Freshwater Fish Larva: A Dangerous Time

When the embryo emerges from the egg it becomes known as a larva. The first stage of its life is over but the pace of its development is maintained. The odds of a fish surviving past the larval stage are often tiny but if it is to have any chance, it must grow — and quickly.

What happens during the first few hours following hatching?

The newly hatched larva is still some way from living an independent life and hunting for its own food. After hatching, most fishes continue to absorb food for the first few days from the yolk sac. During this time, the larva's body continues to develop. The system of muscles in the body links up with the nervous system and the brain. For the first time, the fish is able to co-ordinate its movements, although while it continues to feed on the yolk sac, the larva cannot swim freely. Some larval fishes, keen not to draw unwelcome attention to themselves, remain very still at this stage. Others, particularly those with parents to protect them, continually twitch and wriggle, working their developing muscles like a fledgling bird.

When does the larval fish start to feed itself?

Once the yolk supply is exhausted the larval fish must find food rapidly. Fish at this stage obviously have no fat reserves and can quickly starve if they do not locate food. Less than a week after beginning life as a single fertilized cell, the tiny fish begins to hunt for food. The larva has large eyes and can detect moving prey, often single-celled protozoans such as Paramecium. As it grows, the larva graduates to larger, more substantial food, including tiny aquatic worms and crustaceans.The larva must feed almost continuously to supply the energy demands of its growing body. Many succumb to starvation at this stage, unable to fuel the furnace within. Unlike their parents, larval fish tend to have a continuous fin that extends around the rear half of the body, rather like a frog tadpole. One reason for this is to counteract the density of water, which is 800 times denser than air. For a larval fish with its newly formed muscles, swimming in this viscous environment must be exhausting.

How does a larval fish sense its world?

The crucially important, complex sensory systems are in the final stages of development during the larval stage. However, the connections between the larva's nervous system and the motor cells that control
movement are still incomplete. This is why if you touch a newly hatched larva it does not dart away. Larval fish tend to have huge eyes — a clue to the importance of their sense of sight at this point in their development. Gradually, as the fish is “wired up,” its senses and its movement become coordinated and it becomes better able not only to hunt but also to avoid its predators.