Abstract
The Echiura, or spoon worms, are a group of marine worms, most of which live in burrows in soft sediments. This annelid-like animal group was once considered as a separate phylum because of the absence of segmentation, although recent molecular analyses have placed it within the annelids. In this study, we elucidate the interfamily relationships of echiuran worms and their evolutionary pattern of feeding mode and sexual dimorphism, by performing molecular phylogenetic analyses using four genes (18S, 28S, H3, and COI) of representatives of all extant echiuran families. Our results suggest that Echiura is monophyletic and comprises two unexpected groups: [Echiuridae+Urechidae+Thalassematidae] and [Bonelliidae+Ikedidae]. This grouping agrees with the presence/absence of marked sexual dimorphism involving dwarf males and the paired/non-paired configuration of the gonoducts (genital sacs). Furthermore, the data supports the sister group relationship of Echiuridae and Urechidae. These two families share the character of having anal chaetae rings around the posterior trunk as a synapomorphy. The analyses also suggest that deposit feeding is a basal feeding mode in echiurans and that filter feeding originated once in the common ancestor of Urechidae. Overall, our results contradict the currently accepted order-level classification, especially in that Echiuroinea is polyphyletic, and provide novel insights into the evolution of echiuran worms.
Highlights
Echiurans are a group of marine worms with a sausage-shaped non-segmented body and a highly extensible scoop-like proboscis [1,2,3]
Molecular phylogenetic analysis We collected sequence data for the 18S ribosomal RNA (rRNA), 28S rRNA, H3, and COI genes for the molecular phylogenetic analysis of 15 echiuran species belonging to nine genera, five families, and three orders, as well as eight outgroup species (Fig. 1, Table 1, Table S1)
Maximum likelihood (ML) and Bayesian phylogenetic analyses of the combined data set (18S+28S+H3+COI; Dataset S1) suggested that echiurans are monophyletic within the annelids [Maximum-likelihood bootstrap percentage (MBP) = 100, Bayesian posterior probability (BPP) = 1.00] (Fig. 2) and that Capitellidae is sister to echiurans (MBP = 100, BPP = 1.00) (Fig. 2), as in previous molecular studies [7,8]
Summary
Echiurans (spoon worms or innkeeper worms) are a group of marine worms with a sausage-shaped non-segmented body and a highly extensible scoop-like proboscis [1,2,3]. Most live in burrows in soft sediments, some live in crevices in dead corals or rocks [1,2,3] They are deposit feeders that collect small organic particles by scooping the sediment with their flattened and elongated proboscis, some are filter feeders that create U-shaped burrows and gather food by generating water currents using their peristaltic bodies [1,2,3]. Recent molecular phylogenetic analyses have consistently suggested that echiurans should be placed within the phylum Annelida [6,7,8] They are currently treated as a group of derived annelids [7,8]
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