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The sea anemone Anthopleura elegantissima is a common member of intertidal communities along the west coast of North America, and can experience extended periods of increased temperature during summertime low tides. Internal body temperatures of emersed individuals of A. elegantissima were monitored in a laboratory wind tunnel and in the field, and factors influencing the anemones’ thermal experience were examined. Larger body size and aggregation with conspecifics slowed body temperature increases in controlled wind tunnel conditions. In the field, anemones in the interior of an aggregation stayed cooler than those on the edges, and microhabitat features related to light exposure and surface orientation overshadowed any direct effects of body size. In the warmest month only (July), aggregations of A. elegantissima were significantly larger at the upper limit of their distribution than they were at the mid and lower limits, suggesting aggregation in high intertidal zones may be a behavioral response to desiccation and temperature stress. As this sea anemone can host multiple species of symbiotic algae with different thermal tolerances, the ability to slow body heating may affect the type of algae hosted and thus the potential contribution of this abundant anemone to primary production in the intertidal zone. more...

We evaluated the seasonal stability of two algal symbiont populations in the temperate intertidal sea anemone Anthopleura elegantissima on San Juan Island, Washington, where the relatively thermally tolerant dinoflagellate Symbiodinium muscatinei coexists with the less thermally tolerant chlorophyte Elliptochloris marina. Random collection of anemones along repeatedly sampled transects over four seasons and three shore heights revealed S. muscatinei to be the dominant symbiont, with E. marina mostly limited to anemones in the lower intertidal zone. At the lowest shore height sampled (+0.2 m), the proportion of E. marina was between 40% and 50% of the total symbiont population throughout the year. Symbiont distribution patterns persisted despite considerable seasonal variation in aerial exposure, temperature, irradiance, nutrients, and phytoplankton concentration, as well as a high potential for symbiont shuffling, with mixed-symbiont assemblages occurring in 51% of all anemones sampled. Symbiont density in anemones also changed little despite three- to fourfold-higher division frequencies of both symbionts during July and November. Although the intertidal zonation of these symbionts was stable over an annual period, we predict that their spatial distributions will be responsive to longer-term environmental change, and anticipate that this anemone symbiosis will be a useful and highly tractable barometer for future climate change, with this study serving as a baseline. A diverse array of shallow-water cnidarians, including sea anemones, corals, jellyfish, and hydroids, engage in endosymbioses with phototrophic dinoflagellates (Symbiodinium spp.) that translocate photosynthetic products to their host. Many of these symbioses exhibit plasticity with respect to both the population density and genetic identity of the dinoflagellate partner, allowing them to exploit a wide range of conditions that vary spatially and temporally. In tropical systems, where cnidarian symbioses are particularly well studied, many hosts show depth-related differences in symbiont density and physiology, and also undergo predictable seasonal fluctuations in symbiont density and physiology that are correlated with environmental conditions (Fitt et al. 2000; Warner et al. 2002). In more...

Symbiodinium californium (#383, Banaszak et al. 1993) is one of two known dinoflagellate symbionts of the intertidal sea anemones Anthopleura elegantissima, A. xanthogrammica, and A. sola and occurs only in hosts at southern latitudes of the North Pacific. To investigate if temperature restricts the latitudinal distribution of S. californium, growth and photosynthesis at a range of temperatures (5°C-30°C) were determined for cultured symbionts. Mean specific growth rates were the highest between 15°C and 28°C (μ 0.21-0.26 · d(-1) ) and extremely low at 5, 10, and 30°C (0.02-0.03 · d(-1) ). Average doubling times ranged from 2.7 d (20°C) to 33 d (5, 10, and 30°C). Cells cultured at 10°C had the greatest cell volume (821 μm(3) ) and the highest percentage of motile cells (64.5%). Growth and photosynthesis were uncoupled; light-saturated maximum photosynthesis (Pmax ) increased from 2.9 pg C · cell(-1 ) · h(-1) at 20°C to 13.2 pg C · cell(-1 ) · h(-1) at 30°C, a 4.5-fold increase. Less than 11% of daily photosynthetically fixed carbon was utilized for growth at 5, 10, and 30°C, indicating the potential for high carbon translocation at these temperatures. Low temperature effects on growth rate, and not on photosynthesis and cell morphology, may restrict the distribution of S. californium to southern populations of its host anemones. more...

