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This study asked whether reductive traits in cave organisms evolve at a slower pace (suggesting neutral evolution under relaxed selection) than constructive changes, which are likely to evolve under directional selection. We investigated 11 subterranean and seven surface populations of Sundathelphusa freshwater crabs on Bohol Island, Philippines, and examined constructive traits associated with improved food finding in darkness (increased leg and setae length) and reductive traits (reduced cornea size and eyestalk length). All changes occurred rapidly, given that the age of the most recent common ancestor was estimated to be 722-271 ka based on three mitochondrial markers. In order to quantify the speed of character change, we correlated the degree of morphological change with genetic distances between surface and subterranean individuals. The temporal pattern of character change following the transition to subterranean life was indistinguishable for constructive and reductive traits, characterized by an immediate onset and rapid evolutionary change. We propose that the evolution of these reductive traits-just like constructive traits-is most likely driven by strong directional selection.

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The identity of the rare xanthid crab, Pilumnoplax acanthomerus Rathbun, 1911, originally described from the AmiranteIslands in the western Indian Ocean, is elucidated. Števčić (2005) transferred the species from Pilumnoplax Stimpson,1858, to a new genus, Linnaeoxantho. This monotypic genus is re-diagnosed and new morphological characters are high-lighted. New records from Ryukyu and Line Islands, in the western and central Pacific, respectively, are reported. Linnae-oxantho is compared with the morphologically similar Melybia Stimpson, 1871, from the western Atlantic, and theiraffinities are discussed. Linnaeoxanthinae Števčić, 2005, is here recognised as a valid xanthid subfamily for Linnaeoxantho and Melybia, and is considered to have priority over Melybiidae Števčić, 2005.

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