Blue-green algae associated with ascidians of the Great Barrier Reef

Abstract

IN the rich biota of reef communities, one of the best known symbiotic relationships is that of dinoflagellates known as “zooxanthellae” with corals and giant clams^1–3. In contrast, the presence of algae in ascidians (sea squirts: Phylum Chordata, Subphylum Tunicata), although known for many years, has been studied very little. It is known that the association is confined to tropical ascidians in the family Didemnidae, but even the phylum to which the algae belong, or indeed whether the green cells in question are algae at all, has remained uncertain. During a recent expedition of the RV Alpha Helix to the Great Barrier Reef, we encountered several species of colonial asdidians containing large numbers of bright green, spherical cells. We have established by optical and electron microscopy that these green cells are blue-green algae; their association with primitive chordates represents a considerable extension of the known host range of these prokaryotes.