CLSM analysis of serotonin-immunoreactive neurons in the central nervous system of Nais variabilis, Slavina appendiculata and Stylaria lacustris (Oligochaeta : Naididae)

Abstract

The organization of the nervous system and the distribution of serotoninergic neurons were analysed in three species of Naididae using immunohistochemical labelling methods in conjunction with confocal laser-scanning microscopy. In the posterior region of the ventral nerve cord the serotoninergic neurons are arranged in an alternating pattern in successive ganglia, clearly different from the pattern in the anterior neuromeres. This unique arrangement of 5HT-neurons is presumably an autapomorphic character of the Naididae or at least of a subtaxon including these three species. In all three species the cerebral ganglion is located in segments behind the prostomium, as has previously been found in terrestrial oligochaetes, where the relocation of the brain from the initially anterior position in the prostomium into the following segments can be explained as a functional adaptation to a life spent burrowing through comparatively firm substrates. This structural correspondence is regarded as an indication that the Naididae, being typical limnic oligochaetes, are derived from a terrestrial form.