Synchrony and heterochrony in ontogeny (of fish).

The ontogeny of an organism is a complex process that strongly depends on the timing of developmental processes. In this article, I discuss ontogeny of fish (and other organisms) in temporal terms, based on the hypothesis that organisms as self-organized entities may create their own times for their development, and that this development consists of a sequence of longer stabilized states (steps) with shorter, intermittent less-stable intervals (thresholds). If viewed within the context of structure-to-structure, organ-to-organ and/or organism-to-environment relationships, then the saltatory pattern of ontogeny emerges at each transition from one stabilized state to another. I consider two timing mechanisms essential to ontogeny - synchrony (coordinating) and heterochrony (implementing); their possible roles are discussed. Besides this, a new context and understanding for the term heterochrony is proposed. At least three levels of heterochrony should be distinguished: interspecific, intraspecific and intraindividual. However, the difference among these three types of heterochrony is not in the phenomenon itself but in the way we perceive and classify it.