Reproductive cycle and first maturation of red sea urchin, Pseudocentrotus depressus in Hirado Island, Nagasaki Prefecture, Japan

Seasonal changes of gonad index (GI) and gonadal histology were examined to clarify the annual reproductive cycle of red sea urchin, Pseudocentrotus depressus. Samples were collected in shallow waters at depth of 3-5 m in Hirado, Nagasaki Prefecture, from May 2002 to March 2005. The GI increased from March and attained a high value from May to October and decreased from November to January. Histological observations of the gonad revealed that oogenesis and spermatogenesis started in September, and then the ovary and testis were filled with mature ova and spermatozoa, respectively, from November to January. Spent gonads were observed from November to March. These results suggested that spawning season lasted from November to January. The maturation size was 24mm in test diameter, corresponding to two years of age for wild populations, and one year of age for hatchery-raised released populations, respectively.