Scientists call a previously completely unknown ecosystem in the island kingdom of the Maldives “Trapping Zone”: At a depth of 500 meters, the participants of the Nekton Maldives Mission discovered oases of life in the otherwise rather lifeless expanses of the Indian Ocean, report Alex Rogers from the University of Oxford and Co in a message. Large fish congregate here to feed on the immense amounts of zooplankton that accumulate daily on an underwater mountain.

These animals, called micronekton, are smaller fish, crabs and other organisms between 2 and 20 centimeters in size, which commute back and forth between greater water depths and the surface on a daily basis: a phenomenon that can even be detected acoustically. The micronekton rises at night and swims back down at dawn.

At a seamount called Satho Rahaa, cliffs and shallow terraces of former coral reefs prevent the animals from diving deeper than 500 meters and dispersing. The topography causes them to gather like a trap, which in turn attracts predators. Schools of tuna, sharks, John Dory, alfonsinos and other species, according to the divers’ observations, gather in large numbers and hunt for the micronekton. Above all, the great variety of different shark species amazed the researchers: in addition to hammerhead and tiger sharks, they discovered chin sharks, gill sharks, silky sharks and nail sharks.

However, the discovery raises some questions: Why, for example, do the micronekton animals not migrate beyond the terraces further into the deep sea, where they would be safer? Do other seamounts form similar traps? After all, these sunken mountains are generally considered to be species-rich ecosystems in the sea; Up to now it was assumed that this could be mainly due to the rise of nutrient-rich deep water, which is forced upwards at the obstacles.

The ascent and descent of the nekton, in turn, represents what is probably the largest animal migration on earth. The total weight of the organisms involved is estimated at at least ten billion tons. However, the phenomenon is largely unexplored.

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The original of this article “Hidden world discovered at a depth of 500 meters” comes from