SQUID (LOLLIGUNCULA-BREVIS) LIFE IN SHALLOW WATERS - OXYGEN LIMITATION OF METABOLISM AND SWIMMING PERFORMANCE

Squid (Lolliguncula brevis) were exercised in a tunnel respirometer du ring a stepwise increase in water velocity in order to evaluate the an aerobic threshold, i.e. the critical swimming speed above which anaero bic metabolism contributes to energy production, The average anaerobic threshold was found at speeds of 1.5-2 mantle lengths s(-1). Above th is velocity, alpha-glycerophosphate, succinate and octopine started to accumulate in the mantle tissue, ATP levels fell and phospho-L-argini ne was progressively depleted, while the levels of glucose 6-phosphate and inorganic phosphate rose, The finding of a simultaneous onset of anaerobic metabolism in the cytosol and the mitochondria indicates tha t a limited oxygen supply to the mitochondria elicits anaerobic energy production, This finding is opposite to the situation found in many o ther vertebrate and invertebrate species, in which energy requirements in excess of aerobic energy production are covered by anaerobic metab olism, with mitochondria remaining aerobic, In L. brevis, swimming at higher speeds is associated with a small factorial increase in metabol ic rate based on a high resting rate of oxygen consumption, Pressure r ecordings in the mantle cavity support this finding, indicating a high basal level of spontaneous activity at rest and a small rise in mean pressure at higher swimming velocity, Bursts of higher pressures from the jet support elevated swimming speeds and may explain the early tra nsition to anaerobic energy production which occurs when pressure ampl itudes exceed 1.2-1.5 kPa or when mean pressure rises above 0.22-0.25 kPa. The finding of mitochondrial hypoxia at a low critical speed in t hese squid is interpreted to be related to their life in shallow coast al and bay waters, which limits the necessity to maintain high swimmin g velocities, At increased swimming velocities, the animals oscillate between periods of high and low muscular activity, This behaviour is i nterpreted to reduce transport cost and to permit a longer-term net us e of anaerobic resources when speed exceeds the critical value or when the squid dive into hypoxic waters, The simultaneous onset of anaerob ic metabolism in the cytosol and the mitochondria emphasizes that squi d generally make maximal use of available oxygen under resting conditi ons, when their energy requirements are the highest among marine inver tebrates.