In euryhaline teleosts , salinity may affect gonadal development and reproductive steroidogenesis. The interest on the euryhaline flathead grey mullet ( Mugil cephalus ) is growing due to its highly sustainable and profitable rearing. However, applicable protocols for commercially farming the animals are currently absent as mullet fails to complete gametogenesis in captivity. This study aims at i ) verifying if the transfer of fish from fresh- to saltwater conditions triggers the gonad development; ii) describing the physiological cascade leading to sexual maturation; iii) assessing the lipidomic biochemical changes in gonads and fillets during maturation.
A sub-sample of 3 years old M. cephalus produced at IMC hatchery and farmed in freshwater ponds were transferred at the beginning of the reproductive season (July) into seawater in a lagoon enclosure . We monitored i ) the gonad development ii) the physiological reproductive status ( steroid hormones concentration and gonadotropins receptors gene expression), iii) the lipid composition of gonad and fillet. Equivalent samples were also collected from the population in freshwater and from wild animals during their seasonal reproductive migration.
After the transfer to saltwater, 14% of females developed vitellogenic oocytes. On the other hand, in freshwater only 4 % of females had vitellogenic oocytes while 24% of wild females reached full maturity. In saltwater, t he increase of plasmatic estradiol was significant at the beginning of vitellogenesis (1.14±0.52 ng/ml) , causing a related growth of the oocyte diameters 3 weeks later (377±77 μm). Estradiol followed a similar trend in freshwater and wild females, although with no significant differences during maturation . Fshr expression in vitellogenic stages was down-regulated or not detected in seawater and freshwater farmed fish ; lhr expression was up-regulated in maturing fish in saltwater and maturing wild females. Gonad total lipid content showed an increase in maturing stages. In saltwater females, there was an elevation of PUFAs during the immaturity phase, w hereas SFAs and MUFAs remained stable in both gonadal and muscle tissues. During the late vitellogenesis stage, there was a significant shift in the gonadal fatty acid profile: an increase in mol% of total fatty acids for 18:0 and 18:3n-6, while 20:3n-6 and 18:1n-7 exhibited a decrease. In the muscle tissue of farmed females (both fresh- and seawater), there was a noted deficiency in EPA, ARA, and DHA when compared to their wild counterparts. Conversely, 18:2n-6 and 18:1n-9 were present in higher concentrations, mirroring the dietary inputs and suggesting that the dietary regime may not be optimally tailored to foster an ideal tissue fatty acid composition.
These results suggest that freshwater farmed mullet females can mature similarly to wild individuals in few weeks, if moved during the natural reproductive season to saltwater conditions as described. Our results also shed new light on the physiological and biochemical processes underpinning grey mullet gametogenesis , helping to define new protocols for induction of sexual maturation in captive conditions.