The edible crab Cancer pagurus, brood as many as three million embryos,
twice the usual allometric relationship found in brachyuran crustaceans. The
embryos are tightly packed on the abdomen where they grow and develop until
hatching takes place 6-9 months later. Ovigerous females remain buried for
greater than 25% of embryonic incubation, which exposes the embryos for
prolonged periods to severe hypoxia ('.S 2 kPa).
Synthetic correlates of growth and metabolism, and the influence of short
term hypoxia on these were analysed at four key phases in embryogenesis: the
blastula; limb formation; eye; and hatching phases, which were identified from a
novel developmental scheme. Cellular responses to hypoxia were also analysed.
Whole embryo rates of oxygen uptake ( M02) scaled isometrically with
body size during development, although values were lower than predicted by
tissue mass. Rates of protein synthesis (ks) showed a phase specific relationship
over the same period at 10°C, reaching maximum mean values of24.4 % day-I at
the eye phase, which coincided with the major growth period and increase in
developmental rate. By hatching phase mean ks had decreased to 6.3 % day- I and were similar to the mean values recorded at limb formation. Generally there
wasn't a close relationship between M02 and ks during development, resulting in
variable metabolic costs, ranging between 131 mmol ATP g protein•1 at the eye
phase, to 469 mmol ATP. g protein•1 synthesised at hatching phase.
The physiological effects of Hypoxia on embryonic metabolism were
more pronounced on M02 compared with ks. M02 at the early developmental
phases were less sensitive to hypoxia, particularly the blastula, where metabolism
was relatively quiescent. The eye and hatching phases were oxygen conformers,
which appears to relate to an increase in metabolising tissue, an increase in
growth, and restriction to oxygen uptake. Generally ks values were reduced and
stable in hypoxia at approximately 5 % day- I. At the eye phase progressive
hypoxia resulted in a 5-fold and rapid decrease to normal values, caused by
changes in kRNA, since RNA levels remained unchanged. Hypoxic ks and M02
values were directly correlated, generating a fixed metabolic cost for protein
synthesis of 149 mmol ATP. g protein-1.
Characterisation of the cellular response to hypoxia showed evidence of
high basal levels of heat shock protein 70 and related homologues. Preliminary
evidence was also suggestive of hypoxia specific protein expression patterns,
some of which could also be part of the Hsp70 family. An indication of an arrest
or extension to the cell cycle during development, may fonn another important
element of the cellular defence utilised by embryonic C. pagurus against natural
brooding hypoxias. Taken as a whole, the suppression to rates of protein
synthesis and the cellular responses to hypoxia appear appropriate to the natural
occurrence of hypoxia in the brooding egg mass.