Marine Fish Larva Development
When the embryo hatches from its egg and begins the next stage of its life as a larval fish, the pace of its development remains rapid. The odds of it surviving past the larval stage are still tiny, but to have any chance at all it needs to grow -- and quickly. Eggs released into the water column are carried out on ebbing tides to hatch in the open seas, while those that spent the egg stage in their parent's nest also drift out to sea on currents as they hatch.
So for almost all reef fish, the larval stage of life occurs away from the reef itself and, therefore, away from the armies of potential predators that exist there. In many cases, the newly hatched larvae lack fundamental structures, such as fully developed sensory systems, fins and digestive tract, which can take two to three days to develop. Gradually the fish is "wired up"; its senses and its movement become more coordinated, it becomes better able not only to hunt, but also to avoid its predators. Even so, the continuing danger drives the pace of their development.
Do the larvae just drift randomly in the sea?
It was originally thought that larvae dispersed over long distances, but recent research suggests that this may not be the case. A study that tracked the movements of individually marked larval ambon damselfish (Pomacentrus ambomensis) showed that up to 60% of the larvae returned to the same reef and the same population as their parents. As well as this, it seems that some fish miss out the pelagic stage altogether and undergo the larval stage of their development in the lagoons of their own reefs, remaining close to where they were spawned.
How long do fish spend as larvae?
The exact amount of time varies according to species, temperature, food, and the need of the larvae to find a good place to settle. Clownfish and other parentally guarded species spend among the shortest periods of time as larvae -- perhaps as little as one to two weeks. One possible reason for this is the larger size of their eggs and the longer period the embryos spend developing inside them. Open water spawners, such as angelfish, usually spend considerably longer as larvae. Some of these fishes spend up to 20 weeks at the larval stage before settling.
Do the larvae look like their parents?
In the pelagic stages, reef fishes are very different morphologically from the benthic (bottom-dwelling) adults, because reef fish larvae do not hatch fully developed. For a start, the eggs and larvae are small. The larvae also need different morphological features for their pelagic existence, such as defensive spines and bony plates for protection, and large eyes and mouths for hunting. Larval Atlantic blue tangs (Acanthurus coeruleus) look so different from their parents that they were once thought to be a totally separate kind of fish and were named Acronurus. The larvae of butterflyfishes, known as tholichthys, also bear little resemblance at this stage to what they will later become.
How do the larvae find their way to a suitable reef?
The larvae of most species ride back inshore on an incoming tide at night. To find their way to a good reef they are guided by both sounds and smells. Each reef has a characteristic sound, produced by a combination of wave patterns over the coral heads and a so-called "nocturnal chorus" of feeding invertebrates. The larvae also use chemical cues to locate a good reef and are strongly attracted to the smell of adult conspecifics.