How Many Marine Organisms Are There?

In a landmark paper published in the prestigious international journal Current Biology 220 marine experts worldwide, including Professor Mike Guiry of NUI Galway’s Ryan Institute, have come together to estimate how many species there are in our oceans. The answer will come as something of a disappointment to those who claim more than a million marine species because the authors show that the true number is less than a quarter of that. About 225,000 species have been described to date, and the final tally is likely to be about 500,000.

Previous estimates of a million marine species were based on unreliable figures passed from textbook to textbook and gaining credence when uttered by some famous individual at international conferences.

Species are notoriously difficult to count. Unlike, say, books in a library, they are not discrete objects that can simply be catalogued and a final figure arrived at. The notion of a species is a human construct, essentially a philosophical notion, and the names we use for them, called binomials, are simply for us to communicate, and have no meaning without us.

The numbers in this paper are the best that can be arrived at as, for once, they come from expert opinion, mainly from database curators and world experts in the various groups.

One of these experts is Professor Mike Guiry of the Ryan Institute at NUI Galway. He is the founder of the database AlgaeBase ( funded in 2004-2009 by the Higher Education Authority under the Programme for Research in Third-Level Institutions, itself generously underwritten by Chuck Feeney, recently publically honoured by all the Irish universities, through Atlantic Philantrophies.

The Common Knobbed Star(Pentaceraster mammillatus) and the seagrass Thalassia hemprichii.Zanzibar,Indian Ocean© M.D. Guiry/AlgaeBase.

Professor Guiry has to date catalogued 130,000 names of algae, including 34,000 species. Of these, about half are marine, and include some 9,500 seaweeds. This is the only global species database inIreland.

Professor Guiry comments: “We do not yet know how many species there are in the world, estimates vary from 5-50 million, but the most reliable recent estimates are that 1.9 million, with the insects accounting for more than 1 million.”

Professor Guiry adds: “The marine environment is relatively species-poor, despite claims to the contrary, with less than 12% of all currently described species occurring in the sea. But what the sea lacks in species numbers it makes up spectacularly in diversity and beauty, and in the active part these organisms play in keeping our planet healthy. More than 75% of all photosynthetic oxygen is produced by marine algae.”

The marine Professor’s forthcoming book, A Catalogue of Irish Seaweeds, will show that 7.5% of all seaweeds occur in Ireland, which is extraordinary for the size and geographical spread of the island, and contrasts strongly with our native flowering-plants, which amount to no more than 0.25% of the world’s 350,000-400,000 species.

The marine experts predict that all marine species will have been described by the end of this century, so that the task will have taken the human race 350 years.

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