High biodiversity on coral reefs results in part from tight nutrient cycling among symbiotic organisms, such as within the obligate associations among some damselfishes, cnidarians, and zooxanthellae. Some anemonefish excrete ammonia 20–50× faster than host anemones can
absorb this nutrient, leading to significant growth of the anemones and their zooxanthellae. In contrast, little is known about phosphate transfer in this major coral reef mutualism. We determined rates of phosphate excretion by anemonefish and uptake by giant sea anemones under laboratory
conditions, and compared them with known rates of ammonia transfer in this symbiosis. Immediately after feeding with a phosphate-rich diet, anemonefish excreted phosphate at slow rates of 0.07 ± 0.01 μmol P g–1 d–1, which did not vary significantly
with body size. Starved anemones that had been cultured with phosphate supplements absorbed phosphate at a significantly slower rate (0.18 ± 0.03 μmol P g–1 d–1) than did those cultured with either no supplements or with anemonefish, which absorbed
phosphate at similarly rapid rates (0.54 ± 0.01 and 0.51 ± 0.14 μmol P g–1 d–1 respectively). We conclude that under laboratory conditions, anemones absorb phosphate up to 6.6× faster than the rate at which it is excreted by their anemonefish,
and thus fish do not appear to provide sufficient phosphate to their hosts through this pathway. Anemones may get most of their phosphorus via ingestion of fish feces and/or mucus, or via the ingestion of prey.
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