International audienceThe quasi-extirpation of the cabbage Pringlea antiscorbutica in some locations at the Kerguelen Islands has large impacts on the distribution of the native fly Calycopteryx moseleyi. This insect has long been supposed as being subordinant to P. antiscorbutica until both larvae and adults were observed under seaweed in littoral margins. Physiological plasticity to saline conditions is expected in C. moseleyi, and metabolic rearrangements in salt-exposed specimens should thus be similar between individuals from cabbages and seaweeds. Individuals of C. moseleyi from non-saline (cabbages) and saline (seaweeds) habitats were experimentally subjected to different salinities (0, 35, 70 practical salinity units) and compared to the widely distributed native Anatalanta aptera flies that coexist with C. moseleyi under the seaweeds. A progressive drop of the survival and body water content was observed in C. moseleyi from cabbages exposed to increasing saline conditions. Body water contents remained similar at 35 and 70 practical salinity units in C. moseleyi from seaweeds, while it did not change over the whole saline gradient in A. aptera. Metabolic profiles revealed that both fly populations from the seaweeds accumulated compatible solutes in the form of alanine or glutamic acid when they were exposed to salinity. A distinct pattern was observed in C. moseleyi specimens from cabbages, whose metabolic profiles revealed a progressive loss of metabolic homoeostasis. We conclude that the C. moseleyi specimens from the cabbages and seaweeds differentiated, as also supported by their contrasted morphotypes, and that the limited salinity tolerance of individuals from cabbages may hinder their future expansion to seaweed