Symbiosis and Nitrogen Cycling: Physiological Effects of Anemone Shrimps on Host Sea Anemones in the Caribbean Sea
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Symbioses are characteristic of coral reef communities and can involve networks of species interactions as well as specialized adaptations to reef conditions. Because coral reefs are surrounded by nutrient-poor water, mutualisms provide an opportunity for tight nutrient cycling among reef organisms. However, little is known about how mobile ectosymbionts, such as crustaceans, benefit their hosts, including how they contribute to nutrient cycling in reef cnidarians. I examined the benefits of ammonium contributions by 2 species of obligate ectosymbionts, the spotted cleaner shrimp Periclimenes yucatanicus, and the Pederson’s cleaner shrimp Ancylomenes pedersoni, to selected physiological processes in host Caribbean sea anemones Bartholomea annulata. Rates of ammonia excretion from 3 client fish species were also measured to estimate the potential nitrogen contributions from client fishes which pose and are cleaned by shrimps near host sea anemones. Starved sea anemones were subjected to 1of 3 laboratory treatments for 6 weeks: 1) shrimps present, 2) ammonium present, or 3) neither shrimps nor ammonium present (control). Changes in anemone body size and protein content were measured along with the density, mitotic index, chlorophyll a content and mycosporine-like amino acids of their endosymbiotic microalgae Symbiodinium. Shrimps excreted ammonium at whole-organismal rates that were much lower than those of reef fishes, while sea anemones absorbed ammonium at relatively high rates, indicating that shrimp presence alone cannot meet the needs of host nutrient budgets. The only clear impact of experimental nutrient supplements was enhancement of the mitotic index of Symbiodinium. Shrimp presence also caused a small but statistically insignificant increase in the oral disc size of sea anemones. Future experiments may demonstrate whether shrimp presence causes significant increase in anemone size, resulting in more obvious visual cues for client fishes to locate cleaner shrimp stations on the reef. The ammonia excretion rates of client fishes reveal that they theoretically are able to meet the nitrogen needs of host sea anemones. Thus, a nutrient-driven positive feedback loop may occur among among cleaner shrimps, host sea anemones, their endosymbiotic microalgae, and the client fishes attracted by the shrimps to these cleaning stations. The ectosymbiotic shrimp alone do not appear to provide enough ammonium to support the physiological needs of microalgae in host sea anemones, but the potential client fishes may. These mutualistic interactions may facilitate tight nutrient cycling among multiple levels of species belonging to different phyla in this symbiotic system. Further research on the impacts of ammonia and other nutrients provided by client fishes is necessary to determine benefits from this mutualistic interaction to host sea anemones.