Lionfish on a mesophotic reef off Florida. Photo credit: Mike Echevarria, Florida Aquarium via NOAA
In the last few decades, scientists have come to appreciate the incredible creatures living on the reefs that lie just below conventional diving limits in what is called the mesophotic zone. These incredible biodiversity hotspots are home to more endemic species than shallower reefs, and conservationists are hopeful they may serve as refuges—pockets of relatively pristine habitat out of reach of anthropogenic stressors—where species under threat from pollution, overfishing, and even the effects climate change can hang on while we clean up our act.
Researchers survey bleached corals in the shallow water in Cygnet Bay, Western Australia, during current bleaching event. Photo Credit: Chris Cornwall
April marked the twelfth consecutive month of record-breaking temperatures. That’s an entire year of our planet, on land and in the sea, being hotter-than-ever-recorded since record keeping began in 1880. Such extraordinary warmth is affecting ecosystems globally, but perhaps the hardest hit are coral reefs, whose fundamental organisms are incredibly sensitive to the heat.
Earlier this year, authorities in Australia reported that the Great Barrier Reef was in the midst of its worst bleaching event ever. Surveys above and below the water estimated that over 90% of the reefs were affected by bleaching. Now, as the summer wanes down under, scientists are finally able to begin to assess the lasting damage caused by this event. Their findings are heartbreaking. Continue reading “The Summer One Third of the Great Barrier Reef Died”
The stunning biodiversity of a “coral” reef. Photo by Laura D
Rising carbon dioxide levels in our atmosphere are changing Earth’s climate at an unprecedented rate. Not only is our planet getting warmer on average—in the oceans, a chemical reaction spurred by dissolved CO2 is altering water chemistry, causing a decrease in pH. This effect of climate change, called ocean acidification, can dissolve the calcium carbonate foundations of coral reefs and other calcifying organisms, making it impossible to build and maintain healthy reefs. Luckily, recent studies on how corals react to lower pHs has given scientists hope that they may be more resilient than previously thought. However, to truly understand how reefs will respond to climate change, we have to look at more than just corals.
Reefs are complex ecosystems, the bases of which are comprised of so much more than corals. There are other species which act as calcifiers, adding to the carbonate foundation (such as crustose coralline algae). The contribution of these non-coral species to reef growth, called secondary accretion, helps shape the surface and guide the settlement of larval corals. There are also species that eat away at the reef, including many worms and sponges. These bioeroders can weaken reef structures until they crumble apart. Whether a reef grows or shrinks over time depends on the interplay between its corals, other reef-builders, and the burrowing organisms which eat their way through the reef’s carbonate foundation. Continue reading “Reef “Cat Scans” Reveal Another Way Acidification Speeds Erosion”
The Socotran landscape dominated by dragon’s blood trees. Photo by Tane Sinclair-Taylor.
Some 150 miles off the eastern coast of northern Africa — about 240 miles south of the Arabian peninsula — lies a set of islands that some have called “the most alien-looking place on Earth.” Socotra, which is both the name of the archipelago and the largest island, is a truly bizarre place. More than a third of the plants that erupt from the soil are found nowhere else. Their odd shapes and forms give the islands a Dr. Seussian quality, an almost cartoonish, comical landscape that stands in sharp contrast to the strict religious practices of the people who live there.
Socotra is a part of Yemen, a country currently engaged in a brutal civil war. But, it is also an island apart; the hundreds of miles of water that separate Socotra from the mainland have not only allowed biodiversity to take otherworldly forms, they have also allowed the people of Socotra to become distinct. The 50,000 or so people that live on the islands have their own language, myths and legends, and are their own governorate. Socotra has an archeological history that dates back to some of the earliest civilizations, complete with inexplicable cave art and 2,000 year old tools. The rich culture of the indigenous Soqotri has been insulated, like the flora and fauna they live alongside, for thousands of years — until the past decade, or so, when the Yemeni government has begun to open the islands to tourism.
In 2008, the United Nations Educational, Scientific and Cultural Organization (UNESCO) declared the islands a world heritage site, citing the incredible uniqueness and diversity of the land’s plants and animals — the “Galápagos of the Indian Ocean.” In addition to the hundreds of endemic plants, more than 90 percent of the reptiles and land snails that live on the islands are exclusive to them. Culturally and ecologically, Socotra is a place where uniqueness flourishes.
Coral under siege by the seaweed Chlorodesmis fastigiata
Just below the ocean’s surface, a war is being waged. Coral reefs are under constant assault by seaweeds which seek to take control, stealing the coral’s prime sunlit location for themselves. Many of these plant invaders come equipped with deadly chemical weapons that knock down the coral’s metabolism, which might come off as an unfair fight against a seemingly unarmed foe. But corals aren’t defenseless; as a new paper in Science shows, corals have fish bodyguards at the ready to mount a defense.
Coral reefs are one of the most productive ecosystems on Earth. They’re also one of the most threatened. While managers and scientists struggle to find ways to protect these precious habitats, coral cover has decreased by 50% along the Great Barrier Reef and 80% in the Caribbean. The losses ripple up the food chain, causing declines in fisheries and ecosystem services. But not all organisms mourn coral declines—when corals struggle, seaweeds reap the benefits.
