Monday, August 11, 2014

Are Distributions of Ancient Lineages of Mollusks Shaped by Vicariance or Dispersal?

                It is thought that the study of animal and plant distributions advanced greatly with the development of vicariance biogeography in the 1970s and 1980s.  Rather than assuming related groups of organisms dispersed around the globe across seemingly insurmountable barriers, it was thought that organisms rafted apart from one another on drifting continents via plate tectonics yielding many groups that shared a common Earth history.  Vicariance biogeography revolutionized the discipline and stimulated many studies with an interest in constructing area cladograms and searching for generalized patterns.  Indeed, its influence was so profound that dispersal was considered less and less likely and some even believed it could not be falsified and therefore was unscientific.  Over the past decade, however, numerous studies have been published showing that the distribution of a number of taxonomic groups formerly thought to be due to vicariance are best explained by dispersal.  Indeed, one maverick, Alan de Queiroz, has proposed that most distributions can be attributed to dispersal.  He tells his very interesting story in a recently published book entitled The Monkey’s Voyage – How Improbable Journeys Shaped the History of Life.
                De Queiroz proposes a new paradigm for historical biogeography that entails three steps (1) acceptance of plate tectonics, (2) building evolutionary trees utilizing cladistic thinking or other statistical means, and the step that was missing in vicariance biogeography (3) time.  Time is an absolutely essential component and comes from molecular dating results.  So for example, the New World and Old World monkey molecular timeline is a split between 26 to 51 million years ago, which means they missed the vicariant event separating Africa from South America.  Therefore, Old World monkeys must have dispersed to South America.
                De Queiroz presents numerous case studies and makes a compelling case for dispersal shaping the history of life in a much more significant way than thought of during the vicariance revolution.  But is it possible, the pendulum will swing too far back from vicariance being dominant mechanism to dispersal without thoroughly investigating the matter?   For example, many of the case studies are vertebrate groups that simply are not old enough to play a part in many vicariant events including the ever popular break-up of Gondwana (although this has not stopped investigators from proposing vicariance during the vicariant movement).  However, de Queiroz cites a few exceptions involving two lineages of mite harvestmen (Giribet et al., 2012) and two lineages of centipedes (Murienne et al., 2010) that appear to be Gondwanan relicts dating back to 90 million years ago or earlier.  What about other ancient invertebrate groups?  What about mollusks?  It is known that dispersal plays a fundamental role in the evolution of biodiversity on oceanic islands including terrestrial gastropods (Cowie and Holland, 2006), but what about the break-up of continental landmasses like Gondwana?   Clearly, many mollusk lineages are ancient enough to possibly be shaped by vicariance, but studies are needed to determine whether this is the case or not.  I encourage malacologists to rise to this interesting challenge.

Cowie, R. H., and B. S. Holland.  2006.  Dispersal is fundamental to biogeography and the evolution of biodiversity on oceanic islands.  Journal of Biogeography 33:193-198.

Giribet, G., et al.  2012.  Evolutionary and biogeographical history of an ancient and global group of arachnids (Arachnida: Opiliones: Cyphophthalmi) with a new taxonomic arrangement.  Biological Journal of the Linnean Society 105:92-130.

De Queiroz, A.  2014.  The Monkey’s Voyage:  How Improbable Journeys Shaped the History of Life.  Basic Books, New York. 

Murienne, J., G. D. Edgecombe, and G. Giribet.  2010.  Including secondary structure, fossils and molecular dating in the centipede tree of life.  Molecular Phylogenetics and Evolution 57:301-313.

Friday, May 2, 2014

Towards a Bivalve Tree of Life

An important paper entitled “Investigating the Bivalve Tree of Life – an exemplar-based approach combining molecular and novel morphological characters” was recently published this year in Invertebrate Systematics.  This paper led by Rüdiger Bieler and Paula M. Mikkelsen and their team of 18 collaborators represents a significant product of their Bivalvia project (BivAToL), which is part of the Assembling the Tree of Life program sponsored by the National Science Foundation.  This paper, which uses an exemplar-based approach to generate morphological and molecular data from specimens from the same population or even the same specimen goes a long way towards providing the necessary phylogenetic infrastructure (sensu Bieler et al., 2013 and my April 2013 blog posting) for those interested in using bivalves as model organisms in a wide range of evolutionary and ecological studies. 

