Spawning Season of the Japanese Mantis Shrimp Oratosquilla oratoria in Tokyo Bay

Abstract

Spawning season of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay was ex-amined from seasonal changes in percentage occurrence of mature females and in female gonad index. A total of 15, 871 specimens was sampled in Tokyo Bay from April to December in 1982 and from August 1986 to September 1987.
Analysis showed that spawning season lasted from April to August. Percentage occurrence of mature females during the spawning season showed two peaks, the first one from April to May, and the second from July to August. The first peak was caused by the maturation of females older than 2 years with body lengths above 10cm. In July or August, 1 year old females with body lengths above 8cm matured and contributed to the second peak. The result of the present study showed that the age of maturation of the mantis shrimp in recent years has become rather earlier compared with that in the 1950s in Tokyo Bay.