THE FATE OF BOTRYLLUS (ASCIDIACEA) LARVAE COSETTLED WITH PARENTAL COLONIES: BENEFICIAL OR DELETERIOUS CONSEQUENCES?

Abstract

The consequences of settlement of Botryllus larvae close to or on parental colonies were followed in two sets of experiments. In the first, 28 experimental progeny settled adjacent to 6 parents; 207 other sibling progeny served as controls. Four different types of interactions between parent colony and offspring were observed: fusion and resorption of the offspring, fusion and separation, tunic-to-tunic contact and separation, tunic-to-tunic contact, and the death of offspring. Offspring interacting with parents had significantly higher mortality than control offspring. Resorption was the fastest process (one week on average); the two "separation" processes lasted approximately two months. Twenty of the 21 progeny that died after interacting with parents did not grow at all (even after 75 days). All 7 offspring that separated from their parents grew. In two cases of fusion between offspring and adults, large eggs were found within the progeny zooids. Presumably the eggs translocated from the maternal colony through the connecting blood vessels. Only five progeny survived in this set of experiments, a phenomenon which coincided with the degeneration or the mortality of the parent. In the second set of experiments all 93 progeny which had settled on old, dead tunics of 5 parental colonies died within 8 weeks. These results indicate that cosettlement of offspring proximal to their parental colony is usually deleterious in the long term to the progeny, both when they fuse with or when they merely contact the parent. This phenomenon was also recorded in field observations. We suggest that the phenomenon of gregarious settlement of Botryllus larvae near their parents, although characterized by the loss of many progeny, is nonetheless advantageous in response to biotic interactions such as interspecific competition. In this view resorption may have evolved as a secondary process, as a result of the nature of self/nonself recognition in Botryllus.