Juvenile Reef Fish: The Third Stage In a Fish's Lifecyle

Less than 1% of the fish that started their journey through life inside the egg survive to settle on a coral reef as a juvenile. These larvae come inshore to the reef, guided by its smells and sounds, often borne on incoming tides and under cover of darkness. Yet even for those that make it this far, life remains hazardous and they face a difficult fight for a place in the crowded fish community on the reef itself.

How does the fish change when it becomes a juvenile?

The transition from larva to juvenile involves some significant changes in appearance, diet, and lifestyle. For example, the blue tang Acronurus larvae change over the course of about a week from a silvery color to brown. They become more rounded in shape and develop their characteristic snout. They also gradually lose the spines that previously protected them in the plankton. Then at about 2 inches (5 cm) long, the juveniles finally settle onto the reef. Throughout the larva-to-juvenile period, growth remains extremely rapid, but there are considerable differences between species in the size of these newly settled juveniles. Some species, such as parrotfish, settle when they are only one-fiftieth of the size of fully grown adults. Others, such as some gobies, are almost fully grown at this time.

Why are so many juvenile fish a different color than the adults?

Juvenile fish tend to have different color patterns than adults. The reason for this is that, in the wild, a small, brightly colored fish is unlikely to survive to adulthood; some colors may attract either predators or aggression from mature adults. Quite a few juvenile reef fishes, including some surgeonfish, are bright yellow as juveniles. Although this might seem an unusually bright and conspicuous color to us, recent research suggests that, from a reef fish's perspective, yellow is actually quite a good color for blending into the background.

How do juveniles cope with the intense competition on the reef?

Carving out a niche in the fiercely competitive environment of the coral reef is never easy for a newly settled juvenile. They are disadvantaged by their small size, as well as their need to remain close to the refuge of the reef.

Some species that in later life live a solitary and territorial existence school as juveniles. An example of this is provided by the marine catfish (Plotosus lineatus), which aggregate together in extremely tight groups when young, only to abandon this strategy as they grow. Schooling enables them to break into the territories of larger fish, as well as giving them protection from predators. Another useful strategy that some juveniles use while becoming established on the reef is to have a different diet from the adults, thus avoiding direct competition. Many adult fish that live exclusively as grazing herbivores are at least partially carnivorous as juveniles. This is the case in the seachub (Kyphosus comelli). At first, juveniles feed on small reef invertebrates and plankton, but gradually switch to a more herbivorous diet as they grow. During this time their digestive system alters to accommodate the changes in diet, and the bacteria that will later help them to digest algae start to colonize the gut.