Growth and feeding resilience of green sea urchin ( Strongylocentrotus droebachiensis ) to visible-light quantity and quality

We used a 298-day (July to April) laboratory mesocosm experiment with Newfoundland green sea urchins ( Strongylocentrotus droebachiensis ) to test the prediction that growth and feeding are largely unaffected by light quantity and quality within the visible-light spectrum. Sea urchins experienced either of three light environments simulating differences in illuminance and red/blue light ratios among common habitats for the species: tide pool, shallow subtidal, and deeper subtidal habitats. Sea urchins were fed exclusively kelp ( Alaria esculenta ) and exposed to natural photoperiod. Light had no effect on body diameter and weight. Diameter, but not weight, changed over time with a low, yet statistically significant, 3% increase from August to February. Gonadosomatic index (GSI) of sea urchins in the mesocosms doubled from ~ 10% in Aug to ~ 20% in February, with no further change in April, independently of light. GSI of sea urchins in the field was comparatively lower throughout the experiment, being similarly low, ~ 5%, in August, December, and April, and peaking significantly higher at ~ 9% in February. Light also did not affect feeding on kelp, which, however, differed significantly among the three sampling months, peaking in November. Seasonal patterns of feeding in the mesocosms followed that in natural habitats. Our findings speak highly to the relative insensitivity of the species to its visible-light environment within the range tested. They also reinforce the paradigm that in green sea urchin, visible-light quantity and quality have a much lesser impact on somatic and gonadic growth and feeding than food quality and availability.