A REMARKABLE characteristic of tropical coral reefs, and indeed to some extent of shallow marine waters everywhere, is the profusion of single-cell, yellow-green algae (the so-called symbiotic zooxanthellae) that live within a wide variety of radiolarians, cœlenterates, molluscs, bryozoans, worms, ascidians, etc. With a few exceptions, the definite taxonomic status of zooxanthellae has been difficult because of their somewhat generalized morphology and because of the absence or elusive nature of a motile phase1–3 ; only Kawaguti4 has reported seeing motile forms.