SWALLA, Billie J.; University of Washington: To Settle or to Swim? Ascidian Larval Decisions

Tunicates have a variety of life histories and larval morphologies that make them a fascinating group of animals to study transitions in larval life history. Within one family, the Molgulidae, tailless larvae have evolved many times independently and this transition is correlated with loss of function mutations in larval genes. Coloniality is widespread in ascidians and colonial larvae brood their larvae, releasing large adultated larvae that settle almost immediately. Brooding is always correlated with internal fertilization, where non-self sperm is taken in through the oral siphon and moved through the adult to where the eggs are fertilized. We have been examining triggers to metamorphosis and find that the Stolidobranch ascidians appear to settle to different cues than the Phlebobranch ascidians. Cues to settle seem to be similar in tiny, solitary or well-developed colonial larvae in the Stolidobranch ascidians, even though their morphology is dramatically different. This may be because coloniality has only evolved once in Stolidobranchs, and the colonial species are derived. However, within the Tunicates, coloniality has evolved at least four times independently. These results will be presented and discussed for their implications for chordate evolution.