What makes a competent aquatic invader? Considering saline niches of invertebrates and ray-finned fishes

Aquatic invasive species are of growing concern globally, especially in fresh water. The problem is intensified by climate change, which often causes salinization of coastal fresh waters. Animals deal with salinity through the function of osmoregulation, and osmoregulatory ability can be informative when considering invasive potential. A species is said to be 'euryhaline' if it can tolerate a wide range of salinities, either through osmoregulation (tightly controlling its extracellular fluid osmolality) or osmoconformation (matching the osmotic concentration of its internal fluids with that of the environment). Euryhaline animals display a large fundamental saline niche (FSN); i.e. a wide physiological tolerance of salinity change. However, the range of salinities of the habitats where a species actually occurs define its realized saline niche (RSN). Importantly, aquatic species living in stable habitats (i.e. those with little variation in salinity) will have a small RSN, but may have large FSNs, depending on their evolutionary history. Species with large FSNs are more likely to be successful invaders of new habitats with different salinities. Here, I propose the term 'osmotic comfort' as a concept that is associated with the FSN. The core of the FSN corresponds to ∼100% osmotic comfort, or 'optimum salinity', putatively meaning minimum stress. Physiological markers of osmotic comfort can provide raw data for mechanistic niche modelling in aquatic habitats. A species with a larger FSN is more likely to remain 'osmotically comfortable' in a different saline habitat, and is less likely to suffer local extinction in fresh waters, for example, that undergo salinization.