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The harpacticoid copepod genusTigriopusis characteristic of shallow upper shore rock pools in both hemispheres.Tigriopusare necessarily physiologically tolerant as the pools feature extreme physicochemical conditions; their lower limit on the shore is apparently set by competition/predation. There are several species, differentiated by relatively minor morphological distinctions; speciation appears to be favoured by restricted gene flow between populations, even over short distances. This paper briefly reviews the literature concerned with the taxonomy, physiology and ecology of the genus, and compares environmental conditions and salinity/thermal tolerance data for three species: T.brevicornisfrom Scotland, T.fulvusfrom Madeira, and a previously undescribed population of the Southern Ocean ‘T. angulatus’ group from subantarctic South Georgia. Particularly interesting was a low upper lethal temperature in‘T. angulatus’(21·9°C) that probably restricts the species to relatively large pools that buffer environmental thermal extremes. Unlike the other species,T. fulvuscannot withstand freezing conditions. Evidence is presented to show that the South GeorgianTigriopusdiffers slightly, but significantly, in its morphology from two sibling species that inhabit the subantarctic Kerguelen and Crozet Islands but which cannot interbreed. If the South Georgian copepods are indeed a separate species and do not occur elsewhere (e.g. the Falklands), this indicates rapid speciation within the genus; the coasts of South Georgia were covered by an icecap until 10,000–14,000 years ago and no rock pool habitats were then available. It is suggested thatTigriopusmay have reached South Georgia by rafting withinEnteromorphatubes.

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