Temperate and tropical algal-sea anemone symbioses

In this review, we seek to develop new insights about the nature of algal-s ea anemone symbioses by comparing such associations in temperate and tropic al seas. Temperate seas undergo pronounced seasonal cycles in irradiance, t emperature, and nutrients, while high irradiance, high temperature, and low nutrients are seasonally far less variable in tropical seas. We compare th e nature of symbiosis between sea anemones ( = actinians) and zooxanthellae (Symbiodinium spp.) in both regions to test tropical paradigms against tem perate examples and to identify directions for future research. Although fe wer anemone species are symbiotic in temperate regions, they are locally do minant and ecologically important members of the benthic community compared to the tropics. Zooxanthella densities tend to be lower in temperate anemones, but data are limited to a few species in both temperate and tropical seas. Zooxanthella densities are far more stable over time in temperate anemones than in trop ical anemones, suggesting that temperate symbioses are more resistant to fl uctuations in environmental parameters such as irradiance and temperature. Light-saturated photosynthetic rates of temperate and tropical zooxanthella e arc similar, but temperate anemone hosts receive severely reduced carbon supplies from zooxanthellae during winter months when light is reduced. Sym biont transmission modes and specificity do not show any trends among anemo nes in tropical vs. temperate seas. Our review indicates the need for the following: (1) Investigations of othe r temperate and tropical symbiotic anemone species to assess the generality of trends seen in a few "model" anemones. (2) Attention to the field ecolo gy of temperate and tropical algal-anemone symbioses, for example, how symb ioses function under seasonally variable environmental factors and how zoox anthellae persist at high densities in darkness and winter. The greater sta bility of zooxanthella populations in temperate hosts may be useful to unde rstanding tropical symbioses in which bleaching (loss of zooxanthellae) is of major concern. (3) Study of the evolutionary history of symbiosis in bot h temperate and tropical seas. Continued exploration of the phylogenetic re lationships between host anemones and zooxanthella strains may show how and why zooxanthellae differ in anemone hosts in both environments.