Ascidian Photosymbiosis
In the subphylum Tunicata, algal symbiosis is only known in the tropical species belonging to the family Didemnidae.
While photosymbiotic species are recorded from four genera (Didemnum, Diplosoma, Lissoclinum, and Trididemnum), many of congeners are non-symbiotic. Therefore, the algal symbiosis is supposed to have been established at least once in each genus.
In most species, the photosymbionts are distributed in the common cloacal cavity or in the tunic. In Lissoclinum punctatum, Prochloron cells are distributed in both cloacal cavity and tunic, and the symbionts in the tunic are intracellularly located in the host cells .
The ascidian–cyanophyte symbiosis is supposed to be an obligate association. The photosymbionts are transferred from the parental colony, suggesting the symbionts are necessary for the host ascidians to survive. The mode of the transmission is different among the species, probably depending on the distribution pattern of the photosymbionts in the host colony.
Lissoclinum punctatum
The colony harbors intracellular photosymbionts in the tunic.
Hirose E, Maruyama T, Cheng L, Lewin RA (1996) Intracellular symbiosis of a photosynthetic prokaryote, Prochloron sp., in a colonial ascidian. Invertebr Biol 115:343–348. doi: 10.2307/3227023
Hirose E, Maruyama T, Cheng L, Lewin RA (1998) Intra- and extra-cellular distribution of photosynthetic prokaryotes, Prochloron sp., in a colonial ascidian: Ultrastructural and quantitative studies. Symbiosis 25:301–310.