Morphology and phylogenetic implications of oesophageal modifications in the Limnodriloidinae (Oligochaeta, Tubificidae)

Abstract

Two features have been considered apomorphic for the subfamily Limnodriloidinae (Tubificidae): the lobed prostate glands, which are broadly attached to prostatic pads inside the atrial ampullae, and the ‘modified oesophagus’ in segment IX. The homology of the modified oesophagus may be questioned, as it is not of the same kind in all taxa. In Limnodriloides, Smithsonidrilus, and Tectidrilus, there is a pair of diverticula, whereas in Thalassodrilides, Parakaketio, and Doliodrilus, the oesophagus is dilated to form a barrel-shaped portion. In a few species of Limnodriloides lacking diverticula, a short part of the corresponding oesophagus is swollen. A scrutiny of these features shows that they are morphologically different in several aspects: the thickness and the structure of the epithelium, the granulation, the shape of the cells and the length of the modifications. Moreover, a regular, semi-embedded blood plexus always occurs around the barrel-shaped portion but rarely around the diverticula (if present, plexus irregular), and never around the swollen part of the oesophagus. These different types of modifications fail the similarity test and thus should be regarded as independently evolved structures and coded as different characters in a parsimony analysis. Species from other subfamilies within the Tubificidae were also morphologically investigated, and modifications of the alimentary canal were found in a few of them: Ainudrilus lutulentus, Heronidrilus bihamis, H. fastigatus (all Rhyacodrilinae) and Clitellio arenarius (Tubificinae). The modification in segment VIII of Heronidrilus spp. resembles the barrel-shaped portion in, for example, Thalassodrilides, and should primarily be coded as the same character in a congruence test of homology.