The survival rate during the transition from megalopae to juvenile
crabs was significantly lower than that during transitions between various
zoeal stages. In contrast to the survival rate from the fourth-stage
zoeae to megalopae, abnormal morphologies in the fourth-stage zoeae
and megalopae had no effects on survival during this period. However,
higher CL values in the fourth-stage zoeae improved their survival
after the megalopal stage. In general, the body size of larvae increases
when the culture conditions are optimal and with increasing hatching
size (Anger, 2001b). Therefore, the larval body size was used as an
approximate indicator of the environmental and nutritional conditions
in the larval cultures, which showed that the accumulation of nutrients
and/or energy during the zoeal periodwas important for survival during
the metamorphosis from megalopae into juvenile crabs. Among the
larval culture factors investigated in the present study, later hatching
dates and higher water temperatures significantly reduced the CL in
the fourth-stage zoeae. This agrees with a previous study, which
demonstrated that the size of the first-stage zoeae of the swimming
crab declined with increasing water temperature and later brooding
times throughout the hatching season (Hamasaki et al., 2006; Kurata,
1983). This suggests that the level of nutrients accumulated may be
lower in seed production trials conducted late in the hatching season
and/or with high water temperatures. Based on these results, it is
reasonable to suggest that the reduced survival during the transition
from megalopae into juvenile crabs was caused by a lack of nutrient
accumulation during the zoeal stages.
In this study, we demonstrated that the mortalities during the
transitions from fourth-stage zoeae to megalopae and from megalopae
to juvenile crabs were related to the abnormal immaturity of the
megalopal morphology and the smaller body size of the fourth-stage