Ultrastructure and tubulin immunocytochemistry of the copulatory stylet‐like structure in Childia species (Acoela)

One of the main characters used in acoel taxonomy is the male copulatory organ. Despite this, ul- trastructural studies of this structure are scarce. We studied the ultrastructure of the copulatory organ in eight species of acoels belonging to the taxon Childia. Members of Childia possess a well-developed conical or cylindrical stylet-like structure composed of needles. Immunogold cytochemistry of tubulin was used to deter- mine the composition of the needles. Stylet-like struc- tures of Childia species at the ultrastructural level are basically similar. Stylet needles show intracellular differ- entiations. As shown both by ultrastructural and immu- nocytochemical methods, the stylet needles, in all species studied, are composed of long, parallel microtubules, ei- ther tightly packed or polymerized. We report unusual polymerization of microtubules, resulting in formation of a honeycomb-like structure in cross section. Variations of ultrastructure among Childia species include numbers and arrangement of stylet needles, shape of needles, nee- dle compactness, microtubule polymerization, direction of stylet growth, and presence/absence of different types of granules. The stylet-like structures are homologous within Childia, but are likely to prove nonhomologous with the other needle-like structures found in acoel copu- latory organs. Stylets in Platyhelminthes are not homolo- gous with stylet-like structures in acoels. J. Morphol.