Latitudinal, longitudinal and depth gradients

Most plant and animal species are most diverse at low latitudes. Differences in species richness between latitudes are often truly amazing. Thus, there are about 150 species of marine fish in the North Sea, but there are thousands in tropical seas of much smaller areas. But even at a particular latitude, differences exist along longitude. For example, the south-east Asian waters are particularly rich in faunal diversity, whereas the eastern Pacific is much poorer. Little is known about species numbers in the deep sea, but the few studies that have been conducted suggest that diversity at least of some groups (e.g. nematodes) is very great in the deep-sea benthos. This section evaluates evidence for gradients not only in marine parasite diversity, but also for gradients in reproductive strategies, host specificity and latitudinal ranges.