IMPULSE PROPAGATION AND CONTRACTION IN THE TUNIC OF A COMPOUND ASCIDIAN

Area of Science:

• Marine Biology
• Cellular Physiology
• Ascidian Biology

Background:

• Didemnid ascidians (Diplosoma listerianum and D. macdonaldi) possess a specialized tunic.
• This tunic contains a network of cells identified as myocytes, rich in F-actin.

Purpose of the Study:

• To investigate the contractile properties and nervous system of the ascidian tunic.
• To understand the mechanism of impulse conduction and its role in colony behavior.

Main Methods:

• Histological staining with NBD-phallacidin to detect F-actin.
• Electrophysiological recordings to study tunic potentials (TPs).
• Observation of tunic contractions in response to stimuli.

Main Results:

• Myocytes in the tunic contain high concentrations of F-actin and form sphincter-like bundles around cloacal apertures.
• Electrophysiology revealed a diffuse conduction system in the tunic propagating all-or-none tunic potentials (TPs) with a slow conduction velocity (< 1.5 cm/s).
• TPs correlated one-to-one with tunic contractions, indicating the myocyte network conducts impulses and the tunic acts as an independent behavioral entity.

Conclusions:

• The ascidian tunic possesses an intrinsic, nerve- and muscle-independent system for impulse conduction and contraction mediated by myocytes.
• This system allows regulation of water current and cloacal space volume, functioning as an independent behavioral unit.
• The tunic's contractile system is not integrated with the ascidiozooids' sensory or motor pathways.