The proliferation of artificial structures in coastal regions may be contributing to an increase in jellyfish blooms in these areas, say scientists.
The international team of scientists led by Carlos Duarte, director of the Oceans Institute at the University of Western Australia, found that artificial structures provide the ideal habitat for the polyp stage of many jellyfish to flourish.
"We call this new proposition the 'Trojan Horse' hypothesis," said Duarte.
"The proliferation of artificial structures such as harbors, shipping facilities and aquaculture structures provides a habitat for jellyfish polyps and may be an important driver in explaining the global increase in jellyfish blooms," he said.
The findings have implications for coastal planning, with the need to design coastal structures, manage harbor environments and regulate garbage disposal to reduce suitable habitats for polyp growth.
The researchers surveyed coastal locations from around the world over 20 years and found jellyfish polyps thriving in their millions on the underside of structures such as piers, artificial reefs, buoys, and fish pens.
"Jellyfish polyps existed on the underside of such artificial structures at densities of more than 10,000 individuals per square meter, and sometimes up to 100,000 per square meter," said Duarte.
In some areas, years of surveys revealed no polyps until a new artificial structure was deployed, they report in the journal Frontiers in Ecology and the Environment .
"Polyps were found in high densities on artificial surfaces in harbors, suggesting they could be massive sites of potential release of ephyrae," the researchers say.
The researchers also conducted experiments on two types of jellyfish C. quinquecirrha found in Maryland's Chesapeake Bay, and C. tuerculata found in the Mediterranean sea.
They examined how larvae from these jellyfish settled on oyster shells, flagstones and 16 other surfaces including bricks, ropes, wood, concrete and plastic.
They found that settlement on artificial surfaces such as glass, plastic and bricks were higher than natural substances, particularly when the surfaces were placed close together to prevent attack from predators or in the dark.
The researchers say the "global ocean sprawl" of artificial structures is particularly critical in areas such as the Gulf of Mexico, southeast coast of South America and the Yellow and East China Sea, where natural hard surfaces are scarce.
The giant Nomura jellyfish (Nemopilema nomurai) in South East Asia has seen the most dramatic rise. While the habitat of these jellyfish is largely unknown, the area in which they inhabit is experiencing the fastest growth in aquaculture and shipping in the world.