The numerical abundance of larval stages of many species and the processes that reduce this abundance have been a major focus of ichthyological research. But growth and change continue throughout the life of a fish. Since most commercially important fishes are exploited as adults, juvenile growth and maturation to adulthood have been extensively studied. Additionally, many fishes undergo a postreproductive period when organ systems degenerate, providing a comparative model for studying the aging process and old age, topics of importance in human biology.
Hatching or birth and the onset of exogenous feeding represent two landmark events in the early life of a fish. Also of importance for many species is the change from larval to juvenile habitat, a transition that often involves settling from the water column and the assumption of a nearbenthic existence. Traditionally, the larval phase is considered to end and the juvenile phase begin as larval characters are lost and the axial skeleton, organ systems, pigmentation, squamation, and fins become fully developed, at which time fish look essentially like a miniature adult. This transition can be brief and relatively simple, requiring minutes or hours in some damselfishes, or it can be very long and complicated, taking several weeks in salmons, squirrelfishes, gobies, and flatfishes (see below) (Kendall et al. 1984).
Some complex adaptations that essentially define major taxonomic groups do not appear until the juvenile phase.
One example is the alarm reaction of the Ostariophysi. Minnows and other ostariophysan fishes have a characteristic escape response to alarm substance, a chemical released from the skin of injured conspecifics. The alarm reaction appears relatively late in development, after shoaling behavior develops and after fish can already produce alarm substance in their epidermal club cells. However, the alarm reaction is genetically hardwired. After 51 days posthatching, European Minnows, Phoxinus phoxinus, react to alarm substance in the water the first time they encounter it, regardless of experience with predators (Magurran 1986b).
Although eggs and larvae are by far the most vulnerable stages during the life history of an individual, attainment of the juvenile stage still entails strong selection for successful food acquisition and predator avoidance abilities. The interplay between these factors is exemplified by juvenile Brook Trout, Salvelinus fontinalis. Brook Trout do not undergo the smolt transformation characteristic of many other salmonids, as discussed below. Instead they hatch in spring and take up residence in small, shallow streams. Their chief task is acquiring suffi cient energy stores during their first summer to get through the winter period of low food availability. Larger juveniles have a greater chance of surviving the first winter. To acquire energy and to grow, they must establish and defend a feeding territory. The best territories are in relatively shallow water, exposing the fish to both aquatic and aerial predators. Predators can be avoided by remaining motionless, but motionless fish can not chase prey or repel territorial invaders. Feeding and fighting distract an individual from avoiding predators.
Basically, smaller fish take more risks and tend to feed more extensively and openly, whereas larger fish are less willing to accept predation risks and are more willing to disrupt their feeding by taking evasive actions. The greater likelihood of winter starvation forces smaller juveniles to make the trade-off between predation risk and foraging differently from larger fish of the same age (Grant & Noakes 1987, 1988; Balancing foraging against predatory threat).
Transitional stages complicate the search for universally descriptive terminology about early life history. They also make it difficult to pinpoint when fish change from one developmental form to another. Transitional stages occur most commonly between larval and juvenile and between juvenile and adult periods. The transitional phase between larva and juvenile in reef fishes has been variously referred to as post-larval, late-larval, new recruit, juvenile recruit, pelagic juvenile, transition juvenile, and settler. The transitional phase may be variable in length, even within a species. This variability makes sense when it is realized that a young fish may not find a habitat appropriate to its next stage simultaneously with its ability to make the transition into that stage. Hence if it were forced to settle from the plankton at day 35 of development, or at the moment that skeletal elements became ossified and fin rays fully developed, a larva that was still far out at sea might have no choice but to sink to the bottom several kilometers down and starve or freeze to death.
Variability in larval period is evident in larvae of the Naked Goby, Gobiosoma bosci. These larvae settle from the plankton and take up a benthic, schooling existence for up to 20 days before transforming to solitary juveniles. Other gobies and a wrasse may have a 20–40-day “window of opportunity” during which they can search for appropriate habitat as larvae without transforming into the more sedentary juvenile form. Flatfishes can delay transformation to the juvenile form if they do not encounter an appropriate juvenile habitat; they do this by alternating between settling on the bottom and swimming above it. Substrate preferences, which imply active search for appropriate habitat, have been observed in numerous larvae (e.g., Sale 1969; Kaufman et al. 1992; Sancho et al. 1997). Direct observations of settling coral reef species indicate that such flexibility may be relatively widespread, and that settlement and transition from larva to juvenile should not be viewed as an all-or-nothing decision. Once settling competence is acquired, many larvae may have days or even weeks before they must settle and assume juvenile habits (Victor 1986; Breitburg 1989; Leis 1991; Kaufman et al. 1992; Larval behavior and physiology).
Metamorphoses by definition imply major changes in the anatomy, physiology, and behavior of an animal. The transition from larva to juvenile in many fishes involves complex suites of change that frequently include major changes in feeding habits and habitat. These alterations necessitate a breaking down and reworking of embryonic and larval structures and a rebuilding into adult structures that will function under very different environmental conditions. A brief example involves sea lampreys. Larval lampreys, termed ammocoetes, are sedentary, blind, freshwater animals that reside in burrows in silty bottoms and filter suspended matter from the water. At metamorphosis to the juvenile stage, this animal is transformed into a predator/parasite with a sectorial mouth, rasping tongue, salivary glands that secrete anticoagulants, functional eyes, tidal ventilation, and an ability to live in sea water (Youson 1988). Many other taxa could be mentioned, but details of two well-studied groups, salmon and flatfishes, will serve to exemplify the complexity of the reworkings that go into changing an animal adapted to larval existence into one adapted to meet the challenges of later stages in its life history.