Sexual Maturation of Octopus vulgaris in Northeastern lyo-Nada　of the Seto Inland Sea, Japan

Abstract

Sexual maturation of six cohorts of Octopus vulgaris was investigated in the northeastern area of the Iyo-Nada of the Seto Inland Sea, Japan. The maturation of females was estimated from the gonadal somatic index (GSI). Females with a GSI≧4 were considered to be mature with observations of oocytes in the ovary. The maturation of males was estimated from the maturation index of male (MIM). Males with a MIM≧40 were considered to be mature with observations of spermatophores in the Needham’s sac. MIM could be used to estimate the sexual maturation of males which had ventral mantle lengths (VML) over 50mm. Females began maturation from VML of 78-115mm at 7-11 months of age in each cohort except altrical individuals of the November-hatched cohort. Females of the individuals began maturation from VML of 134mm at 14 months. Males began maturation from VML of 52-73mm at 4-9 months of age in each cohort except altrical individuals of the November-hatched cohort. Males of the individuals began maturation from VML of 123mm at 13 months. Each of the six cohorts of O. vulgaris in the lyo-Nada was considered to be originated from the several different cohorts. Judging from the presence of semen in the oviducts and oviducal balls, the first mating occurred with VML of 75-82mm at 5 months of age of females in the June and July-hatched cohort, and VML of 65-96mm at 7-10 months in the other cohorts.