Species Diversity

In contrast to higher levels of diversity, known species diversity in the sea is much lower than on land - some 250 000 species of marine organisms are currently known, compared with more than 1.5 million terrestrial ones. Much of this difference is because of the large number of described terrestrial arthropods, for which there is no marine equivalent. Amongst fishes, almost as many freshwater species as marine are known, despite the fact that freshwater habitats account for only around one ten- thousandth of the volume of marine ones. Similarly, the most diverse known marine habitats - coral reefs - are far less diverse in terms of species number than the moist tropical forests that are often taken as their terrestrial counterparts.

 

The apparent lower total species diversity of the marine biosphere is because marine environments, particularly deep-water ones, tend to show much less variation in time and space in their physical characteristics than terrestrial ones. This lack of physical variation seems to result in a similar lack of ecological variation over wide areas.

 

The largest marine phyla - Mollusca and Crustacea - each comprise far fewer than 100,000 known marine species, in contrast with the Mandibulata (insects and relatives), of which around 1 million terrestrial species have been identified to date. The only eukaryote phyla with 10,000 or more described species that are believed to have comparable levels of diversity both on land and in the sea are the Platyhelminthes (flatworms), Nematoda, Mollusca and Craniata. As on land, vertebrates are by far the best-known group of marine organisms. Of the 50,000 or so described extant species, around 15,000 may be considered marine, the overwhelming majority of which are fishes and a few are tetrapods.