Improved phylogenetic resolution within Siphonophora (Cnidaria) with implications for trait evolution

Abstract

Siphonophores are a diverse group of hydrozoans (Cnidaria) that are found at most depths of the ocean - from the surface, like the familiar Portuguese man of war, to the deep sea. They play important roles in ocean ecosystems, and are among the most abundant gelatinous predators. A previous phylogenetic study based on two ribosomal RNA genes provided insight into the internal relationships between major siphonophore groups. There was, however, little support for many deep relationships within the clade Codonophora. Here, we present a new siphonophore phylogeny based on new transcriptome data from 29 siphonophore species analyzed in combination with 14 publicly available genomic and transcriptomic datasets. We use this new phylogeny to reconstruct several traits that are central to siphonophore biology, including sexual system (monoecy vs. dioecy), gain and loss of zooid types, life history traits, and habitat. The phylogenetic relationships in this study are largely consistent with the previous phylogeny, but we find strong support for new clades within Codonophora that were previously unresolved. These results have important implications for trait evolution within Siphonophora, including favoring the hypothesis that monoecy arose at least twice.

Shaped to kill: The evolution of siphonophore tentilla for specialized prey capture in the open ocean

“…The gene sequences we used in this study are available online (accession numbers in Appendix 5). Some of the sequences we used were accessioned in (Dunn et al 2005), and others we extracted from the transcriptomes in (Munro et al 2018). Two new 16S sequences for Frillagalma vityazi (MK958598) and Thermopalia sp.…”

“…Given the broader sequence sampling of the transcriptome phylogeny, we ran constrained inferences (using both ML and Bayesian timetree approaches, which produced fully congruent topologies (Appendix 6 and 7)) after fixing the 5 nodes that were incongruent with the topology of the consensus tree in (Munro et al 2018). This topology was then used to inform a Bayesian relaxed molecular clock time-tree in RevBayes, using a birth-death process (sampling probability calculated from the known number of described siphonophore species) to generate ultrametric branch lengths (Appendix 8).…”

“…Contemporary underwater sampling technology provides an unprecedented opportunity to explore the trophic ecology (Choy et al 2017) and functional morphology (Costello et al 2015) of siphonophores. In addition, well-supported phylogenies based on molecular data are now available for these organisms (Munro et al 2018). These advances allow for the examination of relationships between modern siphonophore form, function, and ecology, as well as reconstructing their evolutionary history.…”

Morphology and development of the Portuguese man of war,Physalia physalis

“…Many authors refer to 'cormidia' in Physalia physalis. Cystonectae, the clade to which P. physalis belongs, are sister to Codonophora 6,8 . Cystonects produce all zooids from single buds that arise along the stem, while probud subdivision (all zooids in a cormidium arise from a single bud) is a synapomorphy of Codonophora 28 .…”

“…Colonies are composed of functionally specialized bodies (termed zooids) that are homologous to free living individuals. Most species are planktonic and are found at most depths from the deep sea to the surface of the ocean [5][6][7] . They are fragile and difficult to collect intact, and must be collected by submersible, remotely operated vehicle, by hand while blue-water diving, or in regions with localized upwellings 8,9 .…”