The relative importance of different potential nitrogen inputs in cnidarian symbioses is poorly understood, and virtually no quantitative data exist for the transfer of nitrogen between the animal host and the symbiotic algae. This research examined whether holozoic feeding by anthozoans is influenced by the presence of zooxanthellae and the nutritional history of the symbiosis. A stable isotope tracer (15N) and isotope ratio mass spectrometry (IRMS) were used to describe the assimilation of prey nitrogen in anemones, tropical corals, and temperate corals, and its biochemical distribution within the host and zooxanthellae. In addition, flume studies were conducted to determine whether prey capture rate of the temperate coral Oculina arbuscula depends on the symbiotic status of the coral.

Assimilation efficiency of ingested prey nitrogen was species-specific, but was independent of nutritional history. Zooxanthellae consistently obtained 10-20% of the assimilated nitrogen in temperate and tropical corals but 30-45% in the anemone Aiptasia pallida. Digestion occurred rapidly, and the bulk of prey nitrogen obtained by the symbiotic algae was acquired within 4 h. Zooxanthellae converted prey 15N into macromolecules, while in the host ingested nitrogen remained in the low molecular weight and amino acid pools. This biochemical distribution of nitrogen was independent of nutritional history, which is consistent with host respiration of photosynthetic carbon to conserve internal nitrogen pools. There was no evidence for the translocation of essential amino acids or any other nitrogenous products from zooxanthellae to host tissue. Instead, anemones acquired essential amino acids from the diet or via de novo synthesis.

The presence of zooxanthellae increased assimilation efficiency in the coral O. arbuscula but not A. pallida. Symbiotic condition affected assimilation efficiency, but not prey capture. Capture rate and efficiency were the same for symbiotic and aposymbiotic O. arbuscula, suggesting prey capture ability is independent of photosynthetic energy. High flow speeds increased capture rates and shifted capture location from the front of the colony to the back.