that swell to 20 times their original size with food vacuoles after ingesting a victim. These large cells barely move or are motionless. Gymnodinium fungiforme is attracted to a variety of amino acids and other organic compounds (Spero, 1985). Glycine, taurine, and serine attract the dinoflagellate at threshold detection levels of 10 8 M, followed by dextrose at 10 7 and alanine, proline, and threonine at 10 6 M. Glycine, taurine, and alanine are three of the most abundant free amino acids found in invertebrates and protozoa, which are the major food organisms of G. fungiforme.

Pfiesteria piscicida is a heterotrophic dinoflagellate that is responsible for many of the fish kills along the Atlantic coast of the Southeastern United States (Burkholder and Glasgow, 1997). P. piscicida belongs to the “ambush–predator” group of dinoflagellates which chemically detect their prey and then swarm in a direct attack on the prey. The zoospores have a peduncle that is thin and tapered when the dinoflagellate is not feeding. In the presence of fish, the dinoflagellate zoospores feed on the fish by phagocytizing fish tissue through the peduncle. The peduncle is swollen with haustoria-like penetrating extensions when feeding on the fish (Fig. 7.51(b)).

Symbiotic dinoflagellates

Symbiotic dinoflagellates (zooxanthellae) occur in almost all species of tropical and reef-building corals, jellyfish, and sea anemones (Cnidaria). The dinoflagellates are coccoid spheres in the symbiotic state and have been assigned to the genus Symbiodinium. Studies on nucleic acid makeup have revealed the dinoflagellates to be a diverse group, although the dinoflagellate inhabiting one type of Cnidaria remains a constant (Baillie et al., 2000; Rodriguez-Lanetty et al., 2004).

Symbiodinium cells change into typical gymnodinoid form when placed in culture. Cultured and symbiotic Symbiodinium differ physiologically. The symbiotic cells grow ten times slower than cultured cells. The host exerts strong control over the translocation of metabolites from the dinoflagellate endosymbiont, resulting in 98% of the carbon fixed by the endosymbiont being released to the host. In contrast, cultured cells release only 10% of their photosynthetically fixed carbon (Trench, 1993). The host animal cells secrete the amino acid taurine which causes the