CLUPEIDAE is a family of ray-finned fishes , comprising, for instance, the herrings , shads , sardines , hilsa , and menhadens . The clupeids include many of the most important food fishes in the world, and are also commonly caught for production of fish oil and fish meal . Many members of the family have a body protected with shiny cycloid scales (very smooth and uniform scales), a single dorsal fin , with 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.
DESCRIPTION AND BIOLOGY
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. The family arguably also contains the " Sundasalangidae ", a paedomorphic taxon first thought to be distinct salmoniform family but then found to be deeply nested in Clupeidae. In the fossil record clupeids date back to the early Paleogene .