IMPULSE PROPAGATION AND CONTRACTION IN THE TUNIC OF A COMPOUND ASCIDIAN

Diplosoma listerianum and D. macdonaldi (Fam. Didemnidae) have a network of cells ("monocytes") in the tunic which contain high concentrations of microfilaments and react positively with NBD-phallacidin, indicating the presence of F-actin. The tunic is contractile, especially in the areas around the cloacal apertures, which can be closed completely. Myocytes are concentrated in sphincter-like bundles around these openings, but also are found throughout the tunic. Electrophysiological recordings reveal a diffuse conduction system in the tunic propagating all-or-none impulses ("tunic potentials," TPs) through all parts with a conduction velocity of < 1.5 cm · s^-1, and a refractory period of 1.6 s. TPs correlate one-for-one with contractions. The system is excitable to the touch, but is also spontaneously active, showing steady patterns of potentials as well as regular, `parabolic' bursts. The evidence suggests that the myocyte net itself conducts the impulses triggering the contractions. In the absence of conventional nerves and muscles, the system provides the colony with a way of regulating the effluent water current and hence the volume of a common cloacal space. The TP system is not `wired in' to the ascidiozooids either as a sensory or as a motor pathway. The tunic acts as an independent behavioral entity.