Yolk protein hydrolysis and oocyte free amino acids as key features in the adaptive evolution of teleost fishes to seawater

When teleost fishes made their re-entry into seawater about 150-200 million years ago after some 250 million years of evolution in freshwater (Long 1995), they had to overcome a formidable problem. This problem relates to their osmoregulation and to their oviparous habit of spawning eggs that develop unattended (physiologically speaking) in the water masses. The osmolality of the blood plasma of extant teleosts (Evans 1993) is about 300 mOsm in freshwater and just a little higher in seawater. Indeed, the hyposmotic condition of marine teleosts, a condition that is quite unusual among animals, is taken as evidence for the freshwater origin of the teleost group (Evans 1993; Moyle & Cech 1996). Stable internal osmotic conditions seem to be a demand of all teleost cells including the gonadal cells. Thus, even the eggs and the early embryos of extant marine teleosts are as hyposmotic to seawater as the adult fishes (e.g. Guggino 1980; Riis-Vestergaard 1982, 1987; Hølleland & Fyhn 1986; Mangor-Jensen 1987; Hahnenkamp & al. 1993; Tytler & al. 1993). There are no reasons to believe that the osmotic problem of the newly spawned eggs was any different during the early Mesozoic period when teleosts made their re-entry into the oceans (Fig. 1). Although the adults of the Actinopterygian ancestors of the teleosts with their epidermal armour, fully differentiated organs and physiological mechanisms could gradually adapt to the saline environment and its constraints in the sea, the situation was not so for the newly spawned egg and embryos. These early stages lack the organs and specialised cells that deal with physiological mechanisms involved in the osmotic adaptations to the environment. Yolk protein hydrolysis and oocyte free amino acids as key features in the adaptive evolution of teleost fishes to seawater