The intertidal sea anemone Anthopleura elegantissima in the Pacific Northwest may host a single type of algal symbiont or two different algal symbionts simultaneously: zooxanthellae (Symbiodinium muscatinei) and zoochlorellae (green algae; Trebouxiophyceae, Chlorophyta). A seasonal comparison of zooxanthellate and zoochlorellate anemones showed stable symbiont population densities in summer and winter, with densities of zoochlorellae about 4 times those of zooxanthellae. Photosynthesis-irradiance curves of freshly isolated symbionts show that the productivity (P(max) cell) of freshly isolated zooxanthellae was about 2.5 times that of zoochlorellae during July; comparable rates were obtained in other months. Models of algal carbon flux show that zoochlorellae may supply the host with more photosynthetic carbon per unit anemone biomass than zooxanthellae supply. Zooxanthellate anemone tissue was 2 per thousand ((13)C) and 5 per thousand ((15)N) enriched and zoochlorellate anemone tissue was 6 per thousand ((13)C) and 8 per thousand ((15)N) enriched over their respective symbionts, suggesting that zoochlorellate anemones receive less nutrition from their symbionts than do zooxanthellate individuals. The disparity between predicted contributions from the algal carbon budgets and the stable isotopic composition suggests that short-term measures of algal contributions may not reflect actual nutritional inputs to the host. Isotopic data support the hypothesis of substantial reliance on external food sources. This additional nutrition may allow both algae to persist in this temperate intertidal anemone in spite of differences in seasonal photosynthetic carbon contributions. more...

The sea anemone Anthopleura elegantissima (Brandt) hosts two species of symbiotic dinoflagellates, known as zooxanthellae, which coexist within the host at southern latitudes only. One of these species, Symbiodinium muscatinei LaJeunesse et Trench, has a broad latitudinal distribution, occurring in intertidal anemones from Washington state to Southern California. To investigate whether high thermal tolerance contributes to the ability of S. muscatinei to inhabit anemones from northern and southern regions, the upper thermal tolerance limit for photosynthesis of symbionts in northern (48°24′ N) populations of A. elegantissima was determined by subjecting anemones to a gradual increase in temperature from 12°C to 30°C over a 10-week period. Light-saturated photosynthetic rates of isolated zooxanthellae were the same over the range of 12°C–24°C and declined significantly at 26°C, which is 14°C and 5°C above average summertime seawater temperatures in northern Puget Sound and Southern California, respectively. At 28°C, zooxanthellae isolated from the anemones, and those expelled by their hosts, exhibited extremely low rates of photosynthesis and highly reduced chl content. The photosynthetic rates and chl content of expelled zooxanthellae were lower than those of retained zooxanthellae. The high thermal tolerance of S. muscatinei isolated from northern populations of anemones supports the broad latitudinal distribution of this symbiont, allowing it to coexist with S. californium (#383, Banaszak et al. 1993) in southern populations of anemones. more...

The potentially lethal bleaching of invertebrates through loss of their algal symbionts is a major global conservation issue; cnidarians in such interactions could potentially withstand environmental change by having multiple symbiotic partners whose physiologies vary with critical field parameters such as temperature and light. Spatial variation in symbiont distributions in the field is a strong indicator of such potential. We show that, in an association between an abundant temperate sea anemone and its two endosymbionts, the dinophyte Symbiodinium muscatinei (zooxanthellae) and an unidentified green alga (zoochlorellae), densities and ratios of anemones’ symbionts change along a light gradient generated by intertidal caves in Washington state. In the laboratory, the photosynthetic performance of zooxanthellate anemones exceeded that of zoochlorellate anemones under high irradiance levels (.100 mmol quanta m 22 s 21 ). Further, field sampling revealed that caves are divided into three distinct regions based on anemones’ algal complements: a brown region of zooxanthellate anemones near the mouth of the cave, a green region of zoochlorellate anemones in the middle of the cave, and a white region of algae-free anemones near the back of the cave. These data, together with field reciprocal transplant experiments showing algae-free anemones’ ability to regrow algae consistent with local physical conditions, suggest that algal performance (vs. host selection) determines observed variation in these symbioses. Our results support the idea that temperate, as well as tropical, host‐symbiont associations can respond plastically to environmental change. Our data also suggest that intact symbioses in adult hosts are surprisingly stable; bleaching may be required in rearranging host‐symbiont associations. more...

Along the North American Pacific coast, the common intertidal sea anemone Anthopleura elegantissima engages in facultative, flexible symbioses with Symbiodinium muscatinei (a dinoflagellate) and Elliptochloris marina (a chlorophyte). Determining how symbiotic state affects host fitness is essential to understanding the ecological significance of engaging in such flexible relationships with diverse symbionts. Fitness consequences of hosting S. muscatinei , E. marina or negligible numbers of either symbiont (aposymbiosis) were investigated by measuring growth, cloning by fission and gonad development after 8.5–11 months of sustained exposure to high, moderate or low irradiance under seasonal environmental conditions. Both symbiotic state and irradiance affected host fitness, leading to divergent life-history strategies. Moderate and high irradiances led to a greater level of gonad development in individuals hosting E. marina , while high irradiance and high summer temperature promoted cloning in individuals hosting S. muscatinei and reduced fitness of aposymbiotic anemones. Associating with S. muscatinei may contribute to the success of A. elegantissima as a spatial competitor on the high shore: (i) by offsetting the costs of living under high temperature and irradiance conditions, and (ii) by promoting a high fission rate and clonal expansion. Our results suggest that basic life-history characteristics of a clonal cnidarian can be affected by the identity of the endosymbionts it hosts. more...

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. more...