Species richness of deep-sea wood-boring clams (subfamily Xylophagainae) from the northeast Pacific




Stoeckle, Mathis

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The deep sea. for a long time. had been perceived as a homogenous and stable environment, with little diversity. The discovery of island habitats such as hot vents. cold seeps and whale carcasses revealed the presence of high biomasses in the deep sea. Species richness on island habitats is low however, compared to the species richness on abyssal plains (Tunnicliffe 1991: Stecher et al. 2003). A study addressing species richness on abyssal plains concludes there to be tens of millions of new species (Grassle and Maciolek 1992). Other authors dispute these numbers and suggest a total deep-sea species richness of approximately half a million (May 1993). The discrepancy in these numbers by multiple orders of magnitude emphasize how under-sampled the deep sea is. The Northeast Pacific is recognized as a biogeographic species province both in the intertidal. shallow subtidal (Valentine 1966: Roy et al. 1994) as well as in hydrothermally active areas (Van Dover et al. 2002). The Northeast Pacific hydrothermally active areas are unique in that they appear to foster single dominant species within ecosystems: e.g. Ridgeia piscesae (Vestimentifera. Annelida) (Southward et al. 1995): Lepetodrilus fucensis (Gastropoda, Mollusca) (McLean 1993). Xylophaga washingiona BARTSCH (Bivalvia. Mollusca) is the only described species of deep-sea wood-boring clams in the Northeast Pacific (Bartsch 1921). We investigated the species richness of deep-sea wood-boring clams (Family Pholadidae. Subfamily Xylophagainae) in the Northeast Pacific by means of strategically placed, experimental wood blocks. The objective of the study was to a) assess the morphological diversity and separate the specimens into distinct morphological taxa: b) to use genetic markers (CO 1. 18S) to discern the genetic diversity within the morphological taxa and c) to elucidate the phylogenetic relationships among the new species. We discovered eight morphological taxa (B. F. G. M. R, T. U. X), defined by a set of characters. previously used to describe new pholad species. The most distinguishing characters were the morphologies of the siphon and mesoplax (accessory plates). The CO1 gene sequences showed little diversity within the morphological Taxa B. F, G. U and X. The 18S gene data split Taxa B and T into B1. B2 and T1, T2, respectively. There was no genetic diversity within 18S sequences of Taxon R. Thus. we discovered at least six new species: Xylophaga species B1. F. G, R. U and Xylopholas species X. I cannot confirm the statuses of Taxa M and T due to missing morphological characters as well as the unavailability of their CO1 sequences. The newly discovered species increase the number of deep-sea, wood-boring species from one to seven, in the Northeast Pacific. Globally, a total of 41 species have been described. Given the discoveries in this study, this number is likely an underestimate of the global species richness of the Xylophagainae. The phylogenetic relationships of the genus leveI remain inconclusive due to too few samples. Species relationships emerged. placing Species G and R as most closely related. as well as B1 and X washingtona. Additional samples from all ocean basins are necessary to further investigate the relationships and origins of the deep-sea woodborers. Future avenues that need to be explored are threefold: a) Ancestry: Has there been one or multiple invasions into the Northeast Pacific, with subsequent adaptive radiations? Where do this/these ancestor species come from? b) Support for Diversity: What mechanisms allow the deep-sea wood-boring clams to co-occur? What are the deep-sea wood-boring clams' micro-niches? c) Effects of Diversity : What is the role of the wood-boring clams in species communities inhabiting deep-sea wood?



clams, deep-sea animals, North Pacific Ocean