Tuesday, December 6, 2011
One of the world's largest lakes, Lake Tanganyika (map), has evolved over time to act more like an ocean, housing colorful animals such as jellyfish in water up to a mile (1.5 kilometers) deep.
Many of the African lake's diverse assemblage of freshwater creatures—including snails, crabs, and sponges—bear an uncanny resemblance to their marine counterparts.
(See pictures of colorful sea creatures.)
"You can put Tanganyika snail species next to marine species, and they look more or less identical," said Tony Wilson, an evolutionary biologist at the University of Zurich in Switzerland who has studied the lake's creatures.
Colin Apse, an Africa and U.S. freshwater-conservation advisor for the Nature Conservancy, added, "There are almost 300 species of fish in the lake that you'll find nowhere else in the world, and many of them are brightly colored and dart around just like coral reef fish."
"If you're snorkeling, it can be easy to forget that you're not in the Great Barrier Reef or the Caribbean." (See coral reef pictures.)
The lake, which contains some 17 percent of Earth's available surface fresh water, is an important source of water and food for millions of Africans whose countries—Burundi, Tanzania, Zambia, and the Democratic Republic of the Congo—share its 420-mile-long (676-kilometer-long) coastline.
"The vast majority of people around the lake are living on a dollar a day or less, and they are completely dependent on the lake," Apse said.
But the lake is now at risk from pollution, overfishing, and other human-caused threats, conservationists say.
(Also see "On Africa's Largest Lake, Fishers Suffer Falling Stocks, Rising Demand.")
Freshwater Version of an Ocean
In the early 1900s, scientists proposed that Lake Tanganyika and the ocean were once connected, and that the lake's creatures are descendants of an ancient "marine incursion."
Subsequent studies, however, have shown that the lake and the ocean were never directly connected.
Instead, Lake Tanganyika is a so-called rift lake, a body of water created when sections of Earth's crust—in this case, the eastern and central subsections of the African tectonic plate—drift apart.
Due to the nature of their creation, rift lakes can be extremely deep, and Lake Tanganyika is no exception. (See more lake pictures.)
The lake's incredible depth could also help explain the unusual diversity of its creatures, scientists say.
Because it's so deep, the lake has persisted for millions of years and outlived shallower lakes.
In this way, Lake Tanganyika is like a freshwater version of the ocean, which has endured for eons largely due to its depth.
"The fact that it's persisted for a long time has given opportunities for [the evolution of new species], and for ancient lineages to be maintained within the lake," Wilson said.
Lake Spurred Predator-Prey Evolution
The lake's unusually long "life" may have also allowed an evolutionary arms race to develop, with both predators and prey evolving ever more elaborate ways to best their opponents.
For example, some fish and large-clawed crabs have "developed very specialized feeding mechanisms to crush snail shells," Wilson said.
At the same time, the snails and other invertebrate prey have developed counter-adaptations, such as thicker armor with spikes and knobs that make their exteriors harder to crack.
A similar process is thought to have influenced the evolution of marine snails, something which may help to explain why the two groups of snails resemble one another.
(See "Pictures: How Bubble-Rafting Snails Evolved.")
Wilson thinks that continued study of the lake could yield further insight into how evolution works in watery habitats."
Because Lake Tanganyika is in many ways intermediate between most freshwater lakes and the ocean, it offers unique insights into the origins of aquatic biodiversity."
Overfishing, Deforestation Threats to Lake
But studying the lake might be tough in the future due to certain threats—particularly overfishing.
In addition to being a source of nourishment for local people, the lake's fish are also exported as food and as aquarium pets.
The demand has caused some companies to resort to illegal fishing methods, including the use of small nets along the shore that ensnare baby fish.
"They're scooping up the future generations of fish," the Nature Conservancy's Apse said.
Cutting down surrounding forest for firewood and to make room for farms also ultimately harms the lake.
"If you do that right next to rivers and streams, that leads to a lot of runoff sediments, which go out in big plumes into the lake and disrupt the ability of fish to breed," Apse said.
Save Lake Tanganyika? Treat it Like an Ocean
To address these problems, the Nature Conservancy has teamed up with the Lake Tanganyika Authority, an intergovernmental organization created in 2008 that includes members from the four countries on the lake.
The partnership plans to develop a comprehensive plan to protect parts of the water body.
One possibility is to borrow methods that have proven successful in marine environments.
"Protecting the entire lake would be fantastic but is unfortunately not realistic," Saskia Marijnissen, a Burundi-based technical advisor for the authority, said in an email."
The lake is situated between four countries that include some of the fastest growing human populations on the planet.
"One idea is to protect those sections of the lake that are most vital for fish breeding. These "microscale protected areas" would resemble the marine sanctuaries that help safeguard important sections of the oceans, and could be effective even if they are only a few dozen feet across."
Studies have shown that if you exclude fishing from particular areas of the lake, you can have spill-off benefits to neighboring areas where people fish," the Nature Conservancy's Apse said.
(See "Pictures: Best Marine Park? Booming Fish Leap and Swarm.")
Increasing protection of national parks surrounding Lake Tanganyika—including Gombe National Park in Tanzania, where primatologist Jane Goodall conducted her chimpanzee studies—would also benefit the lake's creatures.
Lake Tanganyika has potential to become a popular ecotourism site, which could provide further incentives for countries to protect it, the Nature Conservancy's Apse added.
"From my perspective, it's the best place for tourism in the world, because you can wander through an amazing forest and see chimpanzees and then jump into the equivalent of coral reef snorkeling—but in fresh water that is kind of the perfect temperature."
Cooperation Key in Lake Protection
The success of any protection plan for the lake will eventually require the cooperation of all four countries that share its borders.
Such a scenario will be difficult, but not impossible, said the University of Zurich's Wilson.
For instance, there's a precedent.
In 1994, the United Nations Development Programme and the Global Environmental Facility funded a five-year program called the Lake Tanganyika Biodiversity Project to develop a sustainable-management plan for the lake.
"This project required a close collaboration among the four countries that make up the Lake Tanganyika coastline and supported the development of scientific infrastructure and training in local communities," Wilson said.
For now, however, the Nature Conservancy and the Lake Tanganyika Authority are focusing their efforts on fisheries management in Tanzania.
(Read about the global fisheries crisis in National Geographic magazine.)
One project involves working with lake communities to develop co-managed protection plans, which would encourage locals to take the lead in developing and monitoring fisheries rules.
"We are developing some really successful examples in a few places," Apse said.
"We want to literally bring people over across the lake, within and between countries, to learn from these examples, so that over time it [catches] fire."