Marine & Environmental Sciences Faculty Articles

Evolution of Anthozoan Polyp Retraction Mechanisms: Convergent Functional Morphology and Evolutionary Allometry of the Marginal Musculature in Order Zoanthidea (Cnidaria: Anthozoa: Hexacorallia)





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BMC Evolutionary Biology



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Phylogenetic comparative methods, Functional morphology, Convergent evolution, Evolutionary allometry, Coelenterata, Hexacorallia, Symbiosis


Background Retraction is among the most important basic behaviors of anthozoan Cnidaria polyps and is achieved through the coordinated contraction of at least six different muscle groups. Across the Anthozoa, these muscles range from unrecognizable atrophies to massive hypertrophies, producing a wide diversity of retraction abilities and functional morphologies. The marginal musculature is often the single largest component of the retraction mechanism and is composed of a diversity of muscular, attachment, and structural features. Although the arrangements of these features have defined the higher taxonomy of Zoanthidea for more than 100 years, a decade of inferring phylogenies from nucleotide sequences has demonstrated fundamental misconceptions of their evolution.

Results Here we expand the diversity of known marginal muscle forms from two to at least ten basic states and reconstruct the evolution of its functional morphology across the most comprehensive molecular phylogeny available. We demonstrate that the evolution of these forms follows a series of transitions that are much more complex than previously hypothesized and converge on similar forms multiple times. Evolution of the marginal musculature and its attachment and support structures are partially scaled according to variation in polyp and muscle size, but also vary through evolutionary allometry.

Conclusions Although the retraction mechanisms are diverse and their evolutionary histories complex, their morphologies are largely reflective of the evolutionary relationships among Zoanthidea higher taxa and may offer a key feature for integrative systematics. The convergence on similar forms across multiple linages of Zoanthidea mirrors the evolution of the marginal musculature in another anthozoan order (Actiniaria). The marginal musculature varies through evolutionary allometry of functional morphologies in response to requirements for additional force and resistance, and the specific ecological and symbiotic functions of individual taxa.







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©2015 Swain et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http:// applies to the data made available in this article, unless otherwise stated.

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NSF grant #s: OCE-0550599, CBET-0937987

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