Morphogenetic Movements and Assembly of Strobilae into Zooidal Systems in Early Colony Development of the Compound...

Large-scale developmental processes, such as the morphogenetic movement of constructional sub-units within colonies of modular organisms, require self-assembling mechanisms. The compound ascidian Polyclinum planum is a modular organism. In the present study, colonies of P. planum were subjected to light-shock, and the development of the released tadpole larvae was followed in the laboratory from settlement, through the formation of oozooids and, finally, small colonies. Oozooids strobilated in 40 to 78 days. The number of post-abdominal strobilae produced per oozooid depended on post-abdomen length, but the timing of strobilation did not. Thoracic and abdominal remnants of P. planum parent zooids regressed completely; only the post-abdominal strobilae regenerated to form new zooids. Regenerating strobilae moved through the tunic into the region once occupied by the parent zooid, and formed a zooidal system around a pocket-like common cloacal cavity. Expansion of the thoraxes of regenerating zooids within the young colony produced the cylindrical to capitate shape of small, single-system colonies. The observed non-regeneration of the thoracic and abdominal remnants of P. planum differs markedly from their reported fates in other polyclinid ascidians. These observations shed light on the "rules of assembly" that govern formation of young P. planum colonies.