Abstract
Terrestrial protein and lipid sources are commonly used as substitutes for marine fishery-derived raw ingredients in fish diets. However, their use is related with several side-effects on marine fish performance, health, or disease resistance. Physical barriers of the skin, gills, and gut constitute the primary defense mechanism of fish. Skin mucus mucosal mucins, water, proteins, ions, and lipids determine the physical, chemical, and protective characteristics of skin mucus. Very little is known about the influence of diet composition on fish skin mucus fatty acid profile. Gilthead sea bream skin mucus contained 10% of total lipids (TL), which consisted of 50–60% neutral (NL) and 40–50% polar lipids (PL) fractions. Σn−3 long chain polyunsaturated fatty acids (LC-PUFA) deposition was preferential in the NL fraction, whereas Σn−6LC-PUFA accumulation was similar in both lipid classes. Docosahexaenoic acid (DHA; 22:6n−3) was the main LC-PUFA stored in skin mucus (14% TL) in relation to eicosapentaenoic acid (EPA; 20:5n−3) (2–3% TL) and arachidonic acid (ARA; 20:4n−6) (2% TL). This study denotes the importance of DHA as component of skin mucus lipids compared to other essential fatty acids, such as EPA and ARA, as well as importance of maintaining an adequate Σn−3/ Σn−6 ratio, regardless of dietary intake.
Highlights
Terrestrial protein and lipid sources are commonly used as substitutes to marine fishery-derived proteins and lipids in gilthead sea bream (Sparus aurata) commercial diets [1,2,3]
The physical, chemical, and protective characteristics of fish skin mucus are determined by its composition [11], its lipid composition and possible functions have been scarcely studied [18]
Regardless of diet composition, gilthead sea bream skin mucus lipid content (10–11% dw: 0.8–1% ww) was higher than that reported in previous studies for several fresh water fish species, such as African catfish (Clarias gariepinus; 5.9% dw) and Malaysia catfish (Clarias sp.1; 4.0% dw) [20], or marine species such as Seriola dumerli (0.62% ww) [21] or Verasper variegatus (0.66% ww) [21]
Summary
Terrestrial protein and lipid sources are commonly used as substitutes to marine fishery-derived proteins and lipids in gilthead sea bream (Sparus aurata) commercial diets [1,2,3]. Even when diets are formulated to cover the theoretical nutrient requirements for essential fatty acids or amino acids [4], variations in their dietary percentage, and in specific ratios among them, have been related with several side-effects on marine fish growth performance, health, welfare, or disease resistance. High levels of dietary plant protein sources may negatively affect the bioavailability of micronutrients [5], contain endogenous anti-nutritional factors, and alter fatty acid profiles, affecting fish growth and fish health and disease resistance [6,7,8,9]. Skin mucus acts as a physical, semipermeable, dynamic, chemical, and biological barrier [12,13]. The physical, chemical, and protective characteristics of mucus are determined by its composition, which is mainly based on water, mucosal mucins, Fishes 2019, 4, 15; doi:10.3390/fishes4010015 www.mdpi.com/journal/fishes
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