Environmental Salinity Modifies Mucus Exudation and Energy Use in European Sea Bass Juveniles

Simple Summary Changes in skin mucus production and composition offer a new means to study how fish cope with changes in the environment. We explored the utility of skin mucus as an indicator of physiological responses and energy use in a reference fish species, the European sea bass. We evaluated the exudation volume of skin mucus and the main stress- and osmoregulation-related biomarkers in both mucus and plasma. We demonstrate the viability to study the exuded volume of skin mucus composition and its parameters as an informative tool of the fish energy waste at different environmental salinities. This study is of great interest for both aquaculture and ecological studies. The European sea bass (Dicentrarchus labrax) is a euryhaline marine teleost that can often be found in brackish and freshwater or even in hypersaline environments. Here, we exposed sea bass juveniles to sustained salinity challenges for 15 days, simulating one hypoosmotic (3 parts per thousand), one isosmotic (12 parts per thousand) and one hyperosmotic (50 parts per thousand) environment, in addition to control (35 parts per thousand). We analyzed parameters of skin mucus exudation and mucus biomarkers, as a minimally invasive tool, and plasma biomarkers. Additionally, Na+/K+-ATPase activity was measured, as well as the gill mucous cell distribution, type and shape. The volume of exuded mucus increased significantly under all the salinity challenges, increasing by 130% at 50 parts per thousand condition. Significantly greater amounts of soluble protein (3.9 +/- 0.6 mg at 50 parts per thousand vs. 1.1 +/- 0.2 mg at 35 parts per thousand, p < 0.05) and lactate (4.0 +/- 1.0 mu g at 50 parts per thousand vs. 1.2 +/- 0.3 mu g at 35 parts per thousand, p < 0.05) were released, with clear energy expenditure. Gill ATPase activity was significantly higher at the extreme salinities, and the gill mucous cell distribution was rearranged, with more acid and neutral mucin mucous cells at 50 parts per thousand. Skin mucus osmolality suggested an osmoregulatory function as an ion-trap layer in hypoosmotic conditions, retaining osmosis-related ions. Overall, when sea bass cope with different salinities, the hyperosmotic condition (50 parts per thousand) demanded more energy than the extreme hypoosmotic condition.

Automated summary

This study demonstrates how changes in skin mucus production and composition can be used as an indicator of physiological responses and energy expenditure in fish living in different environmental salinities, with European sea bass showing increased energy demand in hyperosmotic conditions.