Serotonergic and FMRFamidergic nervous systems in gymnolaemate bryozoan larvae

A growing body of data from nervous systems of marine invertebrate larvae provides an ideal background for comparisons among higher taxa. The currently available data from Bryozoa, however, do not allow for a consistent hypothesis of an ancestral state for this taxon, which would be necessary for phylogenetic inferences. The larval nervous systems of the four gymnolaemate species Flustrellidra hispida, Bugula fulva, Alcyonidium gelatinosum, and Bowerbankia gracilis are examined by means of antibody staining against the neurotransmitters serotonin and FMRFamide, as well as against acetylated α-tubulin. Despite considerable variation, a comparison reveals a common pattern of the distribution of serotonin. The neurotransmitter is found in at least two cells in the apical organ as well as in paired axial and lateral nerves emerging from a central nerve nodule. A ring nerve is present below the corona and at least two serotonergic cells are found between the corona cells. Serotonergic coronal cells might represent unique bryozoan features, whereas the remaining elements show resemblance to the situation found in most spiralian taxa. The data do not provide support for a closer relationship of Bryozoa to Phoronida or Brachiopoda.