Physiological responses of mussel larvae Mytilus edulis to environmental hypoxia and anoxia

Anoxia tolerance of A4fl'lus edufis larvae, expressed in terms of median mortality time, increased from 14 ? h in early prodissoconch larvae (106 ,pm) to 38 ? h in later veliconch larvae (224 pm). Both embryos and early prodlssoconch larvae developed and grew normally at Po2 values 2 3.16 kPa. Feeding activity by the early larval stages generally was maintained or even enhanced under hypoxlc conditions ( 2 3.16 kPa). In contrast, rates of feeding and growth by later stages were depressed at all hypoxic levels examined. Hypoxia had little influence on settlement behaviour of pediveliger larvae, but larvae developed their 'eye-spots' at a smaller size in such circumstances, indicating the uncoupling of growth and morphogenic processes under hypoxlc stress. These physiological responses of mussel larvae were also supported by calorimetric and respirometric measurements of heat dissipation and oxygen uptake Early larval stages were able to maintain their energy metabolism at reduced PO, (critical PO% = 2 kPa), whereas later stages suppressed heat dlssipatlon at moderate hypoxia. There was no significant anaerobic component of total energy expenditure by mussel larvae under hypoxic c o n l t i o n s above ca 1 kPa. Changes occurring during larval development resulted in a more oxygendependent metabolic rate above a critical larval size of 150 to 160 pm, presumably due to a decrease In surface area to volume ratio, and the secretion of the prodissoconch I1 shell, which may limit oxygen diffusion and uptake.