applyparastyle “fig//caption/p[1]” parastyle “FigCapt” Zoological Journal of the Linnean Society, 2020, XX, 1–21. With 7 figures. Downloaded from https://academic.oup.com/zoolinnean/advance-article/doi/10.1093/zoolinnean/zlaa011/5802836 by Iowa State University user on 13 August 2020 Phylogeny and anatomy of marine mussels (Bivalvia: Mytilidae) reveal convergent evolution of siphon traits Jorge A. Audino1*, , Jeanne M. Serb2, , and José Eduardo A. R. Marian1, 1Department of Zoology, University of São Paulo, Rua do Matão, Travessa 14, n. 101, 05508-090 São Paulo, São Paulo, Brazil 2Department of Ecology, Evolution & Organismal Biology, Iowa State University, 2200 Osborn Dr., Ames, IA 50011, USA Received 29 November 2019; revised 22 January 2020; accepted for publication 28 January 2020 Convergent morphology is a strong indication of an adaptive trait. Marine mussels (Mytilidae) have long been studied for their ecology and economic importance. However, variation in lifestyle and phenotype also make them suitable models for studies focused on ecomorphological correlation and adaptation. The present study investigates mantle margin diversity and ecological transitions in the Mytilidae to identify macroevolutionary patterns and test for convergent evolution. A fossil-calibrated phylogenetic hypothesis of Mytilidae is inferred based on five genes for 33 species (19 genera). Morphological variation in the mantle margin is examined in 43 preserved species (25 genera) and four focal species are examined for detailed anatomy. Trait evolution is investigated by ancestral state estimation and correlation tests. Our phylogeny recovers two main clades derived from an epifaunal ancestor. Subsequently, different lineages convergently shifted to other lifestyles: semi-infaunal or boring into hard substrate. Such transitions are correlated with the development of long siphons in the posterior mantle region. Two independent origins are reconstructed for the posterior lobules on the inner fold, which are associated with intense mucociliary transport, suggesting an important cleansing role in epifaunal mussels. Our results reveal new examples of convergent morphological evolution associated with lifestyle transitions in marine mussels. ADDITIONAL KEYWORDS: adaptation – ancestral state – bivalves – correlation – evolutionary convergence – mantle. INTRODUCTION studies, casting light on broader questions concerning adaptive radiation, diversification rates and Apart from their economic importance, mussels evolutionary novelties (Distel, 2000; Owada, 2007; from the family Mytilidae Rafinesque, 1815 exhibit Lorion et al., 2013). remarkable phenotypic and lifestyle diversity. These Many shell features and body plans within bivalves, also known as marine mussels, have an the Mytilidae are putative adaptations to either extensive fossil record, dating back to the Silurian epifaunal or infaunal lifestyles (Stanley, 1972; (~427 Mya) (Berry & Boucot, 1973; Kříž, 2008). From Morton & Dinesen, 2011; Dinesen & Morton, shallow to deep waters, marine mussels represent an 2014; Morton, 2015). Classic examples are the important benthic component in many communities, calcareous borer species in the genera Adula, Botula, playing key ecological roles, such as colonization, bio- Leiosolenus and Lithophaga. Despite a lack of shared erosion, aggregation and supporting associated fauna history, these species have similar patterns of shell (Seed et al., 2000; Dinesen & Morton, 2014). Lifestyles shape and chemical boring methods (Yonge, 1955; are remarkably diverse, including epifaunal, infaunal, Morton & Scott, 1980; Owada, 2007, 2015; Ockelmann semi-infaunal and boring into hard substrates & Dinesen, 2009). Likewise, the adaptive radiations (Morton, 2015). In addition, lineages of Mytilidae of deep-sea mussels on vents, seeps and organic falls have proven to be suitable models for evolutionary are marked by convergent transitions to these deep- sea environments driven, in part, by their bacterial *Corresponding author. E-mail: [email protected] symbiosis (Distel et al., 2000; Jones et al., 2006; © 2020 The Linnean Society of London, Zoological Journal of the Linnean Society, 2020, XX, 1–21 1 2 J. A. AUDINO ET AL. Samadi et al., 2007; Duperron et al., 2009; Lorion et al., pteriomorphian clades, including Arcida and Pectinida 2010; Lorion et al., 2013; Fontanez & Cavanaugh, (Oliver & Holmes 2006; Serb et al., 2011; Serb et al., 2013). Thus, convergence in mytilid morphology and 2017, Audino et al., 2019). Downloaded from https://academic.oup.com/zoolinnean/advance-article/doi/10.1093/zoolinnean/zlaa011/5802836 by Iowa State University user on 13 August 2020 physiology provides compelling insights into the To test our hypothesis that similar mytilid correlation