Size-specific movement of green sea urchins Strongylocentrotus droebachiensis on urchin barrens in eastern Canada

Movement is likely a major factor contributing to the marked size partitioning of populations in relation to depth and habitat, as observed for many coastal invertebrates. Here, 2 approaches were used to examine the relationship of movement to size for the green sea urchin Strongylocentrotus droebachiensis on persistent urchin barrens in the northern Gulf of St. Lawrence, eastern Canada. The first approach involved quantifying the dispersal of tagged urchins from release points. Experiments were started by tagging all urchins in 1.0 m(2) plots with a fluorescent stain, then quantifying their abundance 9 or 40 d later in the release plots and at different distances from the release plots. The rate of dispersal of urchins from the plots varied greatly with size. In the initial plots, we recaptured 69% of <10 mm diameter urchins, compared to 2% for >15 mm diameter urchins, after 9 d, and 25 and 0%, respectively, after 40 d. In 5 of 6 trials executed, movement was directional, albeit in different directions in the different trials. The second approach evaluated urchin movement relative to size by quantifying the numbers of different-sized urchins moving into 1.0 m(2) circular plots, from which the natural population of urchins had been removed 48 h earlier. Trials were made either with or without algae in the center of the plots. After 48 h most urchins which had moved into the plots were large (>15 mm diameter). The movement of urchins was size-dependent, as the presence of algal food affected the rates of movement of large urchins into the plots, but had no detectable effect on small (<15 mm diameter) urchins. Both approaches demonstrated that urchins measuring <15 mm diameter (juveniles) have a relatively sedentary and cryptic lifestyle. Those >15 mm diameter showed increased movement, probably related to the search for food. This ontogenetic change in movement likely affects growth rates and contributes to size partitioning of urchin populations in different subtidal habitats.