Algal Symbionts in the Larval Tunic Lamellae of the Colonial Ascidian Lissoclinum timorense (Ascidiacea, Didemnidae)

Lissoclinum timorense is a colonial ascidian that harbors the prokaryotic alga Prochloron. The algal photosymbionts adhere to the lamellae of the tunic on the posterior half of the trunk of larvae, which aggregate in the common cloacal cavity of the mother colony. Bead-adhesion tests demonstrated that the lamellae are adhesive, whereas the anterior half of the larval trunk Is not. The anterior half is covered with a thin layer of outer tunic, which probably prevents Prochloron cells from attaching and interfering with sensory receptors and adhesive organs. The larval structures and the mode of algal transmission between generations are very similar to those of the Prochloron-harboring ascidian Didemnum molle. Molecular phylogenetic analyses have suggested that photosymbiosis was independently established in each genus, and thus the apparent similarity in the larvae probably resulted from convergence. The distribution pattern of photosymbionts is probably more determinative of algal transmission than phylogenetic constraints.