The osmoregulatory ability in three species of the genusIdotea(Isopoda, Crustacea)

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Abstract
The osmoregulation of Idotea baltica, I. viridis and I. granulosa from Denmark was investigated in an attempt to elucidate their ecological distribution. All were able to keep their haemolymph hyperosmotic relative to the medium in salinities below 30‰. Above this concentration they were isosmotic. The lower salinity limits tolerated varied, as it was 5, 7 and 9‰ for respectively I. viridis, I. baltica and I. granulosa. I. viridis was a significantly better osmoregulator than the two other species. When transferred from high to low salinity water the haemolymph concentration dropped rapidly in all species within the first three hours. After 24 hours an almost steady-state condition was reached. When exposed to a sudden raise in salinity all regulated to the new steady-state situation within three hours. The physiological data found in these experiments fit in with the known ecological distribution of the three species.