The Biological Bulletin, Vol 184, Issue 1 57-78, Copyright © 1993 by Marine Biological Laboratory

ECOLOGY AND EVOLUTION

Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula

A. H. Scheltema
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

Evidence is presented in support of the following phylogenetic hypotheses: (1) Sipuncula are the sister taxon of Mollusca; (2) the two aplacophoran taxa, Neomeniomorpha (= neomenioids) and Chaetodermomorpha (= chaetoderms), are monophyletic with a common neomenioid-like ancestor, and of the two taxa, Chaetodermomorpha are more derived; (3) Aplacophora and Polyplacophora are sister taxa and form a clade, Aculifera; (4) Aculifera are the sister group of the remaining extant mollusks, Conchifera; and (5) Aplacophora are progenetic Aculifera. The evidence is based on homologies of early and late embryological development, adult morphologies, and molecular analyses. Embryological development in sipunculans and mollusks shows a close relationship between them, and embryological development of the shell separates Aculifera and Conchifera. Adult morphologies indicate: (1) monophyly of Aplacophora; (2) sister-group relationship between Aplacophora and Polyplacophora; (3) a molluscan plesiomorphy of nonsegmented serial replication of organs; and (4) progenesis in Aplacophora. Molecular evidence supports the embryological and morphological relationships between Sipuncula and Mollusca. Mollusca are thus hypothesized to be coelomate Eutrochozoa, which share an ancestor that probably had serial replication of organs. Differences in size and structure of the coelom among Eutrochozoa are hypothesized to have been brought about by changes in the timing and the process of cavitation of the mesodermal bands that arise from cell 4d. Through the process of progenesis Aplacophora retained an ovoid embryological shape and several internal structures that, although they appear to be in a primitive state, are actually secondarily derived as is quadrant D specification during early cleavage.

This article has been cited by other articles:

				E. A. Sperling, J. Vinther, V. N. Moy, B. M. Wheeler, M. Semon, D. E. G. Briggs, and K. J. Peterson MicroRNAs resolve an apparent conflict between annelid systematics and their fossil record Proc R Soc B, December 22, 2009; 276(1677): 4315 - 4322. [Abstract] [Full Text] [PDF]

				N. J. Butterfield An Early Cambrian Radula Journal of Paleontology, May 1, 2008; 82(3): 543 - 554. [Abstract] [Full Text] [PDF]

				B. Hausdorf, M. Helmkampf, A. Meyer, A. Witek, H. Herlyn, I. Bruchhaus, T. Hankeln, T. H. Struck, and B. Lieb Spiralian Phylogenomics Supports the Resurrection of Bryozoa Comprising Ectoprocta and Entoprocta Mol. Biol. Evol., December 1, 2007; 24(12): 2723 - 2729. [Abstract] [Full Text] [PDF]

				J. D Sigwart and M. D Sutton Deep molluscan phylogeny: synthesis of palaeontological and neontological data Proc R Soc B, October 7, 2007; 274(1624): 2413 - 2419. [Abstract] [Full Text] [PDF]

				S. Shigeno, T. Sasaki, and G. Haszprunar Central Nervous System of Chaetoderma japonicum (Caudofoveata, Aplacophora): Implications for Diversified Ganglionic Plans in Early Molluscan Evolution Biol. Bull., October 1, 2007; 213(2): 122 - 134. [Abstract] [Full Text] [PDF]

				G. Giribet, A. Okusu, A. R. Lindgren, S. W. Huff, M. Schrodl, and M. K. Nishiguchi Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons PNAS, May 16, 2006; 103(20): 7723 - 7728. [Abstract] [Full Text] [PDF]

				J. L. Boore, M. Medina, and L. A. Rosenberg Complete Sequences of the Highly Rearranged Molluscan Mitochondrial Genomes of the Scaphopod Graptacme eborea and the Bivalve Mytilus edulis Mol. Biol. Evol., August 1, 2004; 21(8): 1492 - 1503. [Abstract] [Full Text] [PDF]

				A. S. Mount, A. P. Wheeler, R. P. Paradkar, and D. Snider Hemocyte-Mediated Shell Mineralization in the Eastern Oyster Science, April 9, 2004; 304(5668): 297 - 300. [Abstract] [Full Text] [PDF]

				R. A. Jenner Unleashing the Force of Cladistics? Metazoan Phylogenetics and Hypothesis Testing Integr. Comp. Biol., February 1, 2003; 43(1): 207 - 218. [Abstract] [Full Text] [PDF]

				K. M. Halanych, T. G. Dahlgren, and D. McHugh Unsegmented Annelids? Possible Origins of Four Lophotrochozoan Worm Taxa Integr. Comp. Biol., July 1, 2002; 42(3): 678 - 684. [Abstract] [Full Text] [PDF]

				J. L. Boore and J. L. Staton The Mitochondrial Genome of the Sipunculid Phascolopsis gouldii Supports Its Association with Annelida Rather than Mollusca Mol. Biol. Evol., February 1, 2002; 19(2): 127 - 137. [Abstract] [Full Text] [PDF]

				S. Johnsen Hidden in Plain Sight: The Ecology and Physiology of Organismal Transparency Biol. Bull., December 1, 2001; 201(3): 301 - 318. [Abstract] [Full Text] [PDF]

				K. M. Halanych and Y. Passamaneck A Brief Review of Metazoan Phylogeny and Future Prospects in Hox-Research Integr. Comp. Biol., June 1, 2001; 41(3): 629 - 639. [Abstract] [Full Text] [PDF]