Abstract
BackgroundNematodes represent the most abundant benthic metazoa in one of the largest habitats on earth, the deep sea. Characterizing major patterns of biodiversity within this dominant group is a critical step towards understanding evolutionary patterns across this vast ecosystem. The present study has aimed to place deep-sea nematode species into a phylogenetic framework, investigate relationships between shallow water and deep-sea taxa, and elucidate phylogeographic patterns amongst the deep-sea fauna.ResultsMolecular data (18 S and 28 S rRNA) confirms a high diversity amongst deep-sea Enoplids. There is no evidence for endemic deep-sea lineages in Maximum Likelihood or Bayesian phylogenies, and Enoplids do not cluster according to depth or geographic location. Tree topologies suggest frequent interchanges between deep-sea and shallow water habitats, as well as a mixture of early radiations and more recently derived lineages amongst deep-sea taxa. This study also provides convincing evidence of cosmopolitan marine species, recovering a subset of Oncholaimid nematodes with identical gene sequences (18 S, 28 S and cox1) at trans-Atlantic sample sites.ConclusionsThe complex clade structures recovered within the Enoplida support a high global species richness for marine nematodes, with phylogeographic patterns suggesting the existence of closely related, globally distributed species complexes in the deep sea. True cosmopolitan species may additionally exist within this group, potentially driven by specific life history traits of Enoplids. Although this investigation aimed to intensively sample nematodes from the order Enoplida, specimens were only identified down to genus (at best) and our sampling regime focused on an infinitesimal small fraction of the deep-sea floor. Future nematode studies should incorporate an extended sample set covering a wide depth range (shelf, bathyal, and abyssal sites), utilize additional genetic loci (e.g. mtDNA) that are informative at the species level, and apply high-throughput sequencing methods to fully assay community diversity. Finally, further molecular studies are needed to determine whether phylogeographic patterns observed in Enoplids are common across other ubiquitous marine groups (e.g. Chromadorida, Monhysterida).
Highlights
Nematodes represent the most abundant benthic metazoa in one of the largest habitats on earth, the deep sea
We lack an accurate characterization of species diversity and phylogeographic patterns in this dominant benthic group–a situation hindered by logistical difficulties in sampling the deep-sea and a persistent shortage of nematode taxonomists
Phylogenies recovered deep-sea species and shallow water species as sister taxa, as clearly seen for Syringolaimus, Bathylaimus, Oxystomina, Halalaimus, and Chaetonema species (Figure 1); deep-sea taxa do not form an independent lineage within the Enoplida
Summary
Nematodes represent the most abundant benthic metazoa in one of the largest habitats on earth, the deep sea. We do not understand what proportion of the deep-sea nematode fauna represents unique taxa (novel species, endemic lineages) and what fraction instead represents a subset of shallow water biodiversity. Evidence from isopods suggests that deep-sea taxa represent a mix of older endemic lineages and fauna derived more recently from shallow water forms. Within the Janiroidea, there is evidence supporting the radiation of shallow-water isopods into deeper habitats through isothermal water columns [7], reinvasions of shallow habitats by deep-sea taxa [7,8], and multiple independent colonizations of the deep-sea [9]–deep-sea taxa within this group probably represent ancient evolutionary lineages that split off early from other taxa [10]. The Flabellifera contains no endemic deep-sea isopod families (in contrast to the Janiroidea which contains seven), and exhibits low diversity in deep habitats [10]; this group is thought to have arisen relatively recently, with shallowwater taxa invading deep-sea habitats subsequent to anoxic oceanic conditions that prompted mass deepwater extinctions in the Palaeocene
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