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Crustacean hyperglycaemic hormone function, roles and importance in the diel and seasonal migration physiology of land crabs

NERC (NE/D00246X/1)

Research Student project associated with this study: see details

Steve Morris, Simon Webster* and Ute Postel
*University of Wales, Bangor

Gecarcoidea natalis, feeding on flowers
of Hibiscus tiliaceus on Christmas Is.

The terrestrial red crabs Gecarcoidea natalis are endemic to Christmas Island (Indian Ocean). Since they retain a marine larval phase the crabs must release their eggs into the ocean, which requires the synchronized annual migration of 65 million crabs walking up to 8 km in 5 days. During the dry season from April to November G. natalis retreat to burrows to avoid dehydration, have low activity and metabolism and thus, have less opportunity to accumulate metabolic fuel reserve. During this time, just 5 min of exercise produces severe and immediate demands on supplementary anaerobiosis. However, with the arrival of the monsoon these land crabs instantly begin their breeding migration, which they accomplish without recourse to anaerobiosis. Concomitantly, these land crabs must switch from water conservation in the dry season to water excretion and ion conservation during the rainy season. To do so they recycle up to 95% of the salts from their primary isosmotic urine, by redirecting it across the gills, which function analogously to kidneys. This uptake can be varied in G. natalis by modulating the urine production rate and the extent of salt resorption. How the change from hypo- to hyper-activity is regulated is unknown, and how water conservation is concomitantly switched to excretion without ion loss has not been studied.
Neuropeptides of the CHH family (crustacean hyperglycemic hormone) produced by the X-organ within the eyestalk and released from the sinus gland are implicated in pivotal roles in regulating both processes energy metabolism and salt/water homeostasis. CHH can promote hyperglycaemia and eyestalk factors are implicated in salt and water balance as well as CHHs are involved in water uptake and putatively, in Na⁺ balance.
This project examines diel and seasonal salt and water balance, energy metabolism, utilization of fuel stores and role of CHH, under naturally extreme conditions in the field as well as in the laboratory. The kinetics of CHH expression employs quantitative RT-PCR, while the circulating concentration of CHH is measured by radio-immunoassay. Metabolic states and status are determined by respirometry, metabolite assay, ¹⁴C tracers and calorimetry. Radio-tracers are used as body fluid volume and filtration markers as well as tracers of in vivo metabolism and ²²Na as an uptake tracer and assay Na⁺/K⁺-ATPase activity. In situ field studies examine free ranging, penned and radio-tracked crabs, underpinned by controlled laboratory studies. The work will contribute to our understanding of hypo- and hyper metabolic states in animals and in switching between them. The role of the CHH neuropeptides and bioamines is fundamental to the understanding of the evolution and distribution of land crabs.