6 Does temperature restrict the latitudinal distribution of symbiotic zooxanthellae within the sea anemone anthopleura elegantissima? Experiments with symbiodinium californium in culture

The intertidal anemone Anthopleura elegantissima hosts two species of symbiotic zooxanthellae with different latitudinal distributions within the anemone host. One of these species, Symbiodinium muscatinei, occurs in anemones from southern California to Washington, while the other species, Symbiodinium californium, is restricted to California and is not found in Oregon or Washington. To investigate if temperature restricts the latitudinal distribution of S. californium, thermal optima and limits to growth and photosynthesis were determined by culturing zooxanthellae at a range of temperatures (5–30 C). Mean growth rates (r) were highest between 15 and 28 C (0.21–0.26 d−1) and extremely low at 5, 10, and 30 C (0.02–0.03 d−1). Light‐saturated maximum photosynthesis (Pmax) increased with temperature between 20 and 30 C (2.93 to 13.18 pg C cell−1·h−1, respectively). The effect of temperature on cell motility and cell volume was assessed by culturing zooxanthellae at 10, 20, and 30 and determining% motility over a 24‐hour period. Cell volumes were also calculated. No apparent change in% motility was observed with respect to time of day. Percent motility and cell volumes were greatest at 10 C (60% and 820 mm3, respectively). S. californium exhibited a dramatic reduction in growth (−89%) and photosynthesis (−64%) at temperatures within the range of average coastal sea temperatures in Oregon and Washington (8–17 C) than at higher temperatures (20–30 C). The results strongly suggest that the latitudinal distribution of S. californium is restricted by its reduced ability to grow or photosynthesize at low temperatures. Comparative studies exploring thermal thresholds of S. muscatinei are needed to conclusively demonstrate the role of temperature in determining the latititudinal distribution of both zooxanthellae in A. elegantissima.