Plenty more fish (and other eukaryotes) in the sea, say boffins

Expert on enormous bottom cucumbers lays it out

While scientists have painstakingly documented the existence of 226,000 marine species to date, there remain two to three times as many sea-dwelling creatures yet to be discovered, a new study has suggested.

Researchers at the University of Florida, who worked with more than 100 taxonomists and biologists to reach their conclusion, said that worldwide there are probably fewer than 1 million eukaryotic marine species - which means any living thing not including bacteria or viruses.

In their paper for the Cell Press publication Current Biology co-author Gustav Paulay, who is an invertebrate curator at the Florida Museum of Natural History on the UF campus, said:

There are issues with any estimate like this, but it's the first time that we really have a solid background at least on what we know, rather than having to speculate about that.

The ocean is a large chunk of the world, so it's useful for biologists to know how diverse the world is - before this, we didn't have this information in one place.

He added that to date less than 300,000 species from dolphins to worms to single-celled algae had been discovered.

"Because it's so connected, there's less diversity in the ocean than on land," Paulay explained. "There's still a lot more to be discovered, but it's a manageable number so we can go after it and really understand the magnitude of marine biodiversity."

Paulay - an expert on sea cucumbers that can grow to about 10 foot long - is a contributor to the World Register of Marine Species, a global marine life inventory dubbed WoRMS, which contains a database listing more than 90 per cent of marine species already found. Researchers used the information to show how diversity is dished out among different types of organisms.

By using that data, those who contributed to WoRMS were able to look at known diversity and then estimate the number of collected species that remain undescribed in museum collections, how many may be cryptic or unrecognisably different, and how many remain undiscovered in the wild.

During the past decade roughly 20,000 species have been discovered. Meanwhile, around 65,000 species remain undescribed in museum collections.

"The most abundant, conspicuous large organisms you see on the ocean floor - which is most of the planet – are echinoderms, especially sea cucumbers," Paulay said.

"They are very important down there, recycling sediment from the ocean bottom or eating suspended particles floating around in the ocean, and they are obviously food sources for other animals."

However, the latest findings about the sea-dwelling species aren't necessarily definitive, noted marine boffin Nancy Knowlton at the Smithsonian Institution's National Museum of Natural History.

"It's really nice to see a very carefully documented analysis that points to these lower numbers, but I think the jury is still out," she said.

"Until different methods start giving us the same answer, it's going to be difficult to know where the truth really lies. But this paper is a very important step in the process of getting an answer for this question." ®

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