Not far from the North Sea beach in Wilhelmshaven, a tropical underwater world opens up that probably only exists in Germany: countless corals, attached to small plates, frolic in several aquariums. Colorful fish swim through the mini coral gardens and individual anemones move their tentacles in the bubbling water.

Special LED lamps bathe the scenery in a bluish light; the air is humid and somewhat stuffy, 22 degrees. The water in the pools is still a little warmer. In the aquarium facility of the Institute for Marine Chemistry and Biology (ICBM) at the University of Oldenburg, biologists Samuel Nietzer and Mareen Möller have fine-tuned everything to the breeding of hard corals.

External influencing factors must be perfect

Scientists have been researching the propagation and breeding of young corals for ten years. Her focus is on the sexual reproduction of animals. The process that otherwise takes place in coral reefs in the sea is difficult to recreate in the laboratory.

“We have gained a lot of sensitivity as to when we have to do what,” says Möller. The researchers keep an eye on many influencing factors: temperature, day length and lunar cycle, water chemistry – everything has to be right for the corals to be stimulated to reproduce naturally under artificial conditions. As in nature, this only happens about once a year in the aquarium.

When spawning, the corals release eggs and sperm into the water at the same time as hermaphrodites. The moon is an important pacemaker: the corals can sense the rhythm of the full and new moon via light receptors, says Nietzer. Thousands of egg and sperm packets then rise to the surface of the water. The egg packets are then collected in the Wilhelmshaven aquariums in order to cross eggs and sperm from different colonies.

Coral trade has not yet been very sustainable

The researchers’ goal is to establish sustainable coral farming. “We want to make the aquarium market more sustainable,” says Möller. To this end, they have founded a start-up that is being funded by the Federal Ministry of Economics. If it is possible to breed corals on a larger scale using sexual reproduction in aquariums, the biologists believe that the coral trade could become less dependent on wild harvesting.

To date, many corals for the aquarium market have come from propagation through fragmentation, i.e. cutting a coral into several parts, and from wild collection from reefs. “That means divers go into the reefs, remove the corals and export them to Europe for the coral trade,” says Möller.

Reef restoration also possible?

According to the researchers, sexual reproduction could also help in the medium term with the reforestation of dead reefs. When asked, Laura Puk from the environmental protection organization WWF also confirmed the potential. “The method in general can certainly be used for reef restoration,” says the coral expert. It is even likely that the technology will be needed because tropical coral reefs are doing “pretty badly” as a result of the climate crisis. The increased water temperatures are deadly for the reefs.

In principle, corals could adapt to higher temperatures. “The problem is the speed at which the temperature is currently increasing. That’s too fast for coral reefs,” says Puk. Research is also being carried out to specifically reproduce heat-resistant corals sexually.