We have analyzed the genetic profiles of dinoflagellate populations obtained from the Pacific coast sea anemone Anthopleura elegantissima (Brandt) at collection sites from Washington to California. Genetic differences within the symbiont populations of California anemones have been uncovered by restriction length polymorphism (RFLP) analysis of the small subunit (SSU) and large subunit (LSU) ribosomal RNA genes, and by denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer region 2 (ITS 2). The existence of two Symbiodinium species is substantiated by sequence analysis of the variable regions V1, V2, and V3 of the SSUrDNA, which also establishes their phylogenetic relatedness to other members of the genus Symbiodinium. Anemones from Washington and Oregon harbor a single dinoflagellate species, for which we propose the name S. muscatinei sp. nov. At these northern locations, S. muscatinei either exists alone or co-occurs with the Chlorella-like green algal symbiont. Our results indicate that S. muscatinei co-occurs with a second dinoflagellate, S. californium, in mixed populations in central and southern California. We suggest that the geographic distribution of these dinoflagellates is related to the temperature cline created by latitude.

Elevated temperature (28-WC) has been hypothesized as the primary cause of the loss of algal endosymbionts in coral reef-associated invertebrates, a phenomenon observed on a world-wide scale over the last decade. In past studies of this "bleaching" phenomenon, there has been an underlying assumption that temperature adversely affects the animal hosts, the algae thereby being relegated to a more passive role. Because photosynthesis is a sensitive indicator of thermal stress in plants and has a central role in the nutrition of symbiotic invertebrates, we have tested the hypothesis that elevated temperature adversely affects photosynthesis in the symbiotic dinoagellate Symbiodinium microadriaicum. The results, based on analyses of light-mediated 02 evolution and in vivo fluorescence, indicate that photosynthesis is impaired at temperatures above 30°C and ceases completely at 34-36WC. These observations are discussed in the context of possible mechanisms that may function in the disaciation of alglinvertebrate symbioses in response to elevated temperature.

The photosynthetic responses to variations in photon flux density were determined for Symbiodinium microadriaticum, the symbiont of the Caribbean jellyfish Cassiopeia xamachana, S. kawagutii, the symbiont of the Indo-Pacific stony coral Montipora verrucosa, and S. pilosum, the symbiont of the Caribbean zoanthld Zoanthus sociatus. Photosynthetic responses were characterized in terms of cellular pigment content, photosynthesis versus irradiance (P-I) relationships, and number and size of the photosynthetic unit (PSU). Analyses of the responses under 2 Light regimes of 40 and 250 p 0 1 quanta m-' S-' indicate that: (1) the 3 different species cultured under identical conditions possess different photosynthetic characteristics; (2) the 3 species acclimate to low photon flux density by simultaneously increasing the number and size of their PSU; (3) PSU characteristics were not correlated with the parameters of their respechve P-lcurves; (4) the 3 species analyzed have different photoacclimatory capabilities which can be correlated with their respective ecological distribution as endosymbionts. As different species demonstrate characteristic responses, photo-acclunatory adjustment in symbiotic dinoflagellates may be under genetic constraints, and thus may represent photo-adaptation (sensu 0. Bjorkman). As a whole, the results suggest that symbiont photo-adaptation may constitute an important axis of niche diversification for the intact associations.

The use of several independent biochemical, physiological, morphological, and behavioral assays has resulted in the reassessment of the taxonomy of the gymnodinioid symbionts of marine invertebrates which belong to the genus Symbiodinuim Freudenthal (Dinophyceae). The formal description of the type species S. microadriaticum Freudenthal is augmented, and three new species are introduced; S. goreauii, symbiotic with the Caribbean sea anemone Ragactis lucida; S. kawagutii, harbored by the Hawaiian stony coral Montipora verrucosa; and S. pilosum, inhabiting the Caribbean zoanthid Zoanthus sociatus.

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