Reproductive cycle and growth of the sand dollar Scaphechinus griseus inhabiting Japanese surf clam Pseudocardium sachalinense fishing grounds in Tomakomai, southwest Hokkaido, Japan.

Bioturbation by the sand dollar Scaphechinus griseus is suspected to inhibit the recruitment of Pseudocardium sachalinense, a commercially important bivalve. This study aimed to develop a standard method for the extermination of the sand dollar by examining its reproductive cycle, relationships between age and growth, and population structure in Tomakomai, southwest Hokkaido, Japan. Histological observations of the gonads indicated that the sand dollar has one spawning season per year, in June–August. Seasonal changes in the appearance of a dark band around the interambulacral plate suggested that the annual ring primarily forms in June–July, with the first ring forming at 1 year of age. Analysis of fitness by growth models showed that the Gompertz curve was better matched to the data than the von Bertalanffy and logistic curves; growth is fast until 1.6 years of age, then slows. The population density was 190–257 individuals/m2, and average test diameters were 39.0–40.9 mm. Three-year-olds comprised the highest percentage of the population; no individuals of 8 years of age or older were detected. In conclusion, to reduce the density of sand dollars efficiently and to prevent their reproduction, individuals with a test diameter of over 30 mm should be exterminated before the spawning season. [ABSTRACT FROM AUTHOR]