On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia

Abstract

Background: A robust phylogenetic hypothesis of euthyneuran gastropods, as a basis to reconstructing their evolutionary history, is still hindered by several groups of aberrant, more or less worm-like slugs with unclear phylogenetic relationships. As a traditional "order" in the Opisthobranchia, the Acochlidia have a long history of controversial placements, among others influenced by convergent adaptation to the mainly meiofaunal habitats. The present study includes six out of seven acochlidian families in a comprehensive euthyneuran taxon sampling with special focus on minute, aberrant slugs. Since there is no fossil record of tiny, shell-less gastropods, a molecular clock was used to estimate divergence times within Euthyneura.

Results: Our multi-locus molecular study confirms Acochlidia in a pulmonate relationship, as sister to Eupulmonata. Previous hypotheses of opisthobranch relations, or of a common origin with other meiofaunal Euthyneura, are clearly rejected. The enigmatic amphibious and insectivorous Aitengidae incerta sedis clusters within Acochlidia, as sister to meiofaunal and brackish Pseudunelidae and limnic Acochlidiidae. Euthyneura, Opisthobranchia and Pulmonata as traditionally defined are non-monophyletic. A relaxed molecular clock approach indicates a late Palaeozoic diversification of Euthyneura and a Mesozoic origin of the major euthyneuran diversity, including Acochlidia.

Conclusions: The present study shows that the inclusion of small, enigmatic groups is necessary to solve deep-level phylogenetic relationships, and underlines that "pulmonate" and "opisthobranch" phylogeny, respectively, cannot be solved independently from each other. Our phylogenetic hypothesis requires reinvestigation of the traditional classification of Euthyneura: morphological synapomorphies of the traditionally defined Pulmonata and Opisthobranchia are evaluated in light of the presented phylogeny, and a redefinition of major groups is proposed. It is demonstrated that the invasion of the meiofaunal habitat has occurred several times independently in various euthyneuran taxa, leading to convergent adaptations previously misinterpreted as synapomorphies. The inclusion of Acochlidia extends the structural and biological diversity in pulmonates, presenting a remarkable flexibility concerning habitat choice.

Figure 1

Origin of Acochlidia within Heterobranchia. RAxML analysis based on the combined dataset (18S, 28S, 16S, COI), masked with Gblocks. Bootstrap support and posterior probabilities given (only BS ≥ 50 and PP ≥ 0.95 are considered relevant and thus shown).

Figure 2

Chronogram of Heterobranchia. Showing estimated divergence times obtained from BEAST v1.5.3 under the uncorrelated lognormal relaxed clock model. Numbers at nodes refer to node ages in this presented tree (maximum-clade-credibility-tree); bars express 95% highest posterior density (HPD) (only presented for nodes with a PP > 0.5). Red circles indicate calibration points. Geological timescale is based on the International Stratigraphic Chart by the International Commission on Stratigraphy (2009).

Figure 3

Proposed reclassification of Euthyneura, discussed groups shown. Nodes with significant support (i.e. > 75% bootstrap support (BS) and > 0.95 posterior probability (PP)) marked by dots; nodes with > 0.95 PP but low BS marked by circles. (Note: "Lower Heterobranchia" does not form a clade in the present study (see Figure 1), the branches are collapsed in the present Figure for illustration purposes.)