For some sea animals, biodiversity peaks at Earth’s mid-latitudes, not the equator

LAWRENCE — For years, most biologists have taught that more numerous and varied species inhabit regions around the Earth’s equator than any other latitude — and, that from this zenith, biodiversity plummets steadily all the way to the North and South poles.

“The typical geographic pattern of biodiversity is something we all learned in school —that the greatest biodiversity is at the equator,” said Daphne Fautin, professor of ecology and evolutionary biology at the University of Kansas. “People point to rainforests where you can find tons of species, as opposed to the poles, where you see masses of very few species. Think of coral reefs — they are models of biodiversity — but at higher latitudes you have a pond with just one kind of fish, like bluegills. The idea is that tropics are more biodiverse. You could say there are more kinds of life — maybe not more life, but more kinds — at low latitudes than high latitudes.”

However, it appears that such a scientific finding does not apply to all life, including sea creatures.

A new study headed by Fautin shows that a greater number of sea anemone species live at the planet’s mid-latitudes — at roughly the position of California, Virginia, Portugal, or New South Wales — and less diversity is apparent within the anemones’ group toward the equator and poles.

In mid-May, Fautin’s research will appear in The Biological Bulletin, vol. 224, no. 2, published by the Marine Biological Laboratory's The Biological Bulletin. The peer-reviewed paper is co-authored by KU colleagues Lacey Malarky and Jorge Soberón.

“I’ve spent the past 10 or 15 years of my life assembling an inventory of all sea anemones in the world,” Fautin said. “I discovered there are fewer species at the equator than at mid-temperate latitudes. That’s where the greatest richness is, and there are fewest species of all at the poles. At first, I thought that maybe we’ve just explored the mid-latitudes more than other places, or there’s more ocean, or more coastline there. But we evaluated all of these factors that might explain if we were missing a bunch of species at the equator — and no. Using statistical inference, we’re able to conclude that at the mid-latitudes there are still more species to be found than at the high latitudes, where there are very few species. The equator is somewhere in between.”

Indeed, Fautin said that along with anemones, other kinds of sea life seem to display peaks of diversity around Earth’s mid-latitudes. Through a review of available research, the KU investigator found a similar geographic distribution pattern for species of planktonic foraminiferans and benthic marine algae.

Anemones are somewhat ideal for such a survey, Fautin said, because they don’t have commercial value, therefore aren’t harvested or propagated. Furthermore, they’re not “charismatic megafauna,” like whales, that are the target of intense preservation efforts.

“An anemone is an animal that has no hard parts, it hasn’t got a bone in its body — it’s not surrounded by a shell,” Fautin said. “It’s like a tin can, attached to a rock or dug into the sand by its bottom. At the top, there’s a central mouth surrounded by tentacles; the mouth leads into a body space — not a stomach. Food goes in there, but it also serves for gas exchange, so it functions like a lung, too. Eggs and sperm form in that cavity. This bag does it all. And waste is expelled through the one body opening.”

With sea anemones and other creatures most diverse at the mid-latitudes, why have scientists assumed there was more marine-life biodiversity around the equator? Fautin said the assumption was based on what is true on land, and a limited number of studies of sea creatures that inhabited shallower waters, and therefore were easy to find.

“It’s difficult to gather the data,” said Fautin. “One of the earliest surveys done was on reef-forming corals, because they’re shallow-water animals. It was easy to document where they occurred because they were accessible even before scuba diving — getting down to 100 meters, and that’s nothing compared to the depths of the ocean. The point is that most marine organisms live out of sight of humans. Reef-forming corals are the charismatic megafauna of the marine environment, and they conformed to this older model, where there are few or none at high latitudes.”

Thus, with new and more comprehensive data, a long-held view of the global distribution of sea life could be falling by the wayside.

The National Science Foundation supported this research.