Innervation of Unpaired Branchial Appendages in the Annelids Terebellides cf. stroemii (Trichobranchidae) and Cossura pygodactylata (Cossuridae)

Abstract

We studied the innervation of unpaired branchial appendages using confocal laser scanning microscopy in two annelid species: Terebellides cf. stroemii (Trichobranchidae) and Cossura pygodactylata (Cossuridae). The branchial filament was found to be innervated by one unpaired segmental nerve coming from the ventral nerve cord at the level of chaetiger 2 in C. pygodactylata. This nerve lies transversally along the body wall and comes laterally to the base of the branchial filament. Such a structure indicates that (1) the branchial filament is unpaired and neither represents merged paired branchiae nor is related in its origin to prostomial appendages or peristomial ones, and (2) the branchial filament is the result of a shift of one of the paired segmental branchiae with parallel reduction of the second branchia in this pair. The branchial organ of T. cf. stroemii is innervated by paired segmental nerves from the first and second chaetigers. The segmental nerves are connected by longitudinal nerves going on the lateral sides of the body closer to the dorsum. One nerve on each side proceeds to the anterior part of the branchial stalk, and another nerve proceeds to the posterior part. All these nerves have transversal connections. The anterior stalk nerves come into the anterior branchial lobes, while the posterior nerves come to the posterior lobes. In the branchial lobes, the nerves give rise to thinner nerves, which go to each of the branchial lamellae. The present study confirms that the branchial organ of Terebellides has a paired nature and originates from segmental branchiae. Based on the number of innervating segmental nerves, we can conclude that the branchial organ of T. cf. stroemii is formed by two pairs of merged branchiae.