Seasonal Variation in Body Growth, Density and Distribution of a Population of Sand Dollars, Mellita Quinquiesperforata (Leske)

Rate of growth, densities and distributions of a population of sand dollars were measured from 1971–1974 at a subtidal beach adjacent to the mouth of Tampa Bay, Florida. The population was found in a continuous band in the study area and was characterized by a negative binomial distribution with clumping on most sampling dates. The population was dominated in numbers of individuals and biomass by the 1971 year class throughout the study period. Settlement of subsequent year classes (1972 through 1974) within the study area was poor. Average number of individuals/m² ranged from 489 at the beginning of the study to 6 on the last sampling date. Highest densities occurred in deeper water. Mortality of the 1971 year class was 95%, 88% and 61% in the first, second and third years of life, respectively. From size-frequency analysis, the average size of an individual spawned in 1971 was 46.5 mm (1.7 g dry weight), 72.0 mm (6.4 g dry weight) and 107.5 mm (19.9 g dry weight) after the first, second and third years of life, respectively. The calculated “K” value (rate of attaining maximum size) was 0.46. Rate of body growth was highest in summer and lowest in fall with negative body growth recorded for larger animals in fall. The body growth rate declined with increasing age. Comparison of body dimensions and weights showed that dry weight was proportional to diameter^3.03 larger animals were slightly flatter and had a higher dry weight:wet weight ratio than smaller animals. Mortality of larvae shortly after settlement was postulated to explain poor representation of year classes after 1971.