Coral reefs not only suffer from the increasingly warm, acidic and polluted water of the oceans. The reefs are also increasingly being overgrown by seaweed and algae. But animal helpers could help: if you increase the density of certain crabs in the reefs, they will effectively graze the overgrowth, as a study has now shown. This also opens up new possibilities for the regeneration of dead coral reefs.
Coral reefs are the habitat of many animal species and are therefore largely responsible for the biodiversity of the oceans. But for a number of years now, scientists around the world have been observing increasing reef deaths. In addition to the pollution of the oceans, the causes are also the effects of climate change. They make the water of many marine areas warmer and this is why coral bleaching occurs again and again on many reefs, in which the organisms repel their algae symbionts. However, without these partners, most cnidarians can only survive for a short time.
Could crabs reduce seagrass growth?
Another problem is seaweed and algae, which are now overgrowing many coral reefs. Among other things, they are no longer adequately eaten by fish and other ocean dwellers due to the overfishing of the seas. Coral reefs in the Florida Keys are also affected by strong seagrass growth, as researchers working with Angelo Jason Spadaro from the College of the Florida Keys have observed over the past 30 years. They also noticed that the nocturnal Caribbean king crab (Maguimithrax spinosissimus), which has so far hardly been researched, eats large amounts of the seaweed – more than other Caribbean fish and invertebrate species. It also grazes on algae species that all other species avoid.
Theoretically, this crab would be a suitable antagonist against the overgrowth of the reefs. The problem, however: by nature, the density of king crabs is not sufficient to keep the seaweed under control. Spadaro and his colleagues have now investigated in more detail whether releasing additional crabs could reduce the growth of seaweed on the coral reefs and what other effects occur. To do this, they divided the reefs of the Florida Keys into twelve isolated areas. Some did not change them at all, in other places they released more than 80 king crabs and at the third location divers first removed the seaweed on the reefs and finally brought additional crabs there too.
Seagrass growth is reduced by up to 80 percent
It showed that the released crabs were actually able to reduce the growth of seaweed on the coral reefs within a year. While the unchanged reef was still 85 percent covered by seaweed one year after the start of the study, the vegetation on the other two test reefs sank significantly. The seagrass density at the second location was reduced to less than 50 percent due to the addition of the crabs. When the reefs were scrubbed first, the seagrass was even reduced by about 80 percent. “When Jason showed me the results, I couldn’t believe it – they looked too good,” says co-author Mark Butler of Florida International University.
“In order to assess the significance of these astonishing results, we carried out the study for another year on twelve additional reefs 13 kilometers from the first investigation site,” the researchers explain. And the repeated experiment also showed similar results after a year: the use of a larger number of king crabs alone reduced the growth by around 50 percent. Coupled with the previous mechanical removal of the growth, the density fell by 70 percent. The crabs also ate certain types of algae on the corals. As a result, the number and diversity of young corals and fish on the reefs had increased nearly five-fold after two years.
Restore reefs with the knowledge
“The experimental increase in the number of large native herbivorous crabs on coral reefs in the Florida Keys led to a rapid decline in algae growth and caused the return of small corals and fish to these reefs over the course of about a year,” says Butler. “This opens up a whole new way of restoring coral reefs.” Because this method complements the usual approach in which new coral fragments are planted in damaged reefs, according to Spadaro and his colleagues.
“But our results mean little if they don’t lead to tangible new recovery efforts,” says Butler. In addition to coral re-breeding, according to the scientists, breeding stations for the king crabs should now also be set up in order to receive more of these animal helpers.
Source: Cell Press, Article: Current Biology, doi: 10.1016 / j.cub.2020.10.097