Age and growth analyses of the purple sea urchin Heliocidaris crassispina inhabiting different feeding environments in the Shimane Peninsula, Japan

The purple sea urchin Heliocidaris crassispina is an echinoid responsible for the denudation of rocky shore seaweed in recent years. We used the gonad index to estimate the spawning period and rotula bones of the mouth to determine the age of H. crassispina individuals living in different feeding environments in the Shimane Peninsula. Seasonal changes in the gonad index suggested that the spawning season for H. crassispina was from May to July, with peak spawning in June. Ring forms in H. crassispina once a year based on the timing of the appearance of the dark zone on the rotula bone; this generally begins around June during the peak spawning period. The age (years) and test diameter (TD, mm) data were fitted to various growth equations. The von Bertalanffy equation that separately used data from the “seaweed bed area” and the “isoyake area” (rocky shore seaweed denudation) resulted in the best fit, with TDt=70.5 (1−e−0.174(t+1.018)) and TDt=60.8 (1−e−0.191(t+1.463)), respectively. This suggested that differences in the feeding environment have a significant effect on the growth of H. crassispina, even in the same bay area. Furthermore, the oldest specimen was estimated to be 25 years old, suggesting that H. crassispina has a higher longevity than that previously reported. Therefore, it is important to manage the H. crassispina population by systematic extermination and not by relying on natural mortality to prevent the decline of seaweed beds.