Our study highlights that the choice of your laboratory experimental conditions can have a major influence on your experimental outcomes. Sounds like common sense, right? 
Looking through literature, however, it becomes apparent that even the most basic of holding conditions, such as stocking density or water flow rates are often not reported in full detail, or even at all within peer-reviewed literature. This therefore makes it difficult to know how comparable a study is relative to other work and most importantly makes it extremely difficult to replicate work within future studies. To highlight how important these conditions are in shaping your experimental outcomes, my students ran a series of trials testing differing rates of seawater flow or differing stocking densities at which animals were kept. We used two species of sea urchin (Psammechinus miliaris and Paracentrotus lividus) as our model species, commonly used for toxicology, anthropogenic and aquaculture studies. We report different somatic outcomes across a gradient of flow rates and stocking densities as well as species specific responses, highlighting the importance of choice of holding conditions which can influence the outcome of any given experimental treatment. We also provided recommendations for future reporting of housing conditions to increase transparency to the research community.
Here’s some more info about our study:
Suckling, CC, Czachur, M, Ellis, J, Davies, AJ. 2020. European sea urchin somatic & gonadal responses to differing stocking densities & seawater flow rates: a case study for experimental design considerations. Journal Shellfish Research. 39: 159-171.
Abstract: Sea urchins are often used as model species in anthropogenic studies and their aquaculture productions has expanded in recent years, yet basic rearing techniques applied in these studies vary and have received little investigation. This study assessed the effect of seawater flow rates and stocking densities on the somatic and physiological characteristics of European sea urchin species, Paracentrotus lividus and Psammechinus miliaris. Two experiments were carried out for each species; one using low, medium and high seawater flow rates (~0.2, 0.8 and 1.60 L min-1 respectively) and another using low, medium and high stocking densities (~ 38, 76 and 115 ind. m-2 or 0.8, 1.6 and 2.4 kg m-2 (wet mass) respectively). Within the flow rate experiment, P. lividus showed no significant treatment effects. In contrast, P. miliaris reared in low seawater flow rates had significantly lower relative spine lengths and gonad indices compared to conspecifics reared under higher flow rates (medium and high). Within the stocking density experiment, P. miliaris reared under low stocking densities demonstrated significantly larger somatic growth (test diameter and whole animal wet mass) relative to those stocked at high densities but were similar to those stocked at medium densities. For both species, gonad indices were largest within low densities compared to higher stocking densities (high only for P. lividus, and medium and high for P. miliaris). This study suggests that careful consideration for general holding conditions and comparison across anthropogenic studies is required, especially across different species. Furthermore, this information could help improve production effort of European sea urchin species whilst achieving high marketable attributes.
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