It is not unusual for tropical organisms to take advantage of sunlight, one of the most celebrated examples being the intra-cellular zooxanthellae providing photosynt-hates that nourish corals and allow many species to be largely independent of ingested food (autotrophic, see Chapter 7). Certain species of the colonial ascidian family Didemnidae have also taken advantage of abundant tropical sunlight developing symbioses with chlorophyll containing symbionts and have become autotrophic. Development of these symbioses is enhanced by a colony form that characteristically has habitats for symbionts in large spaces inside the colony as well as in the test itself. Further, species of Didemnidae contain calcareous spicules in the test that, arranged (by the ascidian host), shield from or expose the contained symbionts to sunlight to regulate the rate that photosynthates are delivered to the host and to reduce photoinhibition (see Chapter 7). This regulation can be differentially imposed so that part of a colony can respond independently to the light falling on it. A colony growing around the side of a rock is green (without spicules) on the shaded underside of the rock; white (with spicules increasingly crowded in the surface) where it grows around the side, and pink (with carotenoid pigment added) on the part of the colony in full sunlight on top of the rock.
Some (Lissoclinum timorense and L. bistratum) didemnid/Prochloron symbioses form vast mosaics covering reef flats, especially in the northern part of the GBR and high tropics of the Indo-West Pacific (where diurnal temperature ranges are not as great as they are in the south). The open reef flat is an unusual place for ascidians, which usually are more cryptic, however, the habitat has obvious advantages for au-totrophic species able to regulate the light their symbi-onts are exposed to. Further, occupation of this habitat is enhanced by the capacity of colonies to divide and move to space themselves evenly, maintaining the mosaic-like pattern on the reef flat.
The vase shaped Didemnum molle is another common species with a remarkable geographic range from the west Indian Ocean to Fiji. It also subdivides and moves, climbing up its substratum (staghorn coral skeletons, aquarium walls and the sides of reefs) toward the light by drawing in tendrils of test from the base of the colony. Didemnum simile, a conspicuous species with a similar geographic range, neither subdivides nor moves. However, it forms extensive sheets and encrusts vast areas of undercut, growing through gaps and holes in the reef flat and binding rubble together. Its colour, affected by its contained Prochloron, varies from bright sapphire blue to emerald green.
The existence of green plant cells in certain tropical species of the Didemnidae was well known by the 1950s although their identity had not been determined, being referred variously to zoochlorellae or zooxan-thellae or Chlorophyta. In 1973, aboard the American research vessel Alpha Helix working in waters off Lizard Island, Eldon Newcomb of the University of Wisconsin recognised the green prokaryotic cells in many ascid-ian colonies as Cyanophyta (blue-green algae). Subsequently a new division of the algae, Prochlorophyta, containing a single genus Prochloron was erected to accommodate these organisms. There was some support for an hypothesis that they were ancestors of green plant chloroplasts, having chlorophyll b in addition to chlorophyll a and lacking phycobiliproteins. However, depending on the ascidian species, the symbionts are either in the common cloacal cavity or embedded in the colony test and are not endosymbionts (like coral zoox-anthellae) as they were claimed to be. In fact they are never even in the ascidian zooid. Later Prochloron was confirmed as a genus of the Cyanophyta and now is grouped (as a 'prochlorophyte') with two other genera containing chlorophyll b.
The number of species in Prochloron is not known and it has not been cultured outside the ascidian host. Obligate symbioses are known in four of the eight known genera of the Didemnidae. Except for symbioses in closely related species of Diplosoma, adaptations of the larval ascidian test to carry the symbionts to the next host generation are diverse. It is most likely that the symbiosis arose at least once and possibly more than once in each genus. About 25 obligate symbioses are now known. Sometimes non-obligate symbionts are on the surface of the colony and are readily brushed off. Also there are chlorophyte symbi-onts in some didemnid species both on their own and with Prochloron.
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