An international team led by Oldenburg has studied tropical copepods in the surface film of the sea for the first time: Thanks to their blue colouring, the animals are apparently well adapted to strong sunlight and camouflaged from predators.
"Pretty blue" thought marine researchers Dr Janina Rahlff and Dr Mariana Ribas Ribas when they came across the three-millimetre-long copepods on an expedition in the tropical Pacific in November 2016. The aim of the scientists from the Institute of Biology and Chemistry of the Marine Environment (ICBM) and their German, US and British colleagues was actually to investigate the chemical and physical properties of the thin film at the boundary between the air and the sea surface. This nutrient-rich layer, which is only a few thousandths of a millimetre thick, forms primarily in calm weather conditions.
The special thing about this film is that the solar radiation is high and the temperature and salt content can fluctuate greatly. What these extreme conditions mean for the organisms in the top millimetre of the sea has hardly been investigated to date - partly because it is not easy to obtain samples from this thin layer. Thanks to a remote-controlled catamaran ("Sea Surface Scanner", S³) developed in the working group of ICBM scientist Dr Oliver Wurl, this is now possible. The expedition team was able to take samples from the upper thin layer of water and also from a depth of one metre. To their surprise, they came across the blue-coloured copepods from the pontellid family, reports Janina Rahlff: "In the thin surface layer, they were sometimes much more abundant than in one metre of water, even in high UV radiation." The researchers also observed that the richer the surface layer was in microalgae - the animals' main food source - the more frequently the small crustaceans appeared.
Copepods are found in all regions of the world's oceans and are an important part of the marine food web. At just over three millimetres, the specimens that the Oldenburg scientists found in the tropical Pacific northwest of the Bismarck Sea were comparatively large - and unusually coloured. The scientists suspect that the blue pigment, consisting of the carotenoid astaxanthin in combination with a protein, protects the animals from the strong UV radiation on the sea surface through its anti-oxidative properties. This is because the researchers sometimes found large quantities of copepods in the surface layer, regardless of the occasionally very high UV indices. In contrast to the results of other publications, the scientists were unable to find any increased mortality, such as would be detectable through larger quantities of empty chitinous shells or dead individuals.
Oceanographer Dr Christopher Zappa from Columbia University, New York, also used a new type of drone to examine the colour of the light reflected from the sea surface and from the blue copepods. This showed that the blue pigment of the small crustaceans mainly reflected the area of the light spectrum that corresponded to the light reflected from the water surface. This means that the animals are difficult to recognise at the air-sea boundary. The scientists assume that the copepods are thus camouflaged from predators. The crustaceans can also evade potential attackers by making short leaps into the air.
"Pontellid copepods can utilise the physically stable surface film in tropical oceans, which is rich in food organisms, thanks to a unique survival strategy," says Ribas Ribas. Unlike most other members of the zooplankton in their size class, which cannot benefit from the surface layer because they are highly visible and cannot withstand the strong UV radiation, the pontellids probably benefit from their pigmentation and adaptability. The researchers' findings, which have been published in the journal Scientific Reports, could help to understand how the small crustaceans adapt to changing environmental conditions in the oceans as a result of climate change.