The specific identity of endosymbiotic dinoflagellates (Symbiodinium spp.) from most zooxanthellate corals is unknown. In a survey of symbiotic cnidarians from the southern Great Barrier Reef (GBR), 23 symbiont types were identified from 86 host species representing 40 genera. A majority (Ͼ85%) of these symbionts belong to a single phylogenetic clade or subgenus (''C'') composed of closely related (as assessed by sequence data from the internal transcribed spacer region and the ribosomal large subunit gene), yet ecologically and physiologically distinct, types. A few prevalent symbiont types, or generalists, dominate the coral community of the southern GBR, whereas many rare and/or specific symbionts, or specialists, are found uniquely within certain host taxa. The comparison of symbiont diversity between southern GBR and Caribbean reefs shows an inverse relationship between coral diversity and symbiont diversity, perhaps as a consequence of more-rapid diversification of Caribbean symbionts. Among clade C types, generalists C1 and C3 are common to both Caribbean and southern GBR symbiont assemblages, whereas the rest are regionally endemic. Possibly because of environmental changes in the Caribbean after geographic isolation through the Quaternary period, a high proportion of Caribbean fauna associate with symbiont taxa from two other distantly related Symbiodinium clades (A and B) that rarely occur in Pacific hosts. The resilience of Porites spp. and the resistance of Montipora digitata to thermal stress and bleaching are partially explained by their association with a thermally tolerant symbiont type, whereas the indiscriminant widespread bleaching and death among certain Pacific corals, during El Niño Southern Oscillation events, are influenced by associations with symbionts possessing higher sensitivity to thermal stress.Cnidarians such as hard corals, soft corals, sea fans, and anemones are the principal faunal constituents of benthic AcknowledgmentsWe thank Jill Torregiani, María del Carmen Gómez-Cabrera, Paul Fisher, and Michael Stat for assisting with the collection and processing of samples, as well as M. J. Van Oppen for her help with determining the specific identity of subclades C1, C2, and C3 (Van Oppen et al. 2001).

The scleractinian coral species, Seriatopora hystrix and Acropora longicyathus, are widely distributed throughout the latitudinal range of the tropical west Pacific. These 2 coral species live in a mutually beneficial relation with symbiotic dinoflagellates (zooxanthellae), which are passed to their progeny by vertical transmission (zooxanthellate eggs or larvae) and horizontal transmission (eggs or larvae that acquire symbionts from the environment), respectively. For S. hystrix, vertical transmission might create biogeographically isolated and genetically differentiated symbiont populations because the extent of its larval migration is known to be limited. On the other hand, horizontal transmission in corals such as A. longicyathus may result in genetically connected symbiont populations, especially if its zooxanthellae taxa are widely distributed. To examine these hypotheses, symbionts were collected from colonies of S. hystrix and A. longicyathus living in the Great Barrier Reef (Australia), South China Sea (Malaysia) and East China Sea (Ryukyus Archipelago, Japan), and were examined using restriction fragment length polymorphism and sequence analysis of large and small subunit rRNA genes. Phylogenetic analysis assigned the symbionts to 1 of 3 taxonomically distinct groups, known as clades. Symbionts from Australian and Japanese S. hystrix were placed in Clade C, and Malaysian S. hystrix symbionts in the newly described Clade D. Seven of 11 Australian and all Japanese and Malaysian colonies of A. longicyathus had symbiotic dinoflagellates that also grouped with Clade C, but symbionts from the remaining Australian colonies of A. longicyathus grouped with Clade A. Analysis of molecular variance of Clade C symbionts found significant genetic variation in 1 or more geographic groups (69.8%) and to a lesser extent among populations within geographic regions (13.6%). All populations of Clade C symbionts from S. hystrix were genetically differentiated according to geographic region. Although Clade C symbionts of A. longicyathus from Japan resolved into a distinct geographic group, those from Australia and Malaysia did not and were genetically connected. We propose that these patterns of genetic connectivity correlate with differences in the dispersal range of the coral or symbiont propagules and are associated with their respective modes of symbiont transmission.

Coral larvae acquire populations of the symbiotic dinoXagellate Symbiodinium from the external environment (horizontal acquisition) or inherit their symbionts from the parent colony (maternal or vertical acquisition). The eVect of the symbiont acquisition strategy on Symbiodinium-host associations has not been fully resolved. Previous studies have provided mixed results, probably due to factors such as low sample replication of Symbiodinium from a single coral host, biogeographic diVerences in Symbiodinium diversity, and the presence of some apparently host-speciWc symbiont lineages in coral with either symbiont acquisition strategies. This study set out to assess the eVect of the symbiont acquisition strategy by sampling Symbiodinium from 10 coral species (Wve with a horizontal and Wve with a vertical symbiont acquisition strategy) across two adjacent reefs in the southern Great Barrier Reef. Symbiodinium diversity was assessed using singlestranded conformational polymorphism of partial nuclear large subunit rDNA and denaturing gradient gel electrophoresis of the internal transcribed spacer 2 region. The Symbiodinium population in hosts with a vertical symbiont acquisition strategy partitioned according to coral species, while hosts with a horizontal symbiont acquisition strategy shared a common symbiont type across the two reef environments. Comparative analysis of existing data from the southern Great Barrier Reef found that the majority of corals with a vertical symbiont acquisition strategy associated with distinct species-or genus-speciWc Symbiodinium lineages, but some could also associate with symbiont types that were more commonly found in hosts with a horizontal symbiont acquisition strategy.

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Symbiodinium spp. are well-known symbionts of corals. A number of key species of bioeroding sponges also contain intracellular dinoflagellate symbionts, but little is known about this relationship. The origin and identity of sponge symbiotic dinoflagellates is unresolved since their discovery in 1964. Symbiotic bioeroding sponges can invade and kill live corals. If the sponges acquire symbionts during this process, they may act as refuges for vulnerable coral symbionts, as bioeroding sponges appear to be more resistant to thermal bleaching than most corals. To test this hypothesis, pairwise tissue samples from the common brown bioeroding sponge Cliona orientalis Thiele, 1900 and from abutting corals were taken. Samples were obtained from 3 disparate regions of northeastern Australia, extending over more than 1300 km. The genetic identities and population diversities of Symbiodinium from sponges and respective host corals were surveyed using 28S RNA gene sequences. Results suggest that C. orientalis consistently harbours the same clade of symbionts, even in very different environmental conditions. However, populations of sponge Symbiodinium did not appear to be genetically connected between the sampled regions, implying maternal transmission of the symbionts. Furthermore, the sponge zooxanthellae were different to those found in corals. We thus rejected the hypotheses that (1) the sponge acquires its symbionts from invaded corals under normal conditions and (2) that it may offer a refuge to coral symbionts. Symbionts from C. orientalis were closely related to, but distinct from symbionts of soritid foraminiferans, and are likely to belong to a new subclade of G-type dinoflagellates.KEY WORDS: Symbiosis · Molecular identity · Dinoflagellata · Symbiodinium · Porifera · Cliona orientalis · Great Barrier ReefResale or republication not permitted without written consent of the publisher

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From an experiment in which corals are transplanted between two depths on a Panamanian coral reef, Baker infers that bleaching may sometimes help reef corals to survive environmental change. Although Baker's results hint at further mechanisms by which reef-building corals may acclimatize to changing light conditions, we do not consider that the evidence supports his inference.