The in vivo swelling and hydration of maturing oocytes of Atlantic halibut Hippoglossus hippoglossus were studied in order to characterise the osmotic mechanism underlying oocyte hydration in oviparous marine teleosts

body fluids (Evans, 1993) is reflected in their eggs (Davenport et al., 1981; Riis-Vestergaard, 1982; Watanabe and Kuo, 1986; Mangor-Jensen, 1987; LaFleur and Thomas, 1991). However, these eggs have none of the advantages of the adult osmoregulatory organs associated with homeostasis, and the typical drinking activity seems to start at, or shortly before, hatching (Guggino, 1980; Mangor-Jensen and Adoff, 1987; Riis-Vestergaard 1987; Tytler et al., 1993). This is also true for Atlantic halibut Hippoglossus hippoglossus embryos (Tytler and Blaxter, 1988). Thus, like other marine teleosts, the eggs of Atlantic halibut, while still protected within the maternal ovary, must be osmotically pre-adapted to the marine environment prior to spawning. The present study therefore examines the intra-ovarian changes that occur in maturing marine teleost oocytes that pre-adapt them to the hyperosmotic condition of sea water in which they will be spawned. It was noted more than a century ago (Fulton, 1898; Milroy, 1898) that the oocytes of marine teleosts undergo a dramatic increase in water content during final maturation. The increase was seen to be greater in species that spawned pelagic eggs (pelagophils) compared to those that spawned benthic eggs (benthophils), and the higher water content of the pelagophil eggs was thought to be the major reason for their flotation (Fulton, 1898; Craik and Harvey, 1987; Mellinger 1994). The observations of Milroy (1898) implicated inorganic ions, 211 The Journal of Experimental Biology 205, 211–224 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3911