Up to three times as many known marine species still undescribed, study finds
The study appearing in Current Biology Nov. 15 shows there are about 226,000 described marine species, but two to three times as many remain undescribed. Based on knowledge of described and estimated unknown species and the rate of new discoveries, researchers predict there may be fewer than 1 million eukaryotic marine species worldwide. Eukaryotes are all living things other than bacteria and viruses. The conclusion differs from past estimates that exceeded 1 million, but unlike previous efforts, researchers based the new approximation on a global inventory.
Researchers say the findings provide a reference point for conservation efforts and may be used as a baseline for understanding environmental changes such as global warming.
"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," said co-author Gustav Paulay, invertebrate curator at the Florida Museum of Natural History on the UF campus. "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."
All study co-authors are contributors to a global marine life inventory known as WoRMS, or the World Register of Marine Species. The database lists more than 90 percent of marine species and researchers used it to create tables to show how diversity is distributed among different types of organisms. Each co-author, an expert on a different group, compiled its known diversity and estimated the number of collected species that remain undescribed in museum collections, how many may be cryptic or unrecognizably different, and how many remain undiscovered in the wild.
More than 20,000 species have been discovered during the last decade and if trends continue, most species will be discovered this century, according to the study. There are also about 65,000 undescribed species already in collections.
"The entire ocean contains less than 300,000 species described to date and that includes everything from dolphins to worms to single-celled algae," Paulay said. "Because it's so connected, there's less diversity in the ocean than on land. 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 is an expert on sea cucumbers, which are grouped together with sea stars, sea lilies and sea urchins in the phylum Echinodermata. Sea cucumbers are bizarre among echinoderms because their skeleton is reduced to microscopic bits so the animals have secondarily become worms, Paulay said. They occur worldwide in the deep sea and range from less than ¼-inch to about 10-feet long. For the study, Paulay organized the more than 2,400 named sea cucumber species and determined about 1,700 are valid, with the remainder being duplicate names proposed for the same species.
"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."
Nancy Knowlton, a marine scientist at the Smithsonian Institution's National Museum of Natural History, said the study is interesting because it differs from conclusions published in a 2011 PLOS Biology study co-authored by Camilo Mora that estimates there are about 2.2 million eukaryotic marine species, with about 91 percent still undescribed.
"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," Knowlton 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."
Provided by University of Florida