Picky cleaning crew
They are considered tropical paradises, but their existence is threatened: Coral reefs. In particular, the increasing pollution of the sea is bad for them. Luckily, there are little helpers who can help keep things tidy.
Blue sea, white beach, shady palm trees swaying in the warm wind - the idea of the South Seas paradise is perfect. And if you dare to look under the water surface, you can enjoy the colorful beauty of the offshore coral reefs.
But the tropical paradises of the seas that line the equator around the globe are under threat. The increasing population on the coasts and the associated agricultural and industrial wastewater are taking their toll: worldwide, more and more coral reefs are fading and eventually dying. One environmental factor that particularly affects reefs is – Sand: Sediment envelops biological structures like a shroud, damaging delicate polyp stocks and depriving their symbiotic algae of vital light.
Although human activities have increased the sediment load in recent years, corals have always had to make sure that their homes don't get dirty. How do they do it? In search of an answer, Hannah Stewart and her colleagues at the University of California at Santa Barbara went to the Society Islands of French Polynesia, more specifically to the island of Moorea.
In fact, the researchers found what they were looking for during their dives in the South Seas paradise: small crustaceans of the species Trapezia serenei and Tetralia nigrolineata, which are only one centimeter wide and counted among the trapezoidal crabs, scurried back and forth between the hard coral stocks. Should these be the cleaners you're looking for?
An underwater field experiment provided clarity. The researchers removed the crabs in a limited area and waited. In fact, being cancer-free did not fare well with the coral stocks: Under a thicker layer of sediment, they grew more slowly, bleached faster - fifty to eighty percent of the stocks died within a month. In contrast, the cancerous control areas continued to do well.
Laboratory experiments confirmed the work commitment of the cleaning crews: "We know that they are very picky"
(Hannah Stewart) The crabs diligently collected sediment and preferred large grains of sand with a diameter of two to four millimeters. It is precisely these grains that damage the delicate coral tissue particularly badly.
"We don't know much about these crabs," explains Stewart. "But we know they are very 'fussy'. They fiddle with their mouthparts and scoop up the sediment."
And what is the reward for this effort? The corals probably provide their household helpers with shelter and protection - possibly worldwide. After all, crabs live on every coral reef on Earth, which might have been even worse off without a symbiotic partnership.
"This relationship is likely found throughout the Pacific and appears to be widespread," Stewart points out. "Crab species that live together with corals may play a much more important role than we previously thought."
