During the last decades, textbook concepts of “Opisthobranchia” have been challenged by morphology-based and, more recently, molecular studies. It is no longer clear if any clear distinctions can be made between major opisthobranch and pulmonate clades. Worm-shaped, mesopsammic taxa such as Acochlidia, Platyhedylidae, Philinoglossidae and Rhodopemorpha were especially problematic in any morphology-based system. Previous molecular phylogenetic studies contained a very limited sampling of minute and elusive meiofaunal slugs. Our recent multi-locus approaches of mitochondrial COI and 16S rRNA genes and nuclear 18S and 28S rRNA genes (“standard markers”) thus included representatives of most mesopsammic ‘opisthobranchs’ within a comprehensive euthyneuran taxon set.
The present study combines our published and unpublished topologies, and indicates that monophyletic Rhodopemorpha cluster outside of Euthyneura among shelled basal heterobranchs, acteonids are the sister to rissoellids, and Nudipleura are the basal offshoot of Euthyneura. Furthermore, Pyramidellidae, Sacoglossa and Acochlidia cluster within paraphyletic Pulmonata, as sister to remaining ‘opisthobranchs’. Worm-like mesopsammic heterobranch taxa have clear independent origins and thus their similarities are the result of convergent evolution. Classificatory and evolutionary implications from our tree hypothesis are quite dramatic, as shown by some examples, and need to be explored in more detail in future studies.
We do not claim that these concatenated “standard marker” gene trees reflect the true phylogeny of all groups; exploring additional, suitable markers is required. We do claim, however, that improved taxon sampling and improved data quality (such as sequences, alignments) were beneficial towards revealing relationships of higher euthyneuran taxa, and that phylogenetic hypotheses based on this data set are converging. The traditional taxon concept of Opisthobranchia is clearly artificial and thus obsolete. Novel phylogenetic hypotheses, as disturbing they may be at first glance, give us the opportunity and perhaps the obligation to refine our approaches and rethink older paradigms. Most importantly, we see no more way to explore morphology, systematics and evolution of “opisthobranchs” separately from “lower heterobranchs” and “pulmonates”.