title: Evolutionary history of Chaetognatha inferred from molecular and morphological data: a case study for body plan simplification.
abstract: BACKGROUND
  Chaetognatha are a phylum of marine carnivorous animals which includes more than 130 extant species. The internal systematics of this group have been intensively debated since it was discovered in the 18(th) century. While they can be traced back to the earlier Cambrian, they are an extraordinarily homogeneous phylum at the morphological level - a fascinating characteristic that puzzled many a scientist who has tried to clarify their taxonomy. Recent studies which have attempted to reconstruct a phylogeny using molecular data have relied on single gene analyses and a somewhat restricted taxon sampling. Here, we present the first large scale phylogenetic study of Chaetognatha based on a combined analysis of nearly the complete ribosomal RNA (rRNA) genes. We use this analysis to infer the evolution of some morphological characters. This work includes 36 extant species, mainly obtained from Tara Oceans Expedition 2009/2012, that represent 16 genera and 6 of the 9 extant families.
  
  
  RESULTS
  Cladistic and phenetic analysis of morphological characters, geometric morphometrics and molecular small subunit (SSU rRNA) and large subunit (LSU rRNA) ribosomal genes phylogenies provided new insights into the relationships and the evolutionary history of Chaetognatha. We propose the following clade structure for the phylum: (((Sagittidae, Krohnittidae), Spadellidae), (Eukrohniidae, Heterokrohniidae)), with the Pterosagittidae included in the Sagittidae. The clade (Sagittidae, Krohnittidae) constitutes the monophyletic order of Aphragmophora. Molecular analyses showed that the Phragmophora are paraphyletic. The Ctenodontina/Flabellodontina and Syngonata/Chorismogonata hypotheses are invalidated on the basis of both morphological and molecular data. This new phylogeny also includes resurrected and modified genera within Sagittidae.
  
  
  CONCLUSIONS
  The distribution of some morphological characters traditionally used in systematics and for species diagnosis suggests that the diversity in Chaetognatha was produced through a process of mosaic evolution. Moreover, chaetognaths have mostly evolved by simplification of their body plan and their history shows numerous convergent events of losses and reversions. The main morphological novelty observed is the acquisition of a second pair of lateral fins in Sagittidae, which represents an adaptation to the holoplanktonic niche.
authors.affiliation: Aix-Marseille Université, CNRS, IRD, Avignon Université, IMBE UMR 7263, 13331 Marseille cedex 3, France ; Université de Tunis El Manar, Faculté des Sciences de Tunis, UR11ES12 Biologie de la Reproduction et du Développement animal, 2092 El Manar, Tunis Tunisie.
authors.forename: Samah
authors.initials: S
authors.lastname: Gasmi
authors.affiliation: Aix-Marseille Université, CNRS, IRD, Avignon Université, IMBE UMR 7263, 13331 Marseille cedex 3, France.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Nève
authors.affiliation: Aix-Marseille Université, CNRS, IRD, Avignon Université, IMBE UMR 7263, 13331 Marseille cedex 3, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Pech
authors.affiliation: Université de Tunis El Manar, Faculté des Sciences de Tunis, UR11ES12 Biologie de la Reproduction et du Développement animal, 2092 El Manar, Tunis Tunisie.
authors.forename: Saïda
authors.initials: S
authors.lastname: Tekaya
authors.affiliation: Aix-Marseille Université, CNRS, IRD, Avignon Université, IMBE UMR 7263, 13331 Marseille cedex 3, France.
authors.forename: André
authors.initials: A
authors.lastname: Gilles
authors.affiliation: Aix-Marseille Université, CNRS, IRD, Avignon Université, IMBE UMR 7263, 13331 Marseille cedex 3, France.
authors.forename: Yvan
authors.initials: Y
authors.lastname: Perez
keywords: Body plan simplification; Chaetognatha; Homoplasy; Phylogenetics; Procrustes surimposition; Systematics