Description of the early ontogeny of the Atlantic salmon, Salmo salar, with a novel system of interval (state) identification

The sequence of morphological changes through intervals from the time of egg insemination until the time of completed yolk absorption was investigated. The studied time span was divided into more than 100 morphologically discrete states. For this purpose, some quantitative characters, such as number of cells, somite pairs, or caudal fin rays, extent of epiboly, or vascularisation, were selected as key characters, since they seem to provide the most adequate picture of the dynamic developmental process. Descriptions and drawings of 41 states (out of the more than 100) were chosen for the present study to illustrate the embryo and alevin periods. The time it takes to form one somite pair (tau(s)) was used as the unit of relative age and duration of the early ontogenetic states. The morphological description of each state was supplemented by the estimation of the relative age of the embryo or alevin at this state as the value of N/tau(s), where N is the time elapsed from egg insemination up to any given state at a given constant temperature. It was determined that the developmental rate of Atlantic salmon embryos depends on the incubation temperature according to the equation: Ig N-t = Ig N-o + (-0.0967 + 0.00207t)t, where N-o and N-t are values of tau(s) at temperatures of 0 and t (degrees C), respectively, taking into account that the values of tau(s) and the duration of the entire embryogenesis vary proportionally within the temperature range tolerated by the species. The methods used here to identify states of embryogenesis and to estimate the relative age of S. salar embryos and alevins might also be applied to other fish species.