Reef Fish Larva Development
Although reef fishes fill specialized feeding niches as adults, their diets are very different during their early life. At this time, almost all coral reef fish species have a pelagic stage, which is spent in the surface waters of the open seas, away from the reef. The larvae live among the plankton, feeding voraciously and growing rapidly.
What do the larvae eat?
Newly hatched larvae are extremely small -- angelfish measure only about one-tenth of an inch (2 mm) when they first emerge from the egg. At this size the fish are limited by the size of their mouths and can only tackle the very smallest foods, including phytoplankton such as diatoms and dinoflagellates. As they grow, so does their ability to overcome larger prey and they switch to zooplankton, such as copepods and amphipods.
How do larvae hunt?
Theoretically, the tiny size of larval fish makes it difficult for them to move around in their environment. At just a few millimeters long, pushing through the water must be like swimming through molasses. Nevertheless, size for size, these larval fish are the fastest swimmers in the fish world, covering up to 20 body lengths per second as they pursue the drifting zooplankton. Their mouths are often huge in relation to their bodies, far larger than when compared to the adults. These huge mouths allow them to hunt the most profitable prey to pack on weight -- many grow by as much as a third of their own body weight each day.
The ability to hunt well is crucial; at this size, fish are extremely vulnerable to almost all predators and can only escape this threat by growing. Moreover, tiny fish have virtually no fat reserves and even a short period without food can be fatal. Living in the midst of a huge food swarm means that this is not usually a problem; studies suggest that starvation is not generally a difficulty encountered by free-living fish larvae. In the aquarium, managing to feed captive-bred larvae was once the greatest obstacle to propagating fish successfully, but the increasing availability of specialist products in aquarium shops means that the problem is rapidly becoming a thing of the past.
How does the diet change when larvae settle onto the reef as juveniles?
As the larvae migrate onto the reefs and begin the transition to juveniles, their diets generally change quite substantially, moving from zooplankton to a wide range of benthic foods. Some species, including butterflyfish that subsist entirely on coral as adults, switch immediately to a diet of coral polyps, even at this early stage.
Juveniles of related butterflyfish species, such as the raccoon butterflyfish (Chaetodon lunula) and the threadfin butterflyfish (Chaetodon auriga), prey on the tentacles of polychaete worms before making the transition to coral eating. Some herbivores continue to eat invertebrates after settling and then switch to algae as they grow. In addition, juvenile fish predators often augment their diet with a range of smaller reef invertebrates until they are sufficiently large to hunt successfully for fish.
In the early years of the tropical marine aquarium hobby, breeding reef fishes successfully was a major problem. Many newly hatched fishes have virtually no reserves and quickly starve if unable to find food.