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

Abstract

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‰), one isosmotic (12‰) and one hyperosmotic (50‰) environment, in addition to control (35‰). 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‰ condition. Significantly greater amounts of soluble protein (3.9 ± 0.6 mg at 50‰ vs. 1.1 ± 0.2 mg at 35‰, p < 0.05) and lactate (4.0 ± 1.0 µg at 50‰ vs. 1.2 ± 0.3 µg at 35‰, 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‰. 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‰) demanded more energy than the extreme hypoosmotic condition.

Keywords: Dicentrarchus labrax; gill Na+/K+-ATPase; mucus exudation; osmoregulation; salinity adaptation.

Figure 1

Gill Na⁺/K⁺-ATPase activity of European sea bass juveniles in response to a chronic osmotic challenge after 15 days. Values are shown as mean ± standard error of mean. n = 10. Letters indicate significant differences among salinities challenges (p < 0.05, ANOVA and post-hoc Tuckey test). The value of 35‰ (white bar) is assumed as control value of seawater salinity.

Figure 2

Gill mucous cell count of cell frequency (A), size (B) and shape (C) and gill mucous cell count of acid (D) and neutral (E) mucins of European sea bass juveniles in response to a chronic osmotic challenge. Image of histological differentiation of acid and neutral mucins (F). Scale bar: 0.5 μm. Values are shown as mean ± standard error of mean. n = 10. Different letters indicate different groups of significance among salinities challenges (3‰, 12‰, 35‰ and 50 ‰) by one-way ANOVA analysis and post-hoc Tuckey’s test (p < 0.05). The value of 35‰ (white bar) is assumed as control value of seawater salinity.

Figure 3

Skin mucus exudation parameters of European sea bass submitted to a chronic osmotic challenge. Values are shown as mean ± standard error of mean. n = 10. Letters indicate significant differences among salinities challenges (p < 0.05, ANOVA and post-hoc Tuckey test). 35‰ (white bar) is assumed as control value of seawater salinity.

Figure 4

Skin mucus osmolality (A) and main osmotic-related ions (B–D) of European sea bass submitted to a chronic osmotic challenge. Values are shown as mean ± standard error of mean. n = 10. (A) Black arrows correspond to measured osmolality of surrounding water (3‰ = 115 mOsmol·kg⁻¹, at 12‰ = 320 mOsmol·kg⁻¹, at 35‰ = 931 mOsmol·kg⁻¹, at 50‰ = 1366 mOsmol·kg⁻¹). Different letters indicate different groups of significance among salinities challenges (3‰, 12‰, 35‰ and 50 ‰) by one-way ANOVA analysis and post-hoc Tuckey’s test (p < 0.05). 35‰ (white bar) is assumed as control value of seawater salinity.

Figure 5

Total exuded biomarkers (A–D) and glucose:lactate ratio (E) in skin mucus of European sea bass juveniles in response to a chronic osmotic challenge. Values are shown as mean ± standard error of mean. n = 10. Different letters indicate different groups of significance among salinities challenges (3‰, 12‰, 35‰ and 50 ‰) by one-way ANOVA analysis and post-hoc Tuckey’s test (p < 0.05). The value of 35‰ (white bar) is assumed as control value of seawater salinity.