The Census for Marine Life has found that the most bio-diverse waters so far studied are around Australia and Japan. These each feature almost 33000 species. However, this could change as some highly diverse areas such as Indonesia and Madagascar have yet to report.

Scientists combined information collected over centuries with data obtained during the decade-long Census to create a roll call of species in 25 biologically representative regions: Antarctica, Atlantic Europe, Australia, Baltic Sea, Brazil, Canada (East, West and Arctic), Caribbean Sea, China, Indian Ocean, Japan, Mediterranean Sea, New Zealand, South Africa, South America (Tropical East Pacific and Tropical West Atlantic), South Korea, the Humboldt Current, the Patagonian Shelf, and the USA (Northeast, Southeast, Hawaii, Gulf of Mexico, and California).

They found that the number of known, named species contained in the 25 areas ranged from 2,600 to 33,000 and averaged about 10,750. On average, about one-fifth of all species were crustaceans which, with mollusks and fish, make up half of all known species across the regions.

Many species appear in more than one region. Current holders of the title “most cosmopolitan” marine species are two opposite kinds: plankton and the seabirds and marine mammals that traverse the oceans throughout their lives.

Among fish, the manylight viperfish (Chauliodus sloani) can be considered the Everyman of the deep ocean. Census data shows the fish has been recorded in more than one-quarter of the world’s marine waters.

The number of unique “endemic” species seen nowhere else on Earth provides another measure of biodiversity. The relatively isolated regions Australia, New Zealand, Antarctica and South Africa have the most endemic species. They may have suffered fewer extinctions from climate cooling thousands of years ago during glaciation. Or, species from regions that escaped glaciers may have reached them more easily when the glaciers melted.

Endemics comprise about half of New Zealand and Antarctic marine species and a quarter of those in Australian and South Africa. The Caribbean, China, Japan, and Mediterranean each have less than 2,000 endemic species, and the Baltic only 1 – a seaweed (Fucus radicans).

To encounter invasive species, visit the Mediterranean. It had the most alien species among the 25 regions with over 600 (4% of the all species inventoried), most of which arrived from the Red Sea via the Suez Canal.

Published by the open access journal PLoS ONE, the landmark collection of papers will help guide future decisions on exploration of still poorly-explored waters, especially the abyssal depths, and provides a baseline for thinly-studied forms, especially small animals.

Further Reading:
What Lives in the Sea? Census of Marine Life