Seasonal, ontogenetic and sexual changes in lipid metabolism of the small-spotted catshark (Scyliorhinus canicula) in deep-sea free-living conditions

• Elasmobranchs exhibit variations in nutritional conditions throughout the year.
• Small-spotted catsharks were blood-sampled and released alive.
• Triglycerides and phospholipids differ ontogenetically in the small-spotted catshark.
• Cholesterol levels change seasonally in the small-spotted catshark.
• Lipid metabolites are more influenced by reproduction than nutrition.

Marine predators, such as elasmobranchs, exhibit variations in nutritional conditions related to both reproductive traits and food availability in the marine environment throughout the year. The main objective of this study was to examine changes in several blood physiological parameters in a demersal shark, the small-spotted catshark (Scyliorhinus canicula), in the wild in relation to season, sex and maturity stage. For this purpose, 108 individuals at different developmental stages were captured and released alive in the western Mediterranean. Blood was obtained from caudal vessels and plasma lipid fractions (total cholesterol, triglycerides and phospholipids) and a ketone body (3-β-hydroxybutyrate) were measured. During summer, plasma triglyceride and phospholipid levels were lower in adults than in juveniles (mainly in females, probably related to breeding season and laying eggs). Plasma cholesterol levels also showed higher values in summer, indicating higher physical activities during summer and revealing that lipid fractions are more related to reproduction than to nutrition. Plasma 3-β-hydroxybutyrate variations showed a different pattern. No differences were found between sex or maturity stage during summer, although the highest values in adult and juvenile males during winter indicates higher physical activity of males. This study, uses an innovative methodology to establish a correlation between lipid fractions and ketone bodies from the blood of wild individuals and changes in sexual and nutritional status. This method was conducted without damage to the target species and provides new information on the physiology of this abundant elasmobranch in the Mediterranean Sea.