Adhesion and reattachment of compound ascidians to various substrata: weak glue can prevent tissue damage.

Sessile, soft-bodied, compound ascidians are successful competitors for substrata in crowded benthic and epibiotic marine communities and can be effective colonists of new sites, through adult rafting and reattachment. Adhesion to the substratum is essential for these ecologically important functions; we therefore studied the material properties of colony attachment to various substrata in the rafting ascidians Botrylloides sp. We found that, compared with the strength of the colony tissues, the glue attaching Botrylloides sp. to the substratum is very weak. This relative weakness may protect the soft-bodied colonies from damage if they are ripped from their host. For sessile animals, such a weak-glue 'strategy' is only effective if the animals can later reattach to a substratum. By detaching Botrylloides sp. colonies from host eelgrass blades and allowing them to reattach, before measuring peel strengths, we learned that the initial reattachment of a colony depends upon rapid new growth of the colony rather than on fresh secretion of glue beneath old zooids. We also found that the propagation peel force necessary to remove Botrylloides sp. from different substrata (e.g. mussel shells, barnacle basal plates or eelgrass blades) depends upon the surface texture of the host. Thus, the overall tenacity of a colony is affected by the types of substrata that it overgrows.