Effects of long-term photoperiod on growth, gonad yield and health status of the sea urchin Strongylocentrotus intermedius

Enhancing growth, gonad yield, and health status in cultured sea urchins is crucial. This study examined the growth, gonadal development, and immune response of the sea urchin Strongylocentrotus intermedius subjected to five distinct photoperiod regimes (light–dark ratio of 0L: 24D, 8L: 16D, 12L: 12D, 16L: 8D, and 24L: 0D) for a duration of 13 months. The findings indicated that sea urchins in the 0L treatment achieved the largest body weight overall, however, they attained the maximum specific growth rate (SGR) only during the initial 0 to 3 months stage. In the latter phases, sea urchins subjected to the 12L or 16L treatment attained the highest SGR. This outcome indicated a heightened requirement for light as body size increases. The photoperiod regime significantly influenced the gonad yield of the experimental sea urchins (P < 0.05), with individuals in the 16L treatment attaining the greatest gonad weight and the highest gonad index by the conclusion of the experiment. The photoperiod regime did not significantly influence the gonadal coloration of the experimental sea urchins. Histological analysis indicated that the proportion of mature individuals decreases with the reduction of daily light duration. The 0L and 8L photoperiods may regulate the excessive gonadal development in young S. intermedius. At the end of the experiment, no significant change in the phagocytic rate or phagocytic index was seen between the treatments. Finally, long-term photoperiods did not influence the survival rate or immunity of S. intermedius. Our findings indicated that in land-based aquaculture systems, the sea urchin S. intermedius may be cultivated in dark or under reduced daily light to achieve rapid growth, followed by cultivation in a 16L photoperiod to enhance gonad output at harvest.