Introduction
Hatchery production of the European flat oyster Ostrea edulis remains constrained by prolonged broodstock conditioning, inconsistent spawning, disease and limited seed availability, restricting aquaculture expansion and restoration efforts (Lynch & Culloty, 2022; zu Ermgassen et al., 2023). Seaweed-derived functional ingredients offer a sustainable strategy to enhance broodstock performance. Fucoidan and ��-carrageenan, sulphated polysaccharides extracted from brown and red macroalgae, together with selenium, possess recognised immunostimulatory, antioxidant and reproductive health-promoting properties (Li et al., 2008; Rudtanatip et al., 2018; Lynch et al., 2021). This study evaluated, for the first time, the effects of these functional ingredients on broodstock reproductive development, fecundity and pathogen prevalence during hatchery conditioning of O. edulis.
Materials and Methods
Two age cohorts of the O. edulis (2- and 3-year-old broodstock) were conditioned over a 10-week laboratory trial under hatchery conditions using gradually increasing water temperatures (14–18°C). Twenty-four 12-L tanks containing 30 oysters per tank were maintained. Oysters were allocated to one of four treatment groups: control, fucoidan (Fucus vesiculosus), ��-carrageenan (Chondrus crispus) or selenium, with three replicate tanks per treatment and age cohort. Treatments were administered weekly for 10 weeks at doses of 150 μg fucoidan g-1 whole wet weight, 300 μg ��-carrageenan g-1 whole wet weight and 167 μg sodium selenite L-1 tank water. Weekly samples were analysed. Broodstock reproductive development was assessed by histological examination of gonadal tissue using the gametogenic staging system of da Silva et al. (2009), together with fecundity estimates based on egg and sperm follicle abundance. Oyster health was evaluated through survival, multiplex PCR screening for Vibrio spp. (V. aestuarianus, V. splendidus, V. alginolyticus and V. anguillarum), PCR screening for Bonamia ostreae, and heart smear examination.
Results
By Week 5, fucoidan- and ��-carrageenan-treated oysters exhibited reproductive development comparable to wild broodstock, indicating accelerated maturation under hatchery conditions. Treated oysters showed a greater frequency of advanced gametogenic stages, increased reproductive activity and enhanced sex switching, while ��-carrageenan produced the highest fecundity. Fucoidan consistently resulted in the lowest cumulative mortality, whereas controls exhibited the highest mortality, particularly in the older broodstock cohort. Vibrio spp. prevalence declined from 68.3% in initial broodstock to 25.0%, 15.0%, 11.7% and 13.3% in the control, fucoidan, ��-carrageenan and selenium treatments, respectively. B. ostreae prevalence similarly decreased from 76.3% initially to 45.0%, 28.8%, 28.3% and 40.7%, with the greatest reductions observed following fucoidan and ��-carrageenan supplementation.
Discussion
Reliable broodstock conditioning remains a major constraint to the hatchery production of O. edulis, limiting consistent seed availability for aquaculture and restoration programmes. These findings demonstrate that macroalgal-derived biocompounds can reduce broodstock conditioning time and associated energy costs while improving spawning reliability. Accelerated maturation has the potential to reduce hatchery operating costs by shortening conditioning time while improving spawning reliability and larval production. The accelerated progression of gametogenesis and increased fecundity observed in treated oysters suggest these sulphated polysaccharides may improve physiological readiness for spawning, while the reduced prevalence of Vibrio spp. and B. ostreae indicates additional health benefits consistent with their recognised immunostimulatory properties (Li et al., 2008; Lynch et al., 2021). Although selenium provided some benefits, the strongest and most consistent responses were associated with the macroalgal-derived biocompounds. These findings support the use of sustainable seaweed-derived functional ingredients as a nutritional strategy for improving broodstock conditioning, hatchery efficiency and reliable seed production of O. edulis. Collectively, these findings support the integration of sustainable seaweed-derived biocompounds into broodstock conditioning protocols to improve hatchery efficiency, reliable seed production and the success of European flat oyster restoration programmes.
Acknowledgements
Research carried out with the support of the Marine Institute, funded under the Marine Research Programme by the Government of Ireland, SusAqua Project ref.: PDOC/21/01/04.
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