Complex life cycles and indeterminate growth
Two general traits shared by most fishes set them apart from the majority of vertebrate species and also underlie many of their more interesting adaptations. These two traits are indeterminate growth and a larval stage. Many fishes emerge from an egg as a larva, which bears little anatomical, physiological, behavioral, or ecological resemblance tithe juvenile or adult into which the fish will eventually transform. In fact, continual growth moves each individual through a progression of life history stages that differ in most traits, creating a spectrum of continually changing structures and characters upon which natural selection hasoperated.
Indeterminate growth describes the continual increase in length and volume that occurs in most fishes throughout their lives. Although this growth may slow considerably as a fish ages, the potential for continuing increase profoundly affects many if not most aspects of a fish’s life. With regard to most traits, larger body size appears to confer an advantage, at least within a species. Reproduction is intimately tied to body size in terms of egg number and size;larger females producing more and bigger eggs (see Life histories and reproductive ecology). Mate choice by both males and females often favors larger individuals, and larger fish are better able to defend a spawning territory(see Sexual selection, dimorphism, and mate choice). Swimming energetics and shoaling interact with body size: fish tend to shoal with individuals of like size(see Responses of aggregated prey), and larger fish can swim faster and migrate over larger distances (see Annual and supra-annual patterns: migrations). Predation rate is typically greater on smaller fish, and small fish may be constrained from feeding in profitable areas by predators or larger conspecifics. Indeterminate growth leads to size-structured populations in which different size individuals essentially function as different species, the so-called ontogenetic niche (Werner & Gilliam 1984; Population dynamics and regulation).Physiological limitations of small body size can be explained by allometric (proportional) growth of many structures, such as the increased visual acuity and sensitivity that occur as a fish grows. Foraging is also affected by body size, not only because many fish are gape-limited and hence only able to eat things they can swallow whole, but also because many prey types are not available to young fishes until muscle attachment sites and muscle masses reach a sizecapable of overcoming prey defenses (see Pharyngeal jaws).
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