Specific features of embryonic development of whitefishes (Coregonidae) as representatives of the “pagophilous” ecological group of reproduction

Abstract

Studies of reproduction and embryonic development in six species of coregonid fishes have revealed the ability of their fertilized eggs to develop normally while being embedded in the ice of a spawning water body (optionally). Such ability is facilitated by the extremely low respiratory activity of embryos at early stages of embryogenesis (from the stage of fission to the stage of organogenesis). The low level of oxygen consumption and carbon dioxide emission is an adaptation to the low diffusion gas permeability of the ice. The main factor controlling the rate of coregonids’ embryonic development is not temperature but the intensity and periodicity of isolation. Without sunlight-an obligatory external factor-normal development is just not possible, Under experimental conditions, developing in the water at near zero temperature, or in the ice, normal morphogenesis of Arctic cisco and Sevan whitefish embryos was observed at an illumination of 50–300 lx. Hemoproteid cytochrome β560, the pigment that has been discovered in the water-soluble part of coregonids oocyte yolk and is treated as a biochemical marker for eggs of the family Coregonidae, in all likelihood performs protective (antioxidant) functions, preventing spontaneous oxidation of embryo’s fatty inclusions. In the oxygen shortage inside the ice envelope, cytochrome β560 probably sets the conditions for oxidation processes of embryo’s tissue respiration. Spherome, being kept until the time of hatching, acts as a temporary hydrostatic organ and ensures larvae buyoancy at the stage of postembryonic metamorphosis. It also serves as an energy store after the downstream migration of larvae from the spawning areas until their shift to exogenous feeding on zooplankton. Conforming to the ecological traits of reproduction and development, and also to revealed morphogenetic, physiological, and biochemical features, it is proposed to ascribe all of the currently known 26 species of whitefishes to the “pagophilous” ecological group.