Hibernation of the colonial ascidian Botrylloides leachi (Savigny): histological observations

In a previous paper the biological cycle of the colonial ascidian Botrylloides leachi, in the Laguna Veneta, was described (BRUNETTI 1976). Such research indicated that during the winter this ascidian undergoes a process of hibernation, which resembles, in some respects, that described by BANCROFT (1903) as “aestivation” in a colony of Botrylloides gascoi (synonymous with Botrylloides leachi, see SALFI, 1931). During hibernation the colony lacks filtering zooids and resembles a homogeneous carpet of ampullae which is usually covered by diatoms and detritus. In spring, with a rise in temperature new zooids are formed, these emerge from the tunic, and gradually reconstruct new “ladder systems”. Due to the opacity resulting from the high density of pigmented cells, the events occurring within the colony during hibernation and vernal reactivation are not easily observed “in vivo”. To overcome this difficulty we have performed a histological study on colonies collected at various periods in order to determine the more characteristic features of this process and follow them in greater detail than is possible with living material.