Skeletal morphology of two controversial Poecilosclerid genera (Porifera, Demospongiae):Discorhabdella andCrambe
© Biologische Anstalt Helgoland 1996
The generaDiscorhabdella andCrambe are characterized by bearing uncommon spicule types, i.e. pseudoastrose acanthostyles and sphaeroclones, respectively. They have traditionally been considered to be unrelated taxa, but the present reexamination made evident that an important amount of skeletal features are shared by both. Some of these morphological features, such as the ornamentation on the point of the ectosomal subtylostyles, are reported for the first time. The study also revealed that a tuberose nature of the tyles of the main choanosomal megascleres could be a common ancestral condition in both genera. The morphology of the multi-toothed anchorate chelae showed a gradual transition across the species, suggesting that the morphological diversity in chelae was generated in these genera through a “palmate-anchorate-arcuate” evolutionary sequence. However, the forward or backward direction of this sequence remained unclear from the available evidence. Important levels of skeletal variability were found to affect many of the skeletal characters, especially in the genusCrambe. In some cases, this variability transgressed the limits theoretically defining a species, making evident that the traditional procedure just based on comparison of the skeletons becomes unreliable when tackling the taxonomy of these genera. Most of the skeletal variability seemed to correspond to genetic polymorphisms, except in the case ofC. acuata. In this taxon, the skeletal variability could be a result of the existence of a cryptic species, originated by a misconceived synonymy betweenC. acuata andC. chelastra. Besides the skeletal variability, the obscure taxonomic meaning of many skeletal features favored the existence of conflicting taxonomic proposals for the suprageneric location of these genera, depending on the author’s criteria. This study made evident that any subsequent attempt of phylogenetic inference should be based on an unweighted analysis of the available skeletal information.