What are the benefits in the ascidian-Prochloron symbiosis?

s Prochloron, a prokaryotic alga, is an obligate symbiont of some tropical ascidian colonies belonging to the family Didemnidae. In this symbiosis, the host (ascidian) is thought to acquire photosynthetic metabolites provided from the symbionts (Prochloron), while the host provides a habitat for the symbionts. Radio-labeling studies suggested that Prochloron cells transfer photosynthetically fixed carbon to the host as soluble compounds of low molecular weight (Pardy and Lewin 1981; Griffith and Thinh 1983). In the most host species, Prochloron cells are extracellularly distributed in the common cloacal cavity of the host and the water current there runs directly toward the outside of the host colony. Therefore, it is unlikely that the metabolites are effectively translocated to the host by means of diffusion, especially since there are no structures that are adapted for nutrient translocation in the cloacal cavity. The intracellular symbiosis of Prochloron cells is known only in Lissoclinum punctatum, but the host cells are free cells distributed in the integument of colony (tunic) and they do not have direct connection with the host zooids. Therefore, the occurrence of the metabolite transfer is questionable since previous studies possibly underestimated the amount of the symbionts remaining in the host tissue when they determined the amount of labeled carbon supposedly incorporated into the host. In Diplosoma virens, the parasitic copepods (notodelphyids) that inhabit in the cloacal cavity are surrounded by the numerous amounts of Prochloron cells. The copepods seem to feed on the tunic matrices but rarely ingest Prochloron cells. This may indicate that Prochloron cells contain toxic compounds to make themselves unsuitable as a food for the copepod. The host colony maybe protected from predation by harbouring the toxic symbionts, Endocytobiosis Cell Res. (2004) 15, 51-62