With increased prominence of sea urchins as models for biomedical research comes a variability of husbandry practices that should be standardized and optimized for lab-scale culture of sea urchins. Housing should be efficient to maximize the use of limited space, which can be accomplished through usage of commercially-available recirculating aquaculture systems (RAS), already common in the culture of other aquatic animal models like the zebrafish, Danio rerio. These commercial RAS provide tanks that are variable in sizes and volumes; however, their value in housing a demersal species has not been determined. It is important to quantify the impacts of tank design on growth and reproductive outcomes of sea urchins.
We evaluated the impacts of tank size (estimated by available surface area and volume) and stocking density on somatic weight gain and gonad development in sea urchins of Lytechinus variegatus. Eighty adult L. variegatus (ca. 34 g, 40 mm diameter) were collected from St. Joseph Bay in T.H. Stone Memorial Park. The sea urchins were proffered a formulated diet suspended in agar gel over a nine-week period. The use of an agar gel feed enabled quantification of feed intake to best ascertain growth demographics across the different treatments. Statistic tests performed were ANOVAs and ANCOVAs, with post-hoc Tukey and Dunnett’s tests, in RStudio.
The smaller tank dimensions were correlated with a reduction in feed intake and less efficient dry matter production (p < 0.05), but no preferential resource allocation was seen among the test, lantern, gut, or gonad at the tank sizes tested. Increased stocking densities resulted in a similar reduction in feed intake as well as decreased dry matter production (p < 0.005). Competitive behaviors were witnessed within the higher density tanks in which a single sea urchin would “hoard” all proffered feed, preventing the others from eating. Additionally, increased densities resulted in preferential allocation of growth away from the gonad (p < 0.05) in favor of increasing somatic skeleton mass, specifically of the test (p < 0.05). There was also a higher percentage of broken spines seen on sea urchins held in higher densities tanks, suggesting intraspecific interactions led to spine breakage (p < 0.005). Interestingly, spine regeneration also exhibited a much higher degree of variability in the highest density treatment, with males and pre-gametic sea urchins showing much greater rates of regeneration compared to females (p < 0.05), again suggesting a preferential allocation of resources to growth. Overall, this study showed that limited space and increased organismal interactions potentially act as environmental stressors and can reduce growth outcomes in laboratory-scale culture.