Sexual Differentiation in Juvenile American Lobster (Homarus americanus)

American lobsters (Homarus americanus H. Milne Edwards, 1837) settling to the bottom after their pelagic larval stage are sexually undifferentiated. Based on stereomicroscope observations, the female's gonopore first appeared for lobsters about 13 mm carapace length (CL) and the dimorphism of the first pair of male pleopods was first observed at about 15 mm CL. Based on histological observations, the internal reproductive system for both males and females began differentiating at sizes >= 20 mm CL. The vasa deferentia were observed for males >30 mm CL indicating that the male reproductive system is fully formed. For females, the ovaries were observed at sizes >= 20 mm CL, but the oviducts were not detected in animals <36 mm CL. The male reproductive system is morphologically complete at a substantially smaller CL than for females.