Marine Biodiversity

Diversity at higher taxonomic levels (phyla and classes) is much greater in the sea than on land or in freshwater. Of the 82 or so eukaryote phyla currently recognised around 60 have marine representatives compared with around 40 found in freshwater and 40 on land. Amongst animals the preponderance is even higher, with 36 out of 38 phyla having marine representatives.

 

Some 23 eukaryote phyla, of which 18 are animal phyla, are confined to marine environments. Most of these are relatively obscure and comprise few species. The major exceptions are the Echinodermata (including starfish, sea urchins and sea stars, sea cucumbers), of which some 6000 species are known, and the Foraminifera, with around 4 000 known, extant species. A number of other important phyla including the Cnidaria (including corals and anemones), sponges (Porifera) and brown and red algae (Phaeophyta and Rhodophyta, respectively) are very largely marine, each with only a small number of non-marine species.

 

The reason for this predominance of marine higher taxa is believed to be because most of the fundamental patterns of organisation and body plan, i.e. the different basic kinds of organism that are distinguished as phyla, originated in the sea and remain there, but only a subset of them has spread to the land and into freshwaters. It is noteworthy that only a third or so of marine phyla are found in the pelagic realm, the remainder being confined to sea bottom (benthic)areas ? the habitat where eukaryotic organisms are believed to have evolved.