Balon Model of Development in Fish

Study Notes

The Balon model of development divides life into five periods: embryonic, larval, juvenile, adult, and senescent.
The embryonic period includes a cleavage phase, covering the time between fertilization and the commencement of organogenesis, and an embryonic phase within which there is intense organogenesis within the egg membrane.
The larval period starts when the transition to exogenous feeding has taken place. The larval period may be divided into two phases: the protopterygiolarval and the pterygiolarval phases.
The juvenile period begins once the fins are fully differentiated, and the temporary larval organs have regressed or been replaced. The juvenile period lasts until the fish mature and the first gametes are produced.
The adult period starts with gonad maturation, culminating in the production of the first gametes. The adult period usually incorporates spawning runs or migrations (see Metcalfe et al., Chapter 8, this volume), specialized reproductive behaviour (see Forsgren et al., Chapter 10, this volume), and changes in external morphology and colour.
The senescent period covers a period of extremely slow growth during which reproductive activity may also be reduced.

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