Abstract
Amphipods of the superfamily Lysianassoidea are ubiquitous at hadal depths (>6000m) and therefore are an ideal model group for investigating levels of endemism and the drivers of speciation in deep ocean trenches. The taxonomic classification of hadal amphipods is typically based on conventional morphological traits but it has been suggested that convergent evolution, phenotypic plasticity, intra-specific variability and ontogenetic variation may obscure the ability to robustly diagnose taxa and define species. Here we use phylogenetic analysis of DNA sequence variation at two mitochondrial (COI and 16S rDNA) and one nuclear (18S rDNA) regions at to examine the evolutionary relationships among 25 putative amphipod species representing 14 genera and 11 families that were sampled from across seven hadal trenches. We identify several instances where species, genera and families do not resolve monophyletic clades, highlighting incongruence between the current taxonomic classification and the molecular phylogeny for this group. Our data also help extend and resolve the known biogeographic distributions for the different species, such as identifying the co-occurrence of Hirondellea dubia and Hirondellea gigas in the Mariana trench.
Highlights
The hadal zone is the deepest part of the ocean, extending from 6000m to c. 11000m
Our analysis revealed some plasticity among different gene trees resolved from nuclear and mitochondrial data. This highlights some of the difficulties associated with producing a definitive phylogeny for deep-sea amphipods, and argues that any attempt requires a considerable body of molecular data from across multiple loci, proper knowledge of the extent of phenotypic plasticity in morphological traits and descriptions of samples that have not been damaged during sampling
We have provided a phylogenetic analysis of the Lysianassoidea which has highlighted several problematic issues in the taxonomic classification of the superfamily alongside informing a better understanding of the biogeographical distributions of key hadal species
Summary
The hadal zone is the deepest part of the ocean, extending from 6000m to c. 11000m. It is comprised of 37 trench systems, primarily located around the Pacific Rim, that are formed at tectonic subduction zones (Jamieson et al, 2010). Most trenches lack adjoining corridors of sufficient depth to provide any connection, and are analogous with both high altitude mountain ecosystems, albeit inverted, and hydrothermal vent systems which are linear spans of distinctive habitat with large intervening abyssal plains. This level of geographic isolation, coupled with potent selection pressures that promote local adaptation, has meant that hadal trenches have traditionally been considered centres of high species endemism (Beliaev, 1989; Wolff, 1970, 1960). Such a perception is difficult to reconcile with the ubiquity of some key cosmopolitan taxa that are found across the abyssal plains and in different trenches, and even more so with the presence of the same putative species in geographically distinct trenches but which are apparently absent from the adjoining abyssal regions (France and Kocher, 1996; Eustace et al, 2013; Fujii et al, 2013; Jamieson, 2015)
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