The BivAToL team examined stomach morphology and other features of the alimentary tract, the morphology of gills and labial palps, shell microstructure, and sperm ultrastructure.  Molecular markers included two nuclear ribosomal genes (18S and 28S rRNA), one nuclear protein encoding gene (histone H3), one mitochondrial ribosomal gene (16S rRNA) and one mitochondrial protein-encoding gene (cytochrome c oxidase subunit I; COI).  Four additional nuclear protein-encoding genes were also generated for a subset of taxa.  This impressive assemblage of data was analyzed using a variety of methods including parsimony, maximum likelihood and Bayesian inference with molecular data alone and in combination with the morphological characters.   The results consistently recovered the monophyly of Autobranchia, Pteriomorphia, Heterochonchia, Palaeoheterodonta, Archiheterodonta, Euheterodonta, Anomalodesmata and a new clade Imparidentia (=Euheterodonta excluding Anomalodesmata).  The authors also discussed the origin and diversification times for each of the major clades and provided a classification scheme for the six major monophyletic groups of modern Bivalvia.  This important paper is certain to stimulate much more research on bivalve systematics and and other interesting evolutionary questions.

Literature Cited

Bieler, R., P. M. Mikkelsen, T. M. Collins, E. A. Glover, V. L. González, D. L. Graf, E. M. Harper, J. Healy, G. y. Kawauchi, P. P. Sharma, S. Staubach, E. E. Strong, J. D. Taylor, I. Tȅmkin, J. D. Zardus, S. Clark, A. Guzmán, E. McIntyre, P. Sharp, and G. Giribet.  Investigating the bivalve tree of life – an exemplar –based approach combining molecular and novel morphological characters.  Invertebrate Systematics 28:32-115.

Bieler, R., P. M. Mikkelsen, and G. Giribet.  2013.  Bivalvia-A discussion of known unknowns.  American Malacological Bulletin 31(1):123-133.

Tuesday, December 17, 2013

A New Classification of the Chromodorid Nudibranchs

With over 300 described species, the chromodorid nudibranchs are one of the most species rich families of gastropods.  Their bright colors and interesting morphology appeal to underwater photographers and scientists alike.  Their aposematic coloration has also drawn the attention of scientists interested in natural products chemistry.  In spite of their conspicuousness and appeal, there has been no comprehensive, well-supported phylogeny of the chromodorid nudibranchs.  This hinders progress being made in other biological disciplines where one may assume that species from the same genus represent a monophyletic group.  In a recent issue of PlosOne, Rebecca Johnson and Terrence Gosliner set out to remedy this situation and generate a phylogeny of the chromodorid nudibranchs and present a classification that accurately reflects the evolutionary history of the group.  

In the paper entitled “Traditional Taxonomic Groupings Mask Evolutionary History: A Molecular Phylogeny and New Classification of the Chromodorid Nudibranchs,” Johnson and Gosliner assembled the most comprehensive dataset to date including 244 specimens (142 new), representing 157 species (106 new) chromodorid species and several other taxa.  They used two mitochondrial genes (16S rRNA and cytochrome oxidase I) to reconstruct a phylogeny of the group.  The results revealed that currently recognized genera were either polyphyletic or nested within another genus rendering the other genus paraphyletic.  Extensive homoplasy of morphological attributes that have been thought to be synapomorphies for a particular genus seems to be the cause of the unnatural groupings that have been recognized in the past.  This pattern of polyphyly of recognized genera has been observed for unionid bivalves as well (Campbell et al., 2004).  

One appealing outcome of the study by Johnson and Gosliner (2012) is that they took the time to propose a new classification of the group based on their findings.  As Johnson and Gosliner (2012) indicate, “the translation of phylogenetic hypotheses into classifications is the best way to communicate results to a larger community” and “communicating these new hypotheses is one of the main contributions systematics can make to the scientific community.”  Their study now provides a sound phylogenetic framework from which morphological, chemical and behavioral attributes can be examined.       

