Phylogenetic Analysis of Thecosomata Blainville, 1824 (Holoplanktonic Opisthobranchia) Using Morphologicaland Molecular Data

© A.Deniaud / Tara Expéditions

Tara Oceans

Emmanuel CORSE, Jeannine RAMPAL, Corinne CUOC, Nicolas PECH, Yvan PEREZ, André GILLES

Plos ONE, April 12, 2103.


Thecosomata is a marine zooplankton group, which played an important role in the carbonate cycle in oceans due to theirshell composition. So far, there is important discrepancy between the previous morphological-based taxonomies, andsubsequently the evolutionary history of Thecosomata. In this study, the remarkable planktonic sampling of TARA Oceansexpedition associated with a set of various other missions allowed us to assess the phylogenetic relationships ofThecosomata using morphological and molecular data (28 S and COI genes). The two gene trees showed incongruities (e.g.Hyalocylis, Cavolinia), and high congruence between morphological and 28S trees (e.g. monophyly of Euthecosomata). Themonophyly of straight shell species led us to reviving the Orthoconcha, and the split of Limacinidae led us to the revival ofEmbolus inflata replacing Limacina inflata. The results also jeopardized the Euthecosomata families that are based onplesiomorphic character state as in the case for Creseidae which was not a monophyletic group. Divergence times were alsoestimated, and suggested that the evolutionary history of Thecosomata was characterized by four major diversifying events.By bringing the knowledge of palaeontology, we propose a new evolutionary scenario for which macro-evolution implyingmorphological innovations were rhythmed by climatic changes and associated species turn-over that spread from theEocene to Miocene, and were shaped principally by predation and shell buoyancy.