Seasonal variation in feed intake under constant temperature and natural photoperiod in the green sea urchin (Strongylocentrotus droebachiensis)

Feed intake, body growth, and gonad growth of adult green sea urchin Strongylocentrotus droebachiensis were investigated at a constant temperature (10 °C) and a simulated natural photoperiod (70 °N) for 848 days. Three size groups of sea urchins with an initial test diameter of 30-40 mm (small, S, N = 32), 40-50 mm (medium, M, N = 36), and 50-60 mm (large, L, N = 32) were cultured individually and fed a formulated moist feed in excess. During the first period of the trial, the test diameter (days 0-560), the feed conversion ratio (days 0-560), and the gonad index (GI, days 0 and 560) were recorded. The feed intake (FI) was registered during the entire period (848 days) and varied strongly with season for all size groups. Hence, the FI was significantly higher in summer (0.5-0.6 g per animal per day) than in winter (0.2-0.3 g per animal per day). The test diameter, on the other hand, increased in a linear manner with no apparent seasonality. The S group displayed the fastest increase in test diameter and the highest ingestion rare followed by size groups M and L. By day 560, the GI had reached 33-35% irrespective of size group, which is the highest GI-value reported for this species. Overall, our findings demonstrate that the feed intake of the adult green sea urchins is strongly linked to season at a constant temperature, and that the seasonality in feed intake is maintained for years in captivity.