangeable. Most euryhaline fishes have an upper salinity tolerance limit of approximately 2× seawater (60 g kg −1). However, some species tolerate up to 130 g kg −1 salinity and they may be able to do so by switching their adaptive strategy when the salinity exceeds 60 g kg −1. The superior salinity stress tolerance of euryhaline fishes represents an evolutionary advantage favoring their expansion and adaptive radiation in a climate of rapidly changing and pulsatory fluctuating salinity. Because such a climate scenario has been predicted, it is intriguing to mechanistically understand euryhalinity and how this complex physiological phenotype evolves under high selection pressure.

Effects of Salinity on Some Haematological and Biochemical Parameters in Nile Tilapia, <i>Oreochromus niloticus</i>

“…There are variations in salinity tolerance among different species [8]. The physiological responses to high or low salinity levels in the aquatic environment have been studied in different fresh and marine species [9][10][11].…”

Feeds, water quality, gut morphology and digestion in Nile tilapia (Oreochromis niloticus)

“…Salinity changes can cause stress to the fish due to interference with physiological homeostasis and routine biological processes (Kültz, 2015). Freshwater fish such as red tilapia (Oreochromis mossambicus x O.niloticus) (Hepher et al, 1983), rainbow trout (Oncorhynchus mykiss) (Oorschot and Boon, 1993) need more energy for osmoregulation maintenance when cultured in a saline environment.…”

Osmolality/salinity-responsive enhancers (OSREs) control induction of osmoprotective genes in euryhaline fish

“…Only a small minority of fish (such as marine hagfish and elasmobranchs) are osmoconformers (3). Altering habitat salinity causes stress and evokes compensatory osmoregulatory responses in fish (3).…”

“…Only a small minority of fish (such as marine hagfish and elasmobranchs) are osmoconformers (3). Altering habitat salinity causes stress and evokes compensatory osmoregulatory responses in fish (3). Fish species that tolerate only a narrow salinity range are known as "stenohaline species"; those that can tolerate a wide salinity range are referred to as "euryhaline species" (4).…”

“…They can sense osmotic stress, leading to the activation of osmosensory signaling mechanisms that, in turn, control osmoregulatory effectors to alleviate osmotic stress (5). A large number of osmoregulatory effector genes and their protein products are regulated when euryhaline fish experience salinity stress (3,6,7). The osmosensory signaling networks that control these effector genes are most potent and apparent in euryhaline species with highly dynamic osmoregulatory ability (8).…”