ENVIRONMENTAL TOLERANCES OF 3 SPECIES OF THE HARPACTICOID COPEPOD GENUS TIGRIOPUS

The harpacticoid copepod genus Tigriopus is characteristic of shallow upper shore rock pools in both hemispheres. Tigriopus are necessarily physiologically tolerant as the pools feature extreme physicochemical conditions; their lower limit on the shore is apparently set by compet ition/predation. There are several species, differentiated by relative ly minor morphological distinctions; speciation appears to be favoured by restricted gene flow between populations, even over short distance s. This paper briefly reviews the literature concerned with the taxono my, physiology and ecology of the genus, and compares environmental co nditions and salinity/thermal tolerance data for three species: T. bre vicornis from Scotland, T. fulvus from Madeira, and a previously undes cribed population of the Southern Ocean 'T. angulatus' group from suba ntarctic South Georgia. Particularly interesting was a low upper letha l temperature in 'T. angulatus' (21.9 degrees C) that probably restric ts the species to relatively large pools that buffer environmental the rmal extremes. Unlike the other species, T. fulvus cannot withstand fr eezing conditions. Evidence is presented to show that the South Georgi an Tigriopus differs slightly, but significantly, in its morphology fr om two sibling species that inhabit the subantarctic Kerguelen and Cro zet Islands but which cannot interbreed. If the South Georgian copepod s are indeed a separate species and do not occur elsewhere (e.g. the F alklands), this indicates rapid speciation within the genus; the coast s 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 th at Tigriopus may have reached South Georgia by rafting within Enteromo rpha tubes.