Invertebrate Systematics Invertebrate Systematics Society
Systematics, phylogeny and biogeography
RESEARCH ARTICLE

Chiton phylogeny (Mollusca : Polyplacophora) and the placement of the enigmatic species Choriplax grayi (H. Adams & Angas)

Julia D. Sigwart A D , Isabella Stoeger B , Thomas Knebelsberger C and Enrico Schwabe B

A Queen’s University Belfast, School of Biological Sciences, Marine Laboratory, 12-13 The Strand, Portaferry BT22 1PF, N. Ireland.

B Bavarian State Collection of Zoology, Münchhausenstrasse 21, 81247 Munich, Germany.

C Senckenberg Research Institute, German Centre for Marine Biodiversity Research (DZMB), Südstrand 44, 26382 Wilhelmshaven, Germany.

D Corresponding author. Email: j.sigwart@qub.ac.uk

Invertebrate Systematics 27(6) 603-621 http://dx.doi.org/10.1071/IS13013
Submitted: 28 March 2013  Accepted: 2 September 2013   Published: 20 December 2013

Abstract

Shallow marine chitons (Mollusca : Polyplacophora : Chitonida) are widespread and well described from established morphoanatomical characters, yet key aspects of polyplacophoran phylogeny have remained unresolved. Several species, including Hemiarthrum setulosum Carpenter in Dall, 1876, and especially the rare and enigmatic Choriplax grayi (Adams & Angas, 1864), defy systematic placement. Choriplax is known from only a handful of specimens and its morphology is a mosaic of key taxonomic features from two different clades. Here, new molecular evidence provides robust support for its correct association with a third different clade: Choriplax is placed in the superfamily Mopalioidea. Hemiarthrum is included in Cryptoplacoidea, as predicted from morphological evidence. Our multigene analysis of standard nuclear and mitochondrial markers demonstrates that the topology of the order Chitonida is divided into four clades, which have also been recovered in previous studies: Mopalioidea is sister to Cryptoplacoidea, forming a clade Acanthochitonina. The family Callochitonidae is sister to Acanthochitonina. Chitonoidea is resolved as the earliest diverging group within Chitonida. Consideration of this unexpected result for Choriplax and our well-supported phylogeny has revealed differing patterns of shell reduction separating the two superfamilies within Acanthochitonina. As in many molluscs, shell reduction as well as the de novo development of key shell features has occurred using different mechanisms, in multiple lineages of chitons.


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