Direct and indirect development in fishes — examples of alternative life-history styles

Synopsis

Fish larvae — like any larva — are characterized by temporary organs and sometimes strikingly different body proportions; some are so different in appearance from the adults that they were initially considered different species. While growing into the definitive phenotype, these larvae have to undergo a more or less drastic metamorphosis. This indirect development is typical for fish with numerous small eggs, little yolk and, in most cases, no parental care; it is especially common in pelagic marine species. With increasing parental care — from egg scatterers to brood hiders to external and internal bearers — the eggs become yolkier and less numerous. A higher amount and density of yolk enables the young to grow to a larger size and to further differentiate before feeding actively and also allows them to develop more directly into the definitive phenotype. Non-guarding egg-scattering fishes share some characteristics with altricial birds: small eggs, little yolk, and smaller and less developed young (at hatching in birds, at onset of exogenous feeding in fishes). In contrast, most guarders and bearers have in common with precocial birds large eggs with a large amount of dense yolk and larger, more developed young. Because of these parallels one might also distinguish between altricial and precocial life-history styles in fishes.