between phenotype and environment morphologies are likely convergent and associated (e.g. Stanley, 1972; Owada, 2007; Morton, 2015). with independent transitions to similar lifestyles (i.e. In this context, the mantle margin is a promising boring, epifaunal or infaunal), we investigate mantle model to study convergent evolution, due to its strong evolution within Mytilidae. A phylogenetic framework association with habitat use and lineage diversification is used to reconstruct the evolution of lifestyles and (Yonge, 1983; Audino & Marian, 2016). In bivalves, this key mantle traits, based on extensive observations of anatomical region is usually organized in three folds preserved specimens, and to test hypotheses of trait responsible for sensory, muscular and secretory roles correlation. In addition, we thoroughly investigate the (Yonge, 1983) and often exhibits great disparity among mantle margin of two epifaunal and two borer species bivalve groups. Although anatomical data for the to explore their detailed structure and associated mytilid mantle margin are available for some species functions. (e.g. Soot-Ryen, 1955; Morton & Scott, 1980; Narchi & Galvão-Bueno, 1983, 1997; Morton & Dinesen, 2010; Morton, 2012; Dinesen & Morton, 2014), these MATERIAL AND METHODS data are lacking for most mytilids. Nevertheless, the observed variation in mantle margin morphology TAXON SAMPLING indicates that the Mytilidae is a suitable model for A detailed history of taxonomic proposals for mytilid testing hypotheses on trait evolution, correlation with subfamilies was summarized by Morton (2015). lifestyles and putative adaptations. Previous studies The classification adopted herein is in accordance suggest that evolutionary convergences may have with Huber (2010, 2015), also adopted by the World underlain morphological diversification of the family Register of Marine Species (WoRMS, http://www. (e.g. Distel (2000)). For example, siphon development marinespecies.org/aphia.php?p=taxdetails&id=211), would be expected among infaunal lineages as an including ten subfamilies and 52 genera. We used a adaptation to burrowing habits in soft sediments combination of five genes from mitochondrial (16S (Stanley, 1968). rRNA and COI) and nuclear (18S rRNA, 28S rRNA The Mytilidae exhibit favourable features for and histone H3) genomes for our phylogenetic evolutionary investigation. The family has been inference of the Mytilidae, totalling 5710 bp. All consistently recovered as monophyletic and molecular data was curated from available sequences phylogenetically placed within the Pteriomorphia, in Genbank, the public repository maintained by along with oysters, scallops and ark clams (Distel, the National Center for Biotechnology Information 2000; Giribet & Wheeler, 2002; Matsumoto, 2003; (NCBI). In total, the molecular character matrix is Owada, 2007; Samadi et al., 2007; Bieler et al., 2014; comprised of 33 mussel species (19 genera) (Table 1), Combosch et al., 2017; Sun & Gao, 2017; Liu et al., including representatives of all but one subfamily (i.e. 2018; Lee et al., 2019). Recent phylogenetic analyses Crenellinae), and has a completeness of 79%. Eight from transcriptome and whole mitochondrial genome pteriomorphian and five non-pteriomorphian bivalve datasets indicate that the mytilids are organized species served as outgroups for the phylogenetic into two major clades with high support for most analyses (Table 1). genera (Gerdol et al., 2017; Liu et al., 2018; Lee et al., Morphology was examined in 21 of those 33 2019). However, while the relationships within some species based on preserved specimens from museum subfamilies are relatively well understood, such as collections. For the remaining 12 sequenced species, the deep-sea Bathymodiolinae (Jones et al., 2006; morphological data was obtained using surrogate Duperron et al., 2009; Lorion et al., 2010; Lorion et al., species within the same genus. In total we examined 2013), traditionally accepted subfamilies such as 43 species (25 genera) from the following collections: the hard-substrate borers of Lithophaginae and the Museum of Comparative Zoology (MCZ), Museum of Mytilinae (Distel, 2000; Owada, 2007; Gerdol et al., Zoology “Prof. Adão José Cardoso” of the University 2017; Kartavtsev et al., 2018; Liu et al., 2018; Lee et al., of Campinas (ZUEC-BIV), Museum of Zoology of 2019) do not appear to be monophyletic. This suggests the University of São Paulo (MZSP), Smithsonian that convergent morphologies and lifestyles may be National Museum of Natural History (USNM) and more prevalent among mytilids than expected. Such Santa Barbara Museum of Natural History (SBMNH). an evolutionary pattern, with pervasive morphological Museum catalogue numbers are listed in Table 1. convergences,