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Journal of Experimental Biology 136,259-272 (1988)
Published by Company of Biologists 1988

Branchial Na⁺/K⁺-ATPase and Osmoregulation in the Isopod, Idotea Wosnesenskii

CHARLES W. HOLLIDAY ¹

¹ Oregon Institute of Marine Biology, Charleston, OR, USA and Department of Biology, Lafayette College, Easton, PA, USA

Pleopod Na⁺/K⁺-ATPase enzyme specific activity (ESA) and osmoregulatory performance were measured in isopods acclimated for 12-14 days in 100%, 75%, 50% and 25% sea water (SW). The animal was 63-65 mosmol kg^-1 hyperosmotic to the medium in 100% and 75% SW, 214 mosmol kg^-1 hyperosmotic in 50% SW and 239 mosmol kg^-1 hyperosmotic in 25% SW. The five pairs of pleopods are biramous but are not similar. The two anterior pairs do not stain with silver, have low and unchanging ESA, are smaller and more rigid than the three posterior pairs and are used for swimming. The posterior three pairs are thinner and more flexible; their endopodites stain with silver and have fivefold higher ESA than their exopodites or the front two pleopods. Only the endopodites of the posterior three pleopods showed large (twofold) increases in ESA in animals acclimated in dilute media. The time course of acclimation from 100% to 50% SW was measured: osmotic equilibrium occurred after 1 day; posterior endopodite ESA was elevated after 1 day and was fully activated after 3 days. Ablation of the endopodites of the posterior three pleopods eliminated the animal's ability to hyperosmoregulate in 50% SW; ablation of the exopodites of the same appendages or of both rami of the first two pairs of pleopods had no significant effect on osmoregulation. Thus, the endopodites of the posterior three pleopods of I. wosnesenskii are the sites of inward ion transport in dilute media.

Key words: Na⁺/K⁺-ATPase, osmoregulation, sodium transport, Crustacea, gill

Accepted on November 25, 1987

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