Population structure of the sand dollar Echinarachnius parma in the subtidal zone of the northern Gulf of StLawrence, eastern Canada

We examined changes in populations of Echinarachnius parma with depth on subtidal sediment slopes in the Mingan Islands. Size distributions showed the presence of two major groups measuring 1-21 and 37-62 mm in length, respectively. Intermediate-sized sand dollars were rare. Juveniles (<28 mm in length) were extremely abundant at 16 and 20 m depth (460-660/m(2)) and decreased in number with decreasing depth, whereas the density of adults was relatively stable at different depths. Juveniles were more frequently buried (95%) than adults (30%). Analysis of growth lines and of shifts in juvenile cohorts indicated that growth rate increased with size up to a maximum between 20 and 40 mm and then decreased sharply. The bimodal size structure is possibly due to the accelerated growth rate at intermediate sizes. We hypothesize that sand dollars move to shallower water with increasing size to take advantage of food resources, possibly benthic diatoms, which are more abundant there. Large individuals are probably better adapted than juveniles to exploiting shallower water because they are less likely to be transported by water turbulence.