Literature Cited
Cambpell, D. C., J. M. Serb, J. E. Buhay, K J. Roe, R. L. Minton, and C. Lydeard.  2005.  Phylogeny of North American amblemines (Bivalvia, Unonoida): prodigious polyphyly proves pervasive across genera.  Invertebrate Biology 124:131-164.
Johnson, R. F., and T. M. Gosliner.  2012.  Traditional taxonomic groupings mask evolutionary history: a molecular phylogeny and new classification of the chromodorid nudibranchs.  PlosOne 7(4):e33479.

Tuesday, November 26, 2013

Increased Litter Decomposition Rates by Terrestrial Gastropods in Hawaii

Terrestrial gastropods are often a major component of various terrestrial ecosystems.  It is thought that litter-dwelling terrestrial gastropods contribute to the cycling of nutrients either directly or indirectly through metabolism and modifying habitat to enhance micro-arthropod or microbial activity, respectively.  However, their role in ecosystem processes is poorly known particularly in tropical forests.  In a recent issue of Biotropica, Wallace M. Meyer III, Rebecca Ostertag, and Robert H. Cowie shed some light on this very issue in a paper entitled “Influence of Terrestrial Molluscs on Litter Decomposition and Nutrient Release in a Hawaiian Rain Forest.”  Meyer et al. (2013) used a field mesocosm approach to examine (1) whether the presence of terrestrial gastropod species increased rates of leaf litter decomposition, (2) whether different terrestrial gastropod species influence the rates of nutrient release differently, and (3) whether terrestrial gastropods facilitate recruitment of mesoinvertebrates.  The results of the experiments showed that the presence of gastropods increased litter decomposition rates and that the highest decomposition rates were those with the greatest gastropod biomass.  Furthermore, although there were differences in the rates of release of some nutrients among treatments, the different gastropod species appeared to influence nutrient release in a similar way.  Finally, there was no evidence that terrestrial gastropods facilitated mesoinvertebrate recruitment.

The authors have shown empirically that terrestrial gastropods can play a major role in litter decomposition.  One interesting aspect of the study is that it was done using the five most abundant species of gastropods in the Hawaiian rain forest: the native Succinea cepulla and four non-native species (Arion intermedius, Deroceras leave, Oxychilus alliarius, and Limax maximus).    The native species had the lowest density among the gastropods studies and is comparatively rare.  Indeed, Hawaii presents a particularly compelling case because some 65-90 percent of the 750+ species (over 99% endemic) are now considered extinct (Solem, 1990; Cowie et al. 1995; Cowie, 2001; Lydeard et al., 2004) so there is the distinct possibility that invasive gastropod species are now conducting important ecological processes that were once carried out by native species and potentially benefitting otherwise native ecosystems.  Regrettably, important information is lacking to fully address this issue such as species richness and densities in historical, native communities.

Literature Cited
Cowie, R. H., N. L. Evenhuis, and C. C. Christensen.  1995.  Catalog of the native aland and freshwater molluscs of the Hawaiian Islands.  Backhuys Publishers, Leiden, The Netherlands.
Cowie, R. H.  2001.  Invertebrate invasions on Pacific islands and the replacement of unique native faunas: a synthesis of land and freshwater snails.  Biol. Invasions 3:119-136.
Meyer III, W. M., R. Ostertag, and R. Cowie.  2013.  Influence of terrestrial molluscs on litter decomposition and nutrient release in a Hawaiian rain forest.  Biotropica 45(6):719-727.
Lydeard, C., R. H. Cowie, W. F. Ponder, A. E. Bogan., P. Bouchet, S. A. Clark, K. S. Cummings, T. J. Frest, O. Gargominy, D. G. Herbert, R. Hershler, K. E. Perez, B. Roth, M. Seddon, E. E. Strong, and F. G. Thompson.  2004.  The global decline of nonmarine mollusks.  Bioscience 54:321-330.
Solem, A.  1990.  How many Hawaiian land snail species are left? And what we can do for them.  Bishop Museum of Occasional Papers 30:27-40.