Abstract
BackgroundMetacrangonyctidae (Amphipoda, Crustacea) is an enigmatic continental subterranean water family of marine origin (thalassoid). One of the species in the genus, Metacrangonyx longipes, is endemic to the Balearic islands of Mallorca and Menorca (W Mediterranean). It has been suggested that the origin and distribution of thalassoid crustaceans could be explained by one of two alternative hypotheses: (1) active colonization of inland freshwater aquifers by a marine ancestor, followed by an adaptative shift; or (2) passive colonization by stranding of ancestral marine populations in coastal aquifers during marine regressions. A comparison of phylogenies, phylogeographic patterns and age estimations of clades should discriminate in favour of one of these two proposals.ResultsPhylogenetic relationships within M. longipes based on three mitochondrial DNA (mtDNA) and one nuclear marker revealed five genetically divergent and geographically structured clades. Analyses of cytochrome oxidase subunit 1 (cox1) mtDNA data showed the occurrence of a high geographic population subdivision in both islands, with current gene flow occurring exclusively between sites located in close proximity. Molecular-clock estimations dated the origin of M. longipes previous to about 6 Ma, whereas major cladogenetic events within the species took place between 4.2 and 2.0 Ma.ConclusionsM. longipes displayed a surprisingly old and highly fragmented population structure, with major episodes of cladogenesis within the species roughly correlating with some of the major marine transgression-regression episodes that affected the region during the last 6 Ma. Eustatic changes (vicariant events) -not active range expansion of marine littoral ancestors colonizing desalinated habitats-explain the phylogeographic pattern observed in M. longipes.
Highlights
Metacrangonyctidae (Amphipoda, Crustacea) is an enigmatic continental subterranean water family of marine origin
Four gene fragments-three mitochondrial (cytochrome oxidase subunit 1, cytochrome b and 16S rRNA) and one nuclear (Histone H3A)-with a total sequence length of about 1.7 Kb were sequenced from 34 Metacrangonyx longipes specimens and the outgroups Metacrangonyx ilvanus, M. remyi and M. sp
Our data suggest that marine transgression-regression cycles may have induced the repeated range expansion, contraction and fragmentation of populations of M. longipes, which appears currently split into several isolated and genetically divergent lineages adapted to a broad spectrum of salinity conditions
Summary
Metacrangonyctidae (Amphipoda, Crustacea) is an enigmatic continental subterranean water family of marine origin (thalassoid). It has been suggested that the origin and distribution of thalassoid crustaceans could be explained by one of two alternative hypotheses: (1) active colonization of inland freshwater aquifers by a marine ancestor, followed by an adaptative shift; or (2) passive colonization by stranding of ancestral marine populations in coastal aquifers during marine regressions. The occurrence of extensive morphological conservatism in subterranean fauna frequently hampers the establishment of phylogenetic inferences based solely on morphological features. Recent molecular phylogenetic and phylogeographic studies on subterranean amphipods emphasize the role played by historical factors (i.e., glacial or drought episodes) in the pattern of genetic diversification and distribution displayed by these animals [5,6,11]. Stygobiont amphipods have a comparatively reduced dispersal potential (as do all peracarid crustaceans), as the females carry offspring in a marsupium and these are brooded and not released into the water column until metamorphosed into diminutive non-natatory adults [15]
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