At present there are approximately 11 500 known species of copepods. The number of species described during the past 27 years is nearly two-thirds of all those described in the previous 100 years. Approximately one-third of marine copepod species are parasites or associates, nearly equally divided between those on fishes and those on invertebrates. Individual copepods are extremely abundant, not only as free-living species or parasites of fishes, but as associates of invertebrates, especially in the tropical regions of the world.