Phylogeny of Molluscs
World Congress of Malacology
Perth, Western Australia
11-16 July 2004
On July 11th 2004 over 300 biologists and paleontologists from more than 40 countries gathered in Perth, Australia for the World Congress of Malacology. Malacology, or the study of molluscs (e.g., snails, clams, squid and octopus), focuses on understanding one of the most diverse groups of organisms on earth. Mollusca are diverse in many ways, including body form and size which can range from giant squids over 20 meters in length to adult snails with body sizes of less than half a millimeter. Molluscs are also thought to represent a significant amount of biodiversity as well with estimates of over 200,000 living species. They also exhibit a great range of physiological, behavioral, and ecological adaptations and habitats, and have an excellent fossil record extending back some 560 million years. With so many lineages over such deep time it is hard to imagine any topical biological or paleontological question that cannot be addressed with a molluscan exemplar or system, and the papers and posters at Perth did little to negate this assumption.
One of the centerpieces of the Perth Congress was the symposium on the Phylogeny of Molluscs organized by Winston F. Ponder (Australian Museum, Sydney) and David R. Lindberg (University of California, Berkeley). This symposium, co-sponsored by the University of California Museum of Paleontology, brought together a remarkable group of scientists to present and discuss current hypotheses on the relationships and evolutionary history of the Mollusca. The three day symposium featured 38 presentations and 9 posters. Speakers were intentionally selected to both complement and contrast different view points, datasets, and methodologies. The organizers were not looking for consensus, but rather a critical review and status report of our understanding of the evolutionary history of this major branch on the Tree of Life. So while Gerhard Hazsprunar (Germany) and Andres Wanninger (Denmark) sought molluscan sister taxa on morphological criteria, Kenneth Halanych (USA) and colleagues looked to molecular data to resolve these deep, early divergences. While the results of these two studies were not congruent, they did establish an awareness of the problems and collaborative approaches needed to resolve such distance events in the history of life. This theme was repeated many times over the course of the symposium.
Molecular approaches figured prominently in the question of relationships amongst the higher molluscan groups (i.e., are clams more closely related to snails than snails are to squid). Multi-gene approaches where highlighted by Gonzalo Giribet and colleagues (USA), while Rae Ueshima (Japan) used comparative genomics of molluscan mitochondrial DNA, and Andreas Wanninger (Denmark) and Bernie Degan (Australia) explored phylogenetic markers associated with morphogenetic and gene expression patterns across major molluscan groups.
Within putative molluscan subclades, Annie Lindgren and Michele Nishiguchi (USA) took a combined evidence approach bringing both molecular and morphological data to bear on the question of cephalopod evolution, while Jan Strugnell and colleagues (UK) examined the relationships of squids using a multi-gene approach. Relationships within the gastropods were a major topic, especially the long-neglected and largest subclade, the Caenogastropoda. The earliest caenogastropods of the Late Paleozoic and Early Mesozoic were examined by Alex Nützel (Germany) and Klaus Bandel (Germany). Don Colgan and colleagues (Australia) examined caenogastropod relationships with a multi-gene approach and Ellen Strong and Jerry Harasewych used a single molecular marker to test the monophyly of the Heteropoda and their placement within the Caenogastropoda. The current state of knowledge regarding caenogastropod phylogeny was summarized by Winston Ponder (Australia). Opisthobranch phylogeny was explored by Heike Wäegel and Annette Klussmann-Kolb (Germany) and Peter Mordan and Chris Wade (United Kingdom) asked what we really know about pulmonate phylogeny.
Additional paleontological perspectives on molluscan evolution were presented by Pavel Parkhaev (Russia) who brought the sometimes familiar, but often bizarre morphologies of 500 Ma old limpets and snails to life, and Jiri Frýda (Czech Republic) who investigated the appearance of feeding larval protoconchs in gastropods during the late Paleozoic. Lastly, Peter Wagner (USA) used a statistical likelihood approach to test alternative phylogenetic hypotheses of early gastropods.
Following a trend seen in the last two World Congresses, the dominance of cladograms, molecular datasets, and combined analyses was striking. The routine utilization of modern comparative methods and molecular datasets are now providing critical reassessments of earlier, less rigorous, attempts to divine molluscan relationships. What was unexpected was the number of papers that deconstructed some of the long-established and seemingly sacrosanct familiar groupings – the very suggestion of which would have been viewed as systematic heresy just a few short years ago. Moreover, these findings ranged widely across the hierarchical ranks from molluscan genera to subphyla and invoked lively discussion rather than outright rejection.
Kenneth Halanych (USA) and his colleagues trampled on conventional wisdom with molecular trees that suggested that the Aplacophora were neither a clade nor grade, but rather convergent molluscan ventures into worm-like morphologies. Gonzalo Giribet’s (USA) study of the evolution and phylogeny of the Bivalvia again raised the question of the monophyly of the Bivalvia with regards to the placement of the protobranch bivalves. Peter Wagner (USA) presented analyses that clearly support diphyletic origins for the extinct bellerophonts, and Akiko Okusu et al. (USA) deconstructed several long recognized familial level taxa in the Polyplacophora based on multi-gene analyses. Brian Simison and David Lindberg (USA) showed possible diphyletic origins of the Gastropoda based on mitochondrial gene order and amino acid sequence analyses of complete genes, and Lindberg committed further heresy with character analyses and models that could not refute diphyly for the true limpets (patellogastropods). Lastly, Susan Williams (United Kingdom) and Tomowo Ozawa (Japan) deconstructed another long recognized group, the Turbinidae, based on multi-gene analyses. Some of these groups have been recognized since Aristotle, and their demise as non-evolutionary lineages was unanticipated by most of the symposium participants although this theme reverberated through these and many other talks. The only other theme that was more common was the cautionary note that “more data is needed.”
For further information:
Winston F. Ponder David R. Lindberg
Australian Museum Dept. of Integrative Biology
6 College Street University of California
Sydney, NSW 2010 Berkeley, CA 94720
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