Ammonium and phosphate excretion in three common echinoderms from Philippine coral reefs

Abstract

The ammonium and phosphate excretion and oxygen consumption of three species of echinoderms (Tripneustes gratilla, Protoreaster nodosus and Ophiorachna incrassata) commonly encountered in Philippine coral reefs were investigated in relation to time of day (i.e. daytime between 10:00 and 12:00 h vs. nighttime between 22:00 and 24:00 h) and their recent feeding history (i.e. recently-collected vs. short-term starvation for 3±1 days). The experiment used whole organism incubations and followed a nested hierarchical design. Ammonium excretion rates were 1447±310 nmol g−1 DW h−1 (mean±S.E., n=24) for T. gratilla, 361±33 for O. incrassata and 492±38 for P. nodosus. Ammonium excretion differed significantly among species, time of incubation and recent feeding history. Interaction between species and recent feeding history was also significant. The organisms excreted more ammonium during daytime except for starved specimens of O. incrassata. In addition, animals that were starved in the laboratory for a few days had a tendency to excrete more ammonium than recently-collected specimens. Phosphate excretion rates were 25±13 nmol g−1 DW h−1 for T. gratilla, 10±2 for O. incrassata and 4±1 for P. nodosus. There were no significant differences in phosphate excretion among the three species of echinoderms, their recent feeding history and time of day. Oxygen consumption rates were 286±24 μg O2 g−1 DW h−1 for T. gratilla, 64±3 for O. incrassata and 54±3 for P. nodosus. Oxygen consumption differed significantly among species and recent feeding history but differed only slightly with time of incubation. There was a significant correlation between oxygen consumption and ammonium excretion (r=0.48, P=0.018), and between oxygen consumption and phosphate excretion (r=0.41, P=0.047) for T. gratilla. The nutrient excretion by tropical echinoderms is another pathway by which inorganic nutrients are regenerated in coral reef communities. However, the quantity of nutrients excreted is dependent on the species of echinoderms, their nutritional status and time of day.