Effects of food availability and hypoxia on bivalves: the value of using multiple parameters to measure bivalve condition in environmental studies

摘要

Suspension-feeding bivalves are often included in environmental monitoring because of their trophic position, widespread occurrence and sensitivity to a wide range of environmental factors. Therefore various indices of bivalve condition and growth have been developed, including mor-phometric condition indices (CI), scope for growth (SFG) and nucleic acid ratios (RNA:DNA ratio, RNA:protein ratio or RNA concentration). The sensitivity of these 3 indices has not previously been compared in the same individual bivalves, yet they are likely to have different temporal scales of detectable response to environmental processes. In 2 laboratory experiments (10 and 24 d duration) we investigated the combined effect of food availability and sublethal hypoxia on RNA concentration, CI and SFG in juvenile clams Paphies australis. RNA and CI increased rapidly (5 d) following exposure to high food treatment, but only CI showed a negative influence of hypoxia. Although SFG was affected by both food availability and hypoxia, the patterns were opposite to those of RNA and CI. This anomaly could be explained by the instantaneous change in feeding rates and a slower response in respiration rate (which was correlated with CI) to the uniform conditions under which SFG was determined. Our results emphasise the importance of combined effects of environmental factors on bivalve growth, highlighting the overriding influence of food availability, and imply that a combination of measures will give a more accurate representation of growth rate.