COWART, J.D.*; PAWLIK, J.R.; University of North Carolina at Wilmington; University of North Carolina at Wilmington: The Ascidian Tunic: A First Line of Defense Against Potential Predators?

Investigations of defensive mechanisms of ascidians have focused primarily on the orders Aplousobranchia and Phlebobranchia. The tunics of members of these orders are known to contain extremely high concentrations of vanadium and to possess bladder cells that release highly acidic fluids when the tunic is damaged. High concentrations of vanadium (≥ 100 μm) and sulfuric acid (pH < 4) have been shown to significantly reduce food consumption by fishes, crustaceans, and gastropods. In contrast, the tunics of ascidians within the order Stolidobranchia lack both vanadium and bladder cells. Stolidobranch ascidians have been identified as having a low susceptibility to predation, even though they do not concentrate vanadium or possess bladder cells. Secondary metabolites as chemical defenses may effectively deter predators in the absence of vanadium and bladder cells. In addition, the tough tunics produced by most solitary stolidobranch ascidians could make consumption difficult for predators. Finally, low nutritive value of the tunic could minimize predation on ascidians. The purpose of this experiment is to determine the physical (tensile strength, elasticity, and nutritive value) and chemical (secondary metabolites) properties of stolidobranch ascidian tunics as a means of determining how effective the tunic is as a first line of defense against predation.