Therewas great variation in the morphologies of the megalopae and
fourth-stage zoeae. However, the megalopal morphology had stronger
effects on survival during the transition from the fourth-stage zoeae to
the megalopae than did the morphology of the fourth-stage zoeae.
The survival rate tended to decreasewhen themegalopae retainedmorphological
features of the zoeal stage, such as the dorsal spine and furcae
on the telson, which suggests that this immature megalopal morphology
is abnormal. Larval samples were collected on the next day after
moulting, and the larval survival rates were estimated during the
night-time. Most of the abnormal immature megalopae died within a
few days after their metamorphosis. Thus, the immature morphological
abnormalities of the megalopae may be the major cause of the mass
mortality during the metamorphosis into the megalopal stage in recent
swimming crab seed production trials in Japanese hatcheries. The
reduction in survival during the metamorphosis into megalopae also