Ultrastructural aspects of naturally occurring wound in the tunic of two ascidians: Ciona intestinalis and Styela plicata (Tunicata)

Efficient wound healing is essential for all animals from insects to mammals. Ciona intestinalis and Styela plicata are solitary ascidians belonging to urochordates, a subphylum that occupies a key phylogenetic position as it includes the closest relative to vertebrates. Urochordate first physical barrier against invaders is the tunic, an extracellular matrix that is constantly exposed to all kinds of insults. Thus, when damage occurs, an innate immune response is triggered to eliminate impaired tissue and potentially pathogenic microbes, and restore tissue functionality. Ultrastructural aspects of the tunic in the wound healing process of two ascidians are described. In the injured areas, we evidenced thinning of the tunic and areas of low fibre density, dense intratunic bacterial and protozoan population, and inflammatory aspects such as the increase in tunic cells, their aggregates, and phagocytosis. This is the first report on tunic physical wounding occurring in the natural habitat.