Plant rake and algal pouch of the larvae in the tropical ascidian Diplosoma similis: An adaptation for vertical transmission of photosynthetic symbionts Prochloron sp.

The embryos of Diplosoma similis are brooded within the thick walled tunic of the colony in isolation from the symbiotic algae Prochloron sp., which a re in the cloacal cavity of the parent colony. Prior to the spawning, the p lant rake, a tassel-like structure, protrudes from the postero-dorsal end o f the larval trunk and extends into the cloacal cavity. The algal cells in the cloacal cavity adhere to the plant rake. When the larvae are spawned, t he trunk tunic extends posteriorly and forms a pouch entirely covering the plant rake. The algal cells are packed in the pouch (algal pouch) envelopin g the basal part of the tail. The cell density of the algae in the pouch is much higher than that in the colony, suggesting that the plant rake functi ons for gathering and concentrating the symbionts into the algal pouch. In the course of metamorphosis, the algal pouch expands and turns into the clo acal cavity of the young colony. The high density of algal cells in the pou ch would ensure that the young colony possesses the symbiotic algae of appr opriate cell density in the cloacal cavity, and the colony can sufficiently receive benefits from the symbionts just after the settlement.