To determine still finer temporal resolution of CHH levels, we
decided to investigate changes in CHH levels during the night-today
transition. Just before dawn, for about 2h, red crabs become
very active. We reasoned that this should place significant energy
demands upon the crabs, and previous observations had suggested
that the night-to-dawn transition was in G. natalis often associated
with hyperglycaemia (S.M., unpublished observations). Dawn
hyperglycaemia has been observed in the prawn Macrobrachium
rosenbergii (Kuo and Yang, 1999), conversely both hyperglycaemia
and CHH release associated with the day-to-night transition has been
seen in a nocturnally active crustacean, Orconectes limosus (Kallen
et al., 1990). However, for the shrimp Penaeus monodon, diel
variations in haemolymph glucose were not observed (Hall and Van
Ham, 1998). Because dawn activity is associated with foraging in
G. natalis, it seems possible that feeding-related hyperglycaemia
could mask any endogenous diurnal rhythmicity in glucose levels,
and consequently could inhibit CHH release. Further laboratorybased
experiments are now needed to resolve this issue.