The breakdown of symbiotic zooxanthellae in the sea anemone Phyllactis (=Oulactis) flosculifera (Actiniaria)

The sea anemone Phyllactis (=Oulactis) flosculifera Lesueur (Actiniaria) has developed specialized adaptations of a structural, behavioural and chemical nature, which allow the farming of its symbiotic zooxanthellae as well as their breakdown and use as a source of nutrition. The presence of an extension of the upper column, the ruff, increases the exposed surface area, and the pattern of expansion and contraction of ruff and tentacles allows the high standing crop of algal symbionts they contain maximum exposure to illumination. A protein extract from the combined ruff, oral disc and tentacles has a destructive effect in vitro on the zooxanthellae of Phyllactis (52%), Aiptasia (37·5%) and Zoanthus (16%). Intracellular degeneration of zooxanthellae is greatest in the phagocytic cells of the trefoil forming the free end of the upper mesentery, but also occurs in other parts of the mesentery. The algal pellet extruded by Phyllactis consists mostly of debris, testifying to the anemone's ability to break down its zooxanthellae. Aiptasia tagetes Duch. & Mich, shows only a simple phototactic response, has no algal-damaging agent and very few degenerate zooxanthellae in its mesenteries, but it extrudes large numbers of its symbionts in all stages of the life history.