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Family of fishes

Clupeidae is a family of clupeiform ray finned fishes, comprising, for instance, the herrings and sprats. Many members of the family have a body protected with shiny cycloid (very smooth and uniform) scales, a single dorsal fin, and a fusiform body for quick, evasive swimming and pursuit of prey composed of small planktonic animals. Due to their small size and position in the lower trophic level of many marine food webs, the levels of methylmercury they bioaccumulate are very low, reducing the risk of mercury poisoning when consumed.

Clupeids are mostly marine forage fish, although a few species are found in fresh water. No species has scales on the head, and some are entirely scaleless. The lateral line is short or absent, and the teeth are unusually small where they are present at all. Clupeids typically feed on plankton, and range from 2 to 75 cm (0.8 to 30 in.) in length. Clupeids spawn huge numbers of eggs (up to 200,000 in some species) near the surface of the water. After hatching, the larvae live among the plankton until they develop a swim bladder and transform into adults. These eggs and fry are not protected or tended to by parents. The adults typically live in large shoals, seeking protection from piscivorous predators such as birds, sharks and other predatory fish, toothed whales, marine mammals, and jellyfish. They also form bait balls. Keeping a high swimming speed during periods of low food availability would not be efficient to maintain over long periods of time as the organisms would not net as much energy as they may need to in order to sustain themselves and increase their fitness. Increasing their swimming speed during feeding periods would allow them to take in more plankton while not suffering consequences from maintaining that speed.