Embryonic development of the pacific cod Gadus macrocephalus (Gadidae)

Incubation of pacific cod eggs was divided into eight series, in which temperatures were set at −0.04°C to +4.03°C and warm and cold conditions alternated. The morphological changes that took place during the embryogenesis were described in detail using the results of the incubation. Twenty-two morphological characters that could be identified easily and that characterized the morphogenesis were defined in the course of development. The results of the incubation and data from the literature showed that the duration of the embryonic period in the Pacific cod’s lifecycle grew exponentially as water temperature decreased. It was found during the experiment that developing cod eggs survived low water temperatures up to freezing, as well as abrupt warming or cooling (over 3°C). According to the widely accepted Rass scale, the first stage of the Pacific cod embryogenesis takes 21% of its total duration, the second stage 23%, the third, 17%, and the fourth, 39%. However, at a temperature below 0°C, the relative duration of the stages of cleavage and embryonic shield was slightly shortened, whereas the mature embryo stage extended to almost half of the embryogenesis period. A more comprehensive analysis of temperature effects on embryogenesis revealed that the reduction of the rate of embryogenesis upon a temperature decrease occurred mostly at later stages of embryo growth. Modeling of development using defined morphological characters showed that the duration of embryogenesis grew linearly as the incubation temperature dropped in the first half of the embryogenesis and exponentially in the second half. A function was selected that described the obtained results most satisfactorily and that could be used for estimating the duration of the entire embryogenesis or any its stages within the range of water temperatures typical for Pacific cod.