Metabolic readjustment in juvenile South African abalone (Haliotis midae) acclimated to combinations of temperature and dissolved oxygen levels

Abstract

When ectotherms are exposed to high temperatures, cellular oxygen demand increases. When oxygen demand exceeds oxygen availability, animals may experience functional hypoxia regardless of environmental oxygen levels.In the current study, we aimed to assess if oxidative or thermal restrictions exist when winter-acclimated Haliotis midae juveniles are re-acclimated to temperatures within their thermal tolerance range, but with one temperature outside the average daily temperatures they would experience in winter. At each acclimation temperature (14, 16 and 19°C), we also manipulated dissolved oxygen levels to obtain oxygen saturation levels of 82%, 98% and 126%.Our results indicate growth restrictions and hypometabolism at all oxygen treatments at 19°C. Efficiency of mitochondrial respiration, as indicated by an increase in cytochrome c oxidase subunits 1 and 2 (cox1 and cox2) transcript level, was improved at 19°C, 98% oxygen. A change in substrate utilisation, from a glycogen/lipid-driven metabolism to almost exclusively protein-driven metabolism provided the fuel for more effective cellular respiration. A further decrease in metabolic rate at 19°C, 126% oxygen, this time without an increase in cox1 and cox2 transcripts, may indicate some improvement in gas exchange, but without improvement in growth.We propose that 19°C is nearing a long-term limiting temperature for winter-acclimated SA abalone, and that animals survive this condition by re-allocating available energy to survival mechanisms instead of growth. This species, already threatened by over-harvesting, may be further threatened by increases in surface seawater temperatures.