Abstract
Brachyuran crabs of the family Bythograeidae are endemic to deep-sea hydrothermal vents and represent one of the most successful groups of macroinvertebrates that have colonized this extreme environment. Occurring worldwide, the family includes six genera (Allograea, Austinograea, Bythograea, Cyanagraea, Gandalfus, and Segonzacia) and fourteen formally described species. To investigate their evolutionary relationships, we conducted Maximum Likelihood and Bayesian molecular phylogenetic analyses, based on DNA sequences from fragments of three mitochondrial genes (16S rDNA, Cytochrome oxidase I, and Cytochrome b) and three nuclear genes (28S rDNA, the sodium–potassium ATPase a-subunit ‘NaK’, and Histone H3A). We employed traditional concatenated (i.e., supermatrix) phylogenetic methods, as well as three recently developed Bayesian multilocus methods aimed at inferring species trees from potentially discordant gene trees. We found strong support for two main clades within Bythograeidae: one comprising the members of the genus Bythograea; and the other comprising the remaining genera. Relationships within each of these two clades were partially resolved. We compare our results with an earlier hypothesis on the phylogenetic relationships among bythograeid genera based on morphology. We also discuss the biogeography of the family in the light of our results. Our species tree analyses reveal differences in how each of the three methods weighs conflicting phylogenetic signal from different gene partitions and how limits on the number of outgroup taxa may affect the results.
Highlights
Deep-sea hydrothermal vent communities contain a high proportion of endemic species, at higher taxonomic levels [1]
Alignment and Datasets We obtained the sequences for all six genes from most taxa with the following exceptions: for Austinograea williamsi, A. alayseae and A. aff. williamsi, we could not obtain the Cytochrome Oxidase I gene (COI), H3A, and NaK genes; and, for Gandalfus yunohana, only the mitochondrial genes were available, leaving all five Bythograea spp. and one representative per genus for the remaining five genera with all six genes sequenced ( = 10 taxa)
RDNA, Nak, H3A, and two concatenated datasets—i.e., 16S rDNA+COI+Cytochrome b gene (Cytb), and 16S rDNA+H3A, suggested that the genus Bythograea is monophyletic (Supporting Table S3), with bootstrap and posterior probability values between 99–100% for all datasets except H3A, for which support was lower (i.e., 70–74% ML bootstrap support; Bayesian analyses were not conducted for this dataset due to the large number of taxa; n = 284; Supporting Table S1)
Summary
Deep-sea hydrothermal vent communities contain a high proportion of endemic species, at higher taxonomic levels [1] This endemism reflects the high degree of specialization required to succeed in one of Earth’s most extreme environments. The brachyuran crab family Bythograeidae Williams, 1980 (superfamily Bythograeoidea) is among the most ubiquitous and abundant group of macroinvertebrates to have colonized the deep-sea hydrothermal vents worldwide (Figure 1; [2]). It is the only group within the diverse infraorder Brachyura (which contains 7000 valid species and subspecies in 93 families; [3]) that is endemic to this extreme environment. A hypothesis based on morphology has been put forth (see below), a molecular phylogenetic analysis of the Bythograeidae is lacking
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