It was a bright February morning, a perfect day for diving in the blue waters off the east coast of Kadmat, Lakshadweep. Two Indian marine biologists, Rucha Karkarey from the Nature Conservation Foundation and Anne Heloise Theo of the Indian Institute of Science, strapped on their tanks and plunged into the waters at a spot near a lagoon called Seven Huts. For five years, Karkarey has been studying groupers in these coral reefs and it was to be another day of routine survey. That day was anything but routine.
The two women were sampling along the reef slope when they looked up to find a black mass approaching from the open ocean. “We first thought it was the shadow of a cloud passing over us,” Karkarey told The Wire. “But as it approached, we realised it was a massive shoal of tiny fish.”
The colourless fish completely surrounded them. “They swelled and moved like a bait ball,” recalls Karkarey, referring to the schooling behaviour of fish under attack from predators. In a recently published paper, the biologists wrote that the event reminded them of a murmuration of starlings in the sky. The fish swirled in a shimmering vortex that grew larger as more came in from the sea. The biologists immediately recognised the larvae of surgeonfish. Later, from photographs and video recordings, they identified the young of three colourful species – orange-striped bristletooth tang, lavender tang and Thompson’s surgeonfish.
“It was incredible,” recalls Karkarey.
Forty-five minutes later, when the murmuration of fish stretched 50 metres in diameter and towered four metres, the fish larvae dove to the reef. “It was like the opening of a dam gate,” says Karkarey.
What were these fish doing? Why did they come from across the sea seeking the reef?
Coral reef fish stay close to their patch and defend their territories. Many species lay eggs that drift with the water currents into the open sea. When the larvae hatch, they live on plankton for up to six weeks until they are two to three centimetres in length. They then make their way back to the reef. They switch to grazing on algae, demarcate their territories, find mates and life comes full circle.
Karkarey and Theo witnessed that critical period when the fish larvae forsake their nomadic open water existence for life in the undersea garden of corals.
Once they reached the floor, they didn’t shelter in the nearest crevices. They splintered and spread out across the reef. “The larvae no longer shoaled, but like school children returning to their classrooms after assembly, followed queues, which took distinct routes,” says Karkarey. As more fish poured in from the ocean surface, they stayed close to these already established trails, like wildebeest marching across the plains of Serengeti in East Africa.
“It was an invisible network of roads and highways,” recalls Karkarey. “They weren’t moving haphazardly.”
The biologists ran out of air 75 minutes later. When they returned after a break of four hours, the eddy of fish had stopped, but the larvae continued to stream across the reef along these distinct pathways.
The corridors appeared to align with natural grooves on the reef, parallel to the slope. These trails extended as far north as Masjid Point, a couple of kilometres from Seven Huts. The two biologists followed a column of larvae along a route. Several fish larvae broke ranks, and started “ducking in and out of corals and crevices, grazing on algae, and attacking others that came close.” They seemed to be scouting for suitable places to settle down without leaving the safety of the group entirely. If they were content with the spots they had chosen, they stayed put. Within a day of breaking away, the fish turned dark from eating algae.
“This could potentially be one of the functions of the invisible pathways – to facilitate the dispersal of larvae to as many parts of the island as possible,” says Karkarey. This is the first study to highlight such pathways.
The two marine biologists dove at different spots around Kadmat island and revisited Seven Huts and Masjid Point until the larval shoals disappeared ten days later. Karkarey and Theo say scientists thought fish larvae settle at whichever point of the reef they reach. “Instead, we saw that ‘entry sites’ and actual settlement sites can be two different reefs altogether,” says Karkarey. “One site may serve to attract larvae from the open ocean, but may not be favourable enough to settle in. These tiny larvae actually made their way up and down multiple reef sites before finding a spot good enough to settle in.”
When all the fish larvae found spots, the women found a greater number of the three species on the east coast of Kadmat than on the west coast. They also discovered a few larvae of other species – goldring bristletooth, Indian Ocean mimic surgeonfish, powder-blue tang and brown tang – had colonised only the east coast. These were in the minority and the biologists couldn’t capture them in photographs at the time of murmuration.
In 1982, three scientists from the Central Marine Fisheries Research Institute Centre, Lakshadweep, saw a similar event more than 300 kilometres away in Minicoy. During the third week of September, they reported the colonisation of reefs by large shoals of the chocolate-coloured larvae of slender-toothed surgeonfish over a fortnight. Fish larvae find their way back to the reef where their parents spawned them. And they navigate by smell, sound, chemical plumes, and ocean currents. This sudden influx of fish larvae migrants threw life on Kadmat reefs off-kilter.
“Lagoon damselfish are herbivores and are the bullies of the reefs,” says Karkarey. They live in dead staghorn coral colonies overgrown with algae that they jealously guard against intruders. “God forbid, they will even attack you if you get too close!”
With hundreds of fish larvae swarming the reef, the normally aggressive damselfish couldn’t police their territories anymore. They gave the fish larvae a free pass through their algae gardens. Lined surgeonfish, another nibbler of algae, formed groups with other species to cope with the sudden competition for food.
Huge swarms of prey fish ought to be a bonanza for predators like peacock grouper. After all, surgeonfish are important herbivores in the reef ecosystem and a mainstay of predators. Biologists from Woods Hole Oceanic Institution, Massachusetts, US, say predators take a high toll of larvae. But Karkarey and Theo say the groupers were relaxed.
“The sudden hyper-abundance of prey in their territories caused their territorial and aggressive interactions with other species to completely cease,” says Karkarey. “They stopped patrolling or even swimming to other parts of their territory. Instead, they lined up along the corridors, just like a crowd of people who leave everything and come to the road to watch a carnival pass by.”
One aspect of her study is the diet of peacock groupers. Before this influx of fish larvae, the predators ate other fish. “But for nearly a month after the recruitment event, 90% of the individuals we caught only had surgeonfish in their guts,” says Karkarey.
Although aggregations of prey attract predators, it is to the advantage of vulnerable young fish. “It serves two purposes – confuses predators and overpowers algal competitors,” says Karkarey.
The authors wonder if corridors are as vital to reef fish as they are to the survival of land mammals. Are they used regularly? Do other species use the same network to travel between reefs? What do fish larvae seek in settlement sites? These questions are likely to remain unanswered for some time. Despite the greater number of divers, few witness these unpredictable marine migrations. Often, they occur after dark, when fish larvae are less likely to fall prey.
“Scientists know how pelagic larva find reefs, but don’t know how they find settlement sites once they get there,” write the authors. What little we know of such events comes from studies conducted after the larvae settled down or by recreating conditions in outdoor aquaria.
This murmuration of fish and their colonisation of the reef are not as novel for one community. “Local fishermen said that this event takes place every two years around the same time,” says Karkarey. “We sampled only Kadmat. But from speaking to fishermen from different islands, we realised that the recruitment event took place all over Lakshadweep. We had come across such high densities of surgeonfish larvae on reefs of Agatti island in March of 2012.”
These biologists are a part of a large team monitoring the reefs of Lakshadweep that suffered a severe bleaching event this summer. The arrival of the monsoon put a stop to the assessment, and the team expects to return in the coming weeks. Karkarey wonders if she’d find the reef off Kadmat still alive and what would become of the undersea invisible corridors should reefs bleach and degrade.
The study was published in the journal Frontiers in Ecology and the Environment on December 1, 2016.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.
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