Testing relationships of a new form of boring bivalve: the interesting kind

Abstract

Sunken wood in marine environments supports a diverse community, the stars of the show being the wood-boring bivalves. Shallow versus deep-sea woodfalls tend to only attract bivalves from one of two clades; Teredinidae and Xylophagaidae respectively. A recent deployment of experimental wood substrates returned a new form of boring bivalve that has morphological characteristics (mesoplax and lack of viscera in the siphons) indicating it as a xylophagaid, yet has hard pallet-like structures that had previously only been observed in the shallow water teredinids. This study sought to understand the phylogenetic placement of this new taxon (Xylophagaid A) with consideration of the various siphon-associated hard parts that have been described across boring bivalve groups. I sequenced the 18S and 28S rDNA nuclear genes from Xylophagaid A and Xylophaga zierenbergi and aligned these with sequences from Distel et al. (2011) for Maximum Likelihood and Bayesian phylogenetic analyses. The resulting trees confirm Xylophagaid A as a member of Xylophagaidae and indicate a close relationship to Xyloredo, a genus that has a calcareous tube. Additional taxonomic sampling and closer morphological investigations are required, but this study indicates preliminary evidence for convergence in siphon-associated hard parts in wood-boring bivalves.