Hibernation of the Colonial Ascidian Botrylloides Leachi (Savigny): Histological Observations

Abstract

Fragments and whole colonies of B. leachi collected during different phases of hibernation have been studied histologically. Hibernation occurs in progressive phases and involves: regression of all the adult zooids; loss of coordination of development in the buds; regression of the buds beginning with those most highly developed; production of a large populations of undifferentiated buds. These events are accompanied by a notable development of the circulatory system of the tunic and an enormous increase in the number of macrophages, which also accumulate reserve materials. In severe winters, the hibernating colonies become reduced to a carpet of ampullae filled with macrophages; among these ampullae are numerous buds that have been blocked at the earliest stages of development. The histological picture suggests a probable vascular origin for these buds, which begin to form as aggregates of hemocytes. However, it is not possible to exclude that at least some of the buds seen are of palleal origin, these having precociously separated from their parents and thus escaped degeneration. At the time of vernal reactivation, some of the bud primordia probably selected by competitive mechanism, give rise to the new zooids in the expanding colony. © 1976 Taylor & Francis Group, LLC.