Variability of hair cells in the coronal organ of ascidians (Chordata, Tunicata).

The tunicate ascidians are non-vertebrate chordates, possessing mechanoreceptor cells in the coronal organ in the oral siphon, which monitor the incoming water flow. Like vertebrate hair cells, the mechanoreceptor/coronal cells are secondary sensory (axonless) cells accompanied by supporting cells, and exhibit morphological diversities of apical specialisations: they are multiciliate in ascidians of the order Enterogona, whereas in the other order (Pleurogona) they are more complex and possess one or two cilia accompanied by stereovilli, also graded in length. In morphology, embryonic origin, and arrangement, coronal sensory cells closely resemble vertebrate hair cells. We describe here the coronal organs of five ascidians (Pyura haustor and P. stolonifera, Styela gibbsii and S. montereyensis; Polyandrocarpa zorritensis), belonging to Pleurogona, also comprising species of one family (Pyuridae), not yet considered, and thus completing our overview of the order. Each species possesses at least two kinds of secondary sensory cells, some of them characterised by stereovilli graded in length. In some species, the coronal sensory cells exhibit secretory activity; in P. haustor, a mitotic sensory cell has also been found. We compare the coronal organ in both ascidians and with other chordate sensory organs formed of secondary sensory cells, and discuss their possible homologies.