Diel observation on the distribution of the sea urchin Strongylocentrotus intermedius under different food availability and shelter conditions in the laboratory

The present study investigated the diel distribution of Strongylocentrotus intermedius in different laboratory food availability and shelter conditions. The diel rhythm of sheltering behaviour was similar between sea urchins in groups 1 (food inside shelter) and 3 (no food available), but significantly different to group 2 (food outside shelter). Among the three treatments, we found significantly more urchins outside shelter in group 2 than in groups 1 and 3 (p?<?0.05), while no significant difference was found between sea urchins in groups 1 and 3 (p?>?0.05). Kruskal–Wallis test confirmed this conclusion. When food was inside shelter, we found significantly more sea urchins staying on food than at other places inside the shelter (p?<?0.001). When food was outside shelter, significantly more individuals chose to stay in shelter rather than on food outside shelter (p?=?0.001). The present study provides new insights into the trade-off of sea urchins between foraging and sheltering behaviours.