Towards Integrative Systematics of Anthozoa (Cnidaria): Evolution of Form in the Order Zoanthidea

A decade of research inferring evolutionary relationships from nucleotide sequences has demonstrated a fundamental misconception of the evolution of form in Zoanthidea. Morphological features that define current taxa are plesiomorphic or homoplastic and do not circumscribe clades of species delineated by ecological and molecular characters. Although molecular data have been critical in exposing this deficiency, their parataxonomic application to Zoanthidea has created a barrier to comprehensive revision within the order. Species descriptions and higher taxon definitions based on nucleotide sequences isolate new taxa from the existing taxonomic system and restrict the application of novel systematics hypotheses to a fraction of the known diversity of taxa. This creates competing taxonomic systems that do not benefit from the knowledge contained in the opposing system. To enable the integration of modern molecular data with more than a century of morphological research, characters that can simultaneously span the parataxonomy barrier, existing taxonomic system and historical record must be identified. Here, we test the utility of morphological characters for integrative systematics by reviewing commonly described and novel morphological characters, assessing independence of character components and analysing character homoplasy and ancestral states on the most comprehensive molecular phylogeny available. The results indicate a rich diversity of form that span the full range of homoplasy values, including more than a dozen independent characters useful to systematics or differentiating closely related species. The least homoplasious characters include traditionally targeted (fifth mesenteries, marginal muscle arrangement, encircling sinus) and novel (fissure morphology, basal canals of the mesenteries) features. These analyses represent a first step in identifying characters necessary for reunification and revision of Zoanthidea systematics

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

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