ABSTRACT: RNA/DNA ratios are used in many organisms as an indicator of growth, biomass or metabolic functioning. We hypothesised that hermatypic corals, as they depend for growth largely on energy transferred from their algal symbionts, may show an effectin the RNA/DNA ratio with factors influencing irradiance. This was tested along 2 environmental gradients: in relation to depth (decreasing irradiance) and in relation to environmental degradation (increased turbidity). RNA/DNA ratios were determined byHPLC in coral samples collected over depth gradients in 3 islands off the coast of north-west Java, Indonesia. The reefs range from shallow reefs (depth <3 m) in very turbid water, which are close to eroded shores, to intermediate reefs (depth <6 m)further from the shore with less turbid water, to offshore clear water control reefs (depth <20 m). RNA/DNA ratio was shown to be negatively correlated with depth in all but the most turbid conditions, suggesting it is an expression of metabolicacclimatisation of the coral host-symbiont community with decreasing irradiance (i.e. photoadaptation). Comparing the RNA/DNA ratio in clear and turbid waters, the decrease with depth tended to be steeper over a shorter depth range under turbidconditions, analogous with the more rapid extinction of light in these conditions. The relationship between the RNA/DNA ratio and light was consistently higher under turbid conditions (p = 0.0037), indicating that RNA/DNA ratio had shifted to a higherlevel, possibly indicating a genetic adaptation in the metabolic functioning of corals in the turbid environment. The positive relation between light and RNA/DNA ratio would suggest the opposite, i.e. lower ratio under turbid conditions. The RNA/DNA ratiomay provide a relatively reliable method to determine the metabolic functioning (health) of individual coral colonies.