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First published online July 14, 2008
Journal of Experimental Biology 211, 2450-2459 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.017947
Physiological and molecular mechanisms of osmoregulatory plasticity in killifish after seawater transfer
1 Department of Zoology, University of British Columbia, Vancouver BC, Canada
V6T 1Z4
2 Department of Biology, McMaster University, Hamilton ON, Canada L8S 4K1
Author for correspondence (e-mail:
scott{at}zoology.ubc.ca)
Accepted 22 May 2008
We have explored the molecular and physiological responses of the
euryhaline killifish Fundulus heteroclitus to transfer from brackish
water (10% seawater) to 100% seawater for 12 h, 3 days or 7 days. Plasma
[Na+] and [Cl–] were unchanged after transfer, and
plasma cortisol underwent a transient increase.
Na+/K+-ATPase activity increased 1.5-fold in the gills
and opercular epithelium at 7 days (significant in gills only), responses that
were preceded by three- to fourfold increases in
Na+/K+-ATPase 
1a mRNA expression.
Expression of Na+/K+/2Cl– cotransporter
1, cystic fibrosis transmembrane conductance regulator (CFTR)
Cl– channel, Na+/H+-exchanger 3
(significant in opercular epithelium only) and carbonic anhydrase II mRNA also
increased two- to fourfold after transfer. Drinking rate increased over
twofold after 12 h and remained elevated for at least 7 days. Surprisingly,
net rates of water and ion absorption measured in vitro across
isolated intestines decreased 
50%, possibly due to reduced salt demands
from the diet in seawater, but water absorption capacity still exceeded the
drinking rate. Changes in bulk water absorption were well correlated with net
ion absorption, and indicated that slightly hyperosmotic solutions (
298
mmol l–1) were transported. There were no reductions in
unidirectional influx of Na+ from luminal to serosal fluid or
intestinal Na+/K+-ATPase activity after transfer.
Overall, our results indicate that gill and opercular epithelia function
similarly at a molecular level in seawater, in contrast to their divergent
function in freshwater, and reveal unexpected changes in intestinal function.
As such they provide further insight into the mechanisms of euryhalinity in
killifish.
Key words: Fundulus heteroclitus, gene expression, intestine, gills, opercular epithelium
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