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, 103 (20), 7723-8

Evidence for a Clade Composed of Molluscs With Serially Repeated Structures: Monoplacophorans Are Related to Chitons

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Evidence for a Clade Composed of Molluscs With Serially Repeated Structures: Monoplacophorans Are Related to Chitons

Gonzalo Giribet et al. Proc Natl Acad Sci U S A.

Abstract

Monoplacophorans are among the rarest members of the phylum Mollusca. Previously only known from fossils since the Cambrian, the first living monoplacophoran was discovered during the famous second Galathea deep-sea expedition. The anatomy of these molluscs shocked the zoological community for presenting serially repeated gills, nephridia, and eight sets of dorsoventral pedal retractor muscles. Seriality of organs in supposedly independent molluscan lineages, i.e., in chitons and the deep-sea living fossil monoplacophorans, was assumed to be a relic of ancestral molluscan segmentation and was commonly accepted to support a direct relationship with annelids. We were able to obtain one specimen of a monoplacophoran Antarctic deep-sea species for molecular study. The first molecular data on monoplacophorans, analyzed together with the largest data set of molluscs ever assembled, clearly illustrate that monoplacophorans and chitons form a clade. This "Serialia" concept may revolutionize molluscan systematics and may have important implications for metazoan evolution as it allows for new interpretations for primitive segmentation in molluscs.

Conflict of interest statement

Conflict of interest statement: No conflicts declared.

Figures

Fig. 1.
Fig. 1.
Details of L. antarctica Warén & Hain, 1992. (A) Shell, dorsal view. Note the limpet-like shape with anterior apex and light reflection caused by prismatic and inner nacreous layers. (B) Scanning electron micrograph of the shell (dorsolateral view from left side). (C) Soft body (shell removed) (dorsal view). Note the characteristic spiral intestine (left) filled with mineral particles, brown-dotted esophageal pouches (right), and serial shell muscles (arrows). (D) Soft body, ventral view. Note the round sucker-like foot (central), serial gills (arrows), and mouth area with tentacles (right).
Fig. 2.
Fig. 2.
Phylogenetic tree depicting the relationships of Monoplacophora to other molluscs based on the combined analysis of all molecular loci. Shown is strict consensus of two most parsimonious trees at 64,679 weighted steps (gap opening cost of 3, gap extension cost of 1, all base transformations cost 2) for the analysis of all data under direct optimization with tree fusing. Numbers on branches indicate jackknife support values. Gastropods (in red) and bivalves (in blue) appear diphyletic. Polyplacophora and Monoplacophora form a well supported clade (95% jackknife support). The monoplacophoran species (purple) appears nested within chitons (dark green), but nodal support for its exact position is low. The tree shows monophyly of molluscs, as well as that of Scaphopoda, Cephalopoda, Caudofoveata, and Solenogastres.
Fig. 3.
Fig. 3.
Alignment of one of the regions of 28S rRNA illustrating that L. antarctica does not share unique chiton synapomorphies (asterisks).

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