Hypoxia accelerates the development of respiratory regulation in brine shrimp - but at a cost

The ability to regulate O(2) uptake during exposure to acutely declining O(2) tensions developed early (stage 6) in the brine shrimp Artemia franciscana and co-occurred with the appearance of a functional heart and gills. Culture under chronic hypoxia (P(O2)=10 kPa) resulted in this regulation being brought forward both in development (to stage 3) and in time (hypoxia stimulated early growth), but still before heart and gill formation took place. Consequently, it was suggested that the hypoxia-related early appearance of respiratory regulation is most probably linked to an increase in haemoglobin concentration that occurred at this time. Brine shrimp cultured under conditions of intermittent hypoxia exposure (16 h of normoxia, 8 h of hypoxia) showed a pattern of regulation development intermediate between that of individuals reared in normoxic and chronically hypoxic culture. This occurrence of hypoxia-related, physiological 'heterochrony' in brine shrimp resulted in a decrease in Darwinian fitness (as indicated by a decrease in individual lifetime reproductive output), indicating that, in some cases at least, relatively small alterations in the expression of physiological traits may well have major ecological, and ultimately evolutionary, consequences.