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At first glance, Cassiopea jellyfish may seem like ridiculous failures. Unlike most jellyfish that swim with their bells pointed up, these so-called upside-down jellyfish spend most of their time with their bells resting on the seafloor of shallow, still coastal waters. There, they persistently pulse as if on a Sisyphean quest to burrow through the planet. Now, new research suggests these creatures’ flailing may play an important role in flooded mangrove forests.
The results of research published as a pre-print study show that the jellies’ pulsing causes a surprising amount of mixing in the water. An average-sized Cassiopea can send a water jet several meters upward, and at median densities on the seafloor, a group of jellies can mix a one-meter column of water every 15 minutes. Cassiopea move an order of magnitude more water than filter-feeding animals such as oysters and mussels, even when accounting for the size differences between species.
The research suggests these jellyfish may be indirectly reshaping their habitats through nutrient and gas mixing, making them ecosystem engineers similar to dam-building beavers or hole-drilling woodpeckers.
“It’s quite clear that there are no other species that create as much turbulence in the mangroves,” says Eric Wolanski, a coastal oceanographer at James Cook University in Australia who was not involved in the research.
Mangrove habitats provide crucial services along tropical coastlines, from flood control to acting as nurseries for young fish and crustaceans. Mangrove plants rely on access to nutrients and dissolved gases, and regular mixing from tides helps with this. But the role of mixing caused by animals is poorly understood.
Cassiopea jellies were a perfect candidate for studying animal-driven, or biogenic, mixing, as their ceaseless beating was already known to create an upward jet of water. To find out precisely how much water was being roiled, researchers at the University of South Florida studied the jellies in mangroves on Long Key, Florida.
There they recorded how densely the Cassiopea were covering the shallow bottom and measured the turbulence they produced. The team also took some jellies into the lab and placed them in an aquarium along with extremely small, reflective glass spheres. Shining lasers through the water and using a high-speed camera to film the light’s reflection off the spheres let them accurately measure the Cassiopea-generated jet.
Wolanski says that the presence of Cassiopea jellies in mangrove forests may also reduce the trees’ dependence on mangrove crabs. When some mangroves suck up water, they leave behind salt in the soil. As the crabs burrow, they churn up the salt so it can be flushed away with the tide. It’s possible the jellies’ pulsing could provide this service too, he says.
Still, says Rupesh Bhomia, a wetland biogeochemist at the University of Florida who also wasn’t involved in the study, tidal movement remains the major driver of mixing in mangrove habitats.
The “jellyfish’s contribution in mixing water is pretty small,” says Bhomia. Though localized impacts from the jellies could be substantial in flat, sheltered areas, he adds.
“In those pockets where the water column generally remains stagnant, having jellyfishes could help in mixing the water and improve the circulation of nutrients,” he says.
Bhomia points out that there may be species other than Cassiopea with overlooked ecological roles in mangrove forests.
“These are very diverse areas and we do not really understand a lot about this ecosystem. We have not studied each and every species and their important contributions.”