Planktivore Feeding Habits

Tides and ocean currents deliver a continuous supply of microscopic plants and animals past coral reefs. This abundance of food supports a huge number of sea creatures, from the corals all the way up to enormous animals such as manta rays and the biggest of them all, the whale shark. A large number of more normally sized fishes also feed on the plankton, facing into the currents above the reef to harvest their needs from the mass of tiny copepods, cladocerans, and invertebrate larvae transported across the reef by water currents.

What kinds offish feed on plankton?

Just about all tropical marine fishes undergo a planktonic stage early in their development, where they live as larvae among the plankton. Once settled on the reef, plenty of species diversify into different diets, but equally, a large number continue with their planktivorous diet. Almost all major families have planktivorous members; there are plenty of examples among the damselfish, basslets, and butterflyfish that feed during the day and among nocturnal feeders, such as squirrelfish and cardinalfish.

How are they adapted to their diet?

Diurnal planktivores feed on relatively small members of the zooplankton, such as copepods. These prey animals are usually transparent, all are usually smaller than 3mm and many measure less than 1mm. Spotting them requires excellent eyesight — one characteristic of the fishes that feed on them. The fish also tend to have small mouths and toothless jaws, with tightly packed gill rakers to prevent the escape of captured prey. They have forked tails and, usually, streamlined currents as they feed. These features also help the fish to make a fast getaway if danger threatens.

Nocturnal planktivores are very different in appearance; their huge eyes allow them to see their prey in the dark and their mouths are equally large. One reason for this latter feature is the size of their prey, which is considerably larger than that of diurnal fish — almost always greater than 2 mm — and includes bigger animals, such as mysid shrimps.

How do they feed on the plankton?

Diurnal planktivores face into the current, especially on the outer walls of reefs, picking at their food as it drifts past.The further away the fish venture from the protection of the reef, the more food they can find, but this comes at the cost of a much greater risk of attack from patrolling predatory fish. For this reason, diurnal planktivores tend to feed in groups for safety, and the aggregations are structured by size — the larger, faster-swimming fish can afford to take more risks and feed furthest away from the reef. Sometimes these fish switch from their regular diet to exploit new types of food: when corals, or even large fish such as parrotfish spawn, sizeable groups of planktivores gather downstream to feast The nocturnal planktivores feed in a very different way. As they leave their huge resting aggregations, they split up into small, loose groups or even go solo, spreading across the reef to gather their superabundant prey.

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