SummaryRecognition is an important stage in the establishment of highly specific mutualistic associations. Yet, for the majority of symbioses, very few of the mechanisms involved in recognition and specificity are known. In this study, we provide evidence for a recognition mechanism at the onset of symbiosis between larvae of the coral Fungia scutaria and their endosymbiotic dinoflagellate algae. This recognition step occurs during initial cellular contact between the symbiotic partners through a lectin/glycan interaction. We determined that an intact algal cell surface was required for successful infection of F. scutaria larvae. Modification of the algal cell surface by enzymatic digestion with trypsin or N-glycosidase significantly reduced infection success, and implicated algal cell surface glycans in recognition. Using flow cytometry, a-mannose/aglucose and a-galactose residues were identified as potential recognition ligands on the algal cell surface. Finally, inhibition of these cell surface glycans significantly reduced infection of F. scutaria larvae by the algae. These data provide evidence that the algal cell surface contains glycan ligands, such as a-mannose/ a-glucose and a-galactose, which play a role in recognition during initial contact at the onset of symbiosis with F. scutaria larvae.

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Many corals that harbor symbiotic algae (zooxanthellae) produce offspring that initially lack zooxanthellae. This study examined late larval development and the acquisition of zooxanthellae in the scleractinian coral Fungia scutaria, which produces planula larvae that lack zooxanthellae. Larvae reared under laboratory conditions developed the ability to feed 3 days after fertilization; feeding behavior was stimulated by homogenized Artemia. Larvae began to settle and metamorphose 5 days after fertilization. In laboratory experiments, larvae acquired experimentally added zooxanthellae by ingesting them while feeding. Zooxanthellae entered the gastric cavity and were phagocytosed by endodermal cells. As early as 1 h after feeding, zooxanthellae were observed in both endodermal and ectodermal cells. Larvae were able to form an association with three genetically distinct strains of zooxanthellae. Both zooxanthellate and azooxanthellate larvae underwent metamorphosis, and azooxanthellate polyps were able to acquire zooxanthellae from the environment. Preliminary evidence suggests that the onset of symbiosis may influence larval development; in one study symbiotic larvae settled earlier than aposymbiotic larvae. Protein profiles of eggs and larvae throughout development revealed a putative yolk protein doublet that was abundant in eggs and 1-day-old larvae and was absent by day 6. This study is the first to examine the onset of symbiosis between a motile cnidarian host and its algal symbiont.

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This study aimed at determining whether fine-scale molecular diversity of Symbiodinium corresponds to functional differences in host/symbiont specificity during the onset of symbiosis in cnidarian/alga mutualisms. Infection experiments were conducted in aposymbiotic larvae of the scleractinian Fungia scutaria using known algal Internal Transcribed Spacer (ITS)-2 types within Clade C. Our results show that algal type is directly related to specificity during the onset of symbiosis in F. scutaria larvae. The 3 ITS symbiont types (C 1f, C 15 and C 31) within the major Symbiodinium Clade C engaged differentially in symbiosis with coral larvae. The homologous symbiont (C 1f), found in adult F. scutaria from the field, showed a significantly better association with the host larvae than the other 2 heterologous symbionts (C 15 extracted from Porites compressa, and C 31 from Montipora capitata). This is the first evidence that fine-scale ITS diversity of Symbiodinium confers functionality in the symbiosis.KEY WORDS: Symbiosis · Specificity · ITS-2 types · Symbiodinium · ScleractiniaResale or republication not permitted without written consent of the publisher