Phylogeny of Echiura updated, with a revised taxonomy to reflect their placement in Annelida as sister group to Capitellidae

Abstract

Echiura (commonly called spoon worms) are derived annelids that have an unsegmented sausage-shaped body with a highly extensible anterior end (i.e. a proboscis). Echiura currently contains two superfamilies: Echiurioidea (with Echiuridae, Urechidae and Thalassematidae) and Bonellioidea (with Bonelliidae, and Ikedidae). Ikedidae contains only Ikeda, which is distinctive in having a huge trunk, a highly elongate proboscis with stripes or dots, and numerous gonoducts. A recent molecular phylogeny of Echiura recovered Ikedidae as the sister group to Bonelliidae. However, due to relatively low support values for the monophyly of Bonelliidae, this relationship remains problematic. In this study, we reinvestigated the relationship of Bonelliidae and Ikedidae using an expanded dataset with more taxa and genes. In contrast to the previous results, our analyses strongly support that Ikeda is nested within Bonelliidae due to the placement of Maxmuelleria. On the basis of this result, we synonymise Ikedidae with Bonelliidae and transfer Ikeda to the latter, the diagnosis of which is amended. In addition, we synonymise Urechidae with its sister group Echiuridae because they share the synapomorphy of having anal chaetae rings. Furthermore, considering that recent phylogenetic studies have consistently recovered Echiura as the sister group to Capitelliidae within Annelida, we drop the rank of the echiuran clade to family-level and propose a revised classification: Thalassematidae with two subfamilies, Thalassematinae (with two tribes Echiurini and Thalassematini) and Bonelliinae. In addition, we identified a sample collected from the deep sea (~1820 m) of Monterey Bay, California, based on its molecular data. This terminal unexpectedly formed the sister group to the eight genera of Thalassematini, most members of which are inhabitants of littoral zones.

Knots, spoons, and cloches: DNA barcoding unusual larval forms helps document the diversity of Neotropical marine annelids

“…For ease of presentation, only the Thalassematinae are shown in Figure 7, with the tree rooted between the Thalassematini and the Echiurini following the results of Goto et al. (2020). One of the most common OTUs had >99% sequence similarity with a published sequence of Ochetostoma baronii (Greff, 1872) (published under the junior synonym Ochetostoma edax Fisher 1946) from the Gulf of California (Figure 7).…”

“…We could not determine whether any of our larvae belonged to the two species that have been previously reported from Panama. Sequences are not available for A. panamensis and the available sequence for A. belyaevi is for an animal collected from near South Orkney Islands (Goto et al., 2020), which brings into question the veracity of the record of this species in Panama. Such significant increases in species diversity are typical of barcoding studies focusing on larvae from poorly studied groups of invertebrates (Barber & Boyce, 2006; Collin, Venera‐Pontón, Driskell, Chan, et al, 2019; Collin, Venera‐Pontón, Driskell, MacDonald, et al., 2019; Mahon et al., 2010).…”

“…For the thalassematid tree, we used all of the Thalassematinae from the dataset in Goto et al. (2020) plus two Bonellinae. We included all of the larval specimens that had sequences for at least one marker, as we did not want to exclude unique OTUs, some of which were only successfully sequenced for one marker.…”

“…However, developmental studies have now shown some evidence of metameric organization during larval development, which is subsequently lost in the adults (Hou et al., 2019). Molecular analyses have placed echiurans well inside the Annelida (e.g., McHugh, 1997; Stuck et al, 2007; Wu et al., 2009) and they are newly considered to belong to the family Thalassematidae (Goto et al., 2020). The diversity of the group is poorly known, as many species live in the deep sea, and their soft bodies and propensity to shed the proboscis make them difficult to collect intact.…”

Analysis of the relationship between geography and body color with the genetic diversity in the Echiura worm Urechis unicinctus based on the mitochondrial COI and D-loop sequences

“…Echiura worm Urechis unicinctus is a benthic invertebrate mainly distributed along the coast of Russian, Japan, Korea, and the Bohai Gulf of China (Zheng et al 2006). It taxonomically belongs to Echiurini in the family Thalassematidae, which is the only Echiurini species found in China (Goto et al 2020). Due to its delicious taste and high nutritional value, the demand for U. unicinctus as food is rapid growing (Wang et al 2007).…”