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1 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan
2 Morlab, School of Biological Sciences, Woodland Road, University of Bristol, Bristol BS8 1UG, UK
* To whom correspondence should be addressed. E-mail: ishka{at}ori.u.tokyo.ac.jp
With the arrival of the monsoonal rains and after months of inactivity during the dry season, the terrestrial crab Gecarcoidea natalis embarks on its annual breeding migration to the coast. The physiological demands of the migration were assessed by determining respiratory gases in the hemolymph, key metabolites, and energy stores in G. natalis during two migratory seasons. At the end of each day of migration the pulmonary hemolymph Po2 decreased by 1-2.5 kPa, but the hemocyanin remained saturated with O2 and the venous reserve was largely unchanged (O2 > 0.4 mmol · l-1). The breeding migration of red crabs was accomplished without recourse to anaerobiosis, even though at times walking speeds (up to 6.2 ± 0.5 m · min-1) exceeded those that promoted anaerobiosis in non-migrating crabs and in crabs exercised in the laboratory. In contrast to all previous studies, at the end of each day of migrating, red crabs experienced an alkalosis (up to 0.1 pH units) rather than any acidosis. This alkalosis was removed overnight when the crabs were inactive.
Although there were seasonal fluctuations in the glycogen, glucose, and triglyceride stores, crabs engaging in the migration did not draw on these stores and must have fed along the way. In contrast, crabs returning from breeding activities on the shore terraces had significantly depleted glycogen stores. Additionally, in 1993, the male crabs returning from the breeding activities on the terraces were dehydrated and experienced a decrease in muscle tissue water of 11%. In contrast to the breeding migration per se, fighting for burrows in which breeding occurs produced severe anaerobiosis in males, especially the victors: after 135 s of combat, the maximum L-lactate concentration in the hemolymph was 35 mmol · l-1. It appears that burrowing, courtship, and mating are more demanding than the migration itself. Furthermore, the data provide evidence that the metabolic responses of migrating individuals of G. natalis might be different from those at other times of the year.
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