Fish correspond to the most diversified phylum among vertebrates with a large variety of species. Even if general features are distinguishable during the embryogenesis, several differences in term of timing, organ implementation or step progression always occur between species. Moreover, the developmental timing of wild non-model fish often presents variability within a species. In that context, it is necessary to define a model of developmental table flexible enough to describe fish development by integrating this variability and allow intra- and inter-specific comparisons. The elaboration of a model passes by the definition of new stages that could be easily observable on individuals. The present study aims at proposing such a model and describing accurately the Eurasian perch (Perca fluviatilis) embryogenesis using microscopic techniques among which time lapse video and histological studies. The Eurasian perch belongs to the Percidae family that includes 235 species classified in 11 genera. It is a member of the Perca gender and inhabits the Northern part of Europe and Asia. At 13 °C, P. fluviatilis development elapses for 15 days from the fertilization to the first oral feeding. The staging division first took into account the cellular status to define periods, then the acquisition of new abilities by the embryo to further define stages. It allowed distinguishing two main stages during the cell cleavage period depending on the synchronization of the cell divisions, two stages during the gastrulation period depending on the cell speed migration and five stages during the organogenesis according to the acquisition of key abilities as proposed in the saltatory theory. During each stage, organs implementation was carefully followed with a particular attention for the visual and digestive systems. In addition, our study shows that embryos hatch at various developmental stages while they all begin to feed at a fixed date, 15 days after the fertilization whatever the spawn and the hatching date. These data give arguments to propose the first oral feeding as the best definition of the embryonic-to-larval transition. The present model of developmental table combines flexibility and accuracy allowing detailed description of non-model fish species and intra- and inter-specific comparisons.

It has been hypothesized that a lack of dioxygen could be the trigger of hatching. Here we address the question of whether developmental heterochronies could be an indirect way to study adaptation to lack of dioxygen. To answer this question, we aim to explore the potential of an original method which compares the relative rate of anatomical development across an extended set of 19 species using a hierarchical analysis of ontogenetic time. Then we compare heterochronic rates with the relative time at hatching and information about dioxygen availability. Pairwise independence between dioxygen richness, rate of development and relative time at hatching are all rejected at the 5% threshold. This suggests a link between these three variables, and provides consilient evidence for the hypothesis that lack of dioxygen is a trigger of hatching. A possible scenario is the following: hatching early relatively to developmental time (because of a lack of dioxygen) exposes larvae to predation, and concomitant accelerated early anatomical development could be the result of a selective constraint which consists of being able to escape predators. We conjecture that accelerated embryonic development might be linked to adaptation against predation in dioxygen-poor waters.