Corals and seaweeds are in a constant struggle for dominance. On healthy reefs, seaweeds are kept in check by plant-eating fishes and invertebrates which keep the algae from overtaking their coral homes. When these herbivores are lost, like when sea urchins underwent a massive die off in the Caribbean in the 1980s, the algae run rampant, reducing habitat complexity and leaving many fish homeless. Up until recently, it appeared that corals are relatively passive in this ongoing battle. But now, scientists from Georgia Tech have found that corals actively recruit defenders to fight algal invasions.
“We had been studying coral-seaweed interactions to determine which seaweeds were most damaging to which corals and the mechanisms involved,” explains study co-author and professor in the School of Biology at Georgia Tech, Mark Hay. He and his post-doc Danielle Dixson discovered that the seaweed Chlorodesmis fastigiata is particularly chemically toxic to corals, emitting lipid toxins that harm corals that they come in contact with. Yet in Fiji, where the experiment was conducted, corals seemed to be holding their own. Given the important role of herbivores, the team decided to see if the fishes living in the corals might be fighting back on behalf of their homes. So, they took Acropora nasuta colonies (an important reef-building coral) that had resident gobies and removed the fishes from some of them. They then placed algae or an algae mimic made of nylon line on the corals and watched the corals over a few days to see what happened.
The mutualistic fish Gobidon histrio in its home coral Acropora nausuta, coming out to investigate the presence of the toxic green alga Chlorodesmis fastigiata
While the fake algae (which physically covered the coral but lacked the chemical weaponry of the algae itself) had no effect, the corals where algae was transplanted were all damaged by the introduction of the competitive plant. But, the scientists noted, the corals that retained their fish residents were much better off. After three days, the amount of seaweed left on the corals was reduced by 30%, and the corals themselves suffered only 20% – 30% the damage of the corals without their fish colonies.
Assured of the important role of the fishes living in the corals, the team further investigated the interplay between the fish, coral and algae. The team introduced Chlorodesmis fastigiata to corals again, but this time they watched how the fish reacted. Within minutes, small gobies—only inches in size—emerged from their hiding places to pick at and eat the seaweed. “These little fish would come out and mow the seaweed off so it didn’t touch the coral,” said Hay.
But to really understand what was going on, the scientists took water samples from next to undamaged corals, corals damaged by algae while the algae was still present, corals damaged by algae after the algae was removed, and the algae alone away from coral. They exposed gobies to these water samples and watched how they responded. In less than 15 minutes, gobies were drawn to the water from damaged corals, but didn’t react to the chemical signature of algae by itself. “We found that the gobies were being “called to” the area damaged by the algae, and that the “call” was coming from the damaged coral, not from the seaweed.”
“This species of coral is recruiting inch-long bodyguards,” explained Hay. “This takes place very rapidly, which means it must be very important to both the coral and the fish. The coral releases a chemical and the fish respond right away.” The scientists even tested the gobies by using water from seaweed damage of a different but closely related coral species, but the fish didn’t react. “The gobies came to the rescue of their host coral but did not respond to a related coral’s chemical cues,” said Hay.
The gobies aren’t being entirely selfless. Gobies don’t just eat seaweed—they also eat mucus from the coral itself. “The fish are getting protection in a safe place to live and food from the coral,” explains Hay. “The coral gets a bodyguard in exchange for a small amount of food. It’s kind of like paying taxes in exchange for police protection.”
In addition to defending their homes, the team found that one of the little fish species gets an extra defensive boost from eating the invading seaweed. “One of the gobies was known to produce a toxic skin secretion,” explained Hay. “This goby consumed the toxic seaweed and became more toxic,” thus helping to protect it from potential predators. The other main species of goby found in the coral doesn’t have this defense, but it still fought off the attacker. “It trimmed back the seaweed from its host coral but did not consume the seaweed – it apparently just trimmed it and spit it out.”
The scientists were even able to narrow down what in the seaweed is causing the coral’s cry for help. The team took different fractions of seaweed chemicals and applied them to fake nylon mimics. Only the extract containing the known lipid chemical weapon triggered the fish defense system.
“I’m an ecologist that studies chemically-mediated interactions, but the wonderfully subtle, nuanced, and specific chemical dance being conducted here is still shocking to me,” said Hay. He noted that these findings highlight the significance of mutualistic interactions on coral reefs. “Competition among some seaweeds and corals has been important enough to drive the evolution of this wonderfully well-tuned signaling among a coral and its mutualistic fishes.” While similar mutualistic defense systems are well described in terrestrial species, this is the first time such an interaction has been shown in a marine environment.
Hay also emphasized that, at least when it comes to ecosystems, size really doesn’t matter. “Organisms need not be large or abundant to be ecologically important,” said Hay. “These tiny, inconspicuous fishes are important in slowing, or preventing, the damage that seaweeds do to corals and thus are important, but unappreciated, in stabalizing reef corals and preventing coral loss and reef decline.”
