ABSTRACT
We examined changes in density and size structure of sand dollars, Echinarachnius parma, at different depths on subtidal sediment slopes in the Mingan Islands, northern Gulf of St. Lawrence. Size distributions showed the presence to two major groups, measuring 1 to 21 mm and 37 to 62 mm in length, respectively. Intermediate-sized sand dollars were rare. A market size partitioning occurred with depth. Cohorts of juveniles, < 28 mm in length, were extremely abundant at 16 to 20 m (460–660 individuals m−2), decreased in abundance at 12 and at 4 m, and were rare at shallower depths. By contrast, the density of adults, > 28 mm, between 2 and 20 m in depth was consistently low (25 to 80 m−2). Most juveniles were buried (84% at 2 m to 98% at 20 m in depth) whereas 70% of adults were at the surface. Analysis of growth lines and of shifts in positions juvenile cohorts from 1991 to 1993, indicated that the growth rate increased with size up to about 28 mm, the size at sexual maturity. Rapid growth continued until about 40 mm and then slowed for larger individuals. The maximum size attained was 50–60 mm, and the oldest sand dollars sampled were 18 to 26 year old. The ratio of males to females was 1.19 and did not vary with depth. The bimodal size-structure may be due to the accelerated rate of growth at intermediate sizes (25–40 mm). Alternatively, the drop in individuals beginning at about 21 mm may be due to increased vulnerability to predators, perhaps associated with an increased tendency to live at the surface. 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 depths because, due to their increased body mass relative to length, they are less likely to be transported by water turbulence.
