Pocillopora damicornis rapidly depleted ammonium from the surrounding water. Rates of ammonium acquisition ranged between 5.1 and 91.8 nmol N cm)2 h)1 (on a per area of coral tissue basis) or 5 and 60 nmol 106 cell)1h)1 (on a per symbiotic dino¯agellate basis). Rates were unaected by whether measurements were made inthe light or dark (P > 0.05), but were signi®cantly higher when measured at 10 lM than at 2 lM (P < 0.05; Fig. 2). There was no eect of the long-term exposure to ammonium, except for colonies of the pink morph at substrate concentrations of 10 lM (Fig. 2). In this case, uptake rates were lower in those colonies that been exposed to higher ammonium concentrations in the preceding 2 yr, decreasing from 80.5 to 42.8 nmol cm)2 h)1 (P < 0.05).