Reflections on Some Decisive Events in the Early Life of Fishes

Abstract

A scholarly life-history model, applicable to various ontogenies, requires precise definitions of the major periods and their boundaries. The theory of saltatory ontogeny enables one to construct such a model based on natural boundaries between consecutive intervals of development. This theory stipulates that development does not proceed by a continuous accumulation of inconspicuous, small changes but is a sequence of rapid changes in form and function alternating with prolonged intervals (steady states) of slower development during which complex structures are prepared for the next rapid change. These times of rapid changes from one steady state to the next are called thresholds and represent decisive events of life history. Saltation explains some of the most misunderstood events of ontogeny. For example, it explains why only activation should be accepted as the beginning of ontogeny, and why insemination, fertilization, and hatching are not the appropriate clues for timing ontogeny. Similarly, it is argued that according to the definition of the larva period–lasting from onset of exogenous feeding until metamorphosis–not all fishes have larvae. In fishes with an increased endogenous food supply and parental care (increased reproductive cost per embryo), the embryo develops permanent organs directly, bypassing the larva period with its remodelling of temporary structures. Hatching is rarely a developmental threshold. The transition to exogenous feeding, rather than hatching, is the decisive threshold of ultimate survival value.