The Alliance Between Jellyfish And Baby Fish Fails The Acid Test

Between the tentacles and the bell, tiny fish find safety. James Brooks/University of Adelaide

At least 80 species of fish depend on jellyfish to survive while young, hiding from predators within their stinging tentacles. In acidic environments, however, this relationship breaks down. The researchers who discovered this don't know why this occurs, but it is further bad news for the future of the oceans.

The marine environment, like other ecosystems, depends on a complex web of interactions between different species. “But, apart from the well-known relationship between coral and microalgae and what happens during a bleaching event, little is known about how climate change and predicted ocean acidification will affect such relationships,” said Dr Ivan Nagelkerken, from the University of Adelaide, in a statement

Nagelkerken explored the way juvenile fish hide within the stinging tentacles of jellyfish, and how this changes when carbon dioxide concentrations rise, making waters more acidic. “This is the first study that demonstrates how climate change will disturb such a symbiotic relationship between two animals that interact closely for survival,” he said

In Proceedings of the Royal Society B, Nagelkerken reveals that 86 percent of juvenile yellowtail scad, mulloway, and barramundi placed in a tank with a jellyfish took refuge among the tentacles when the water matched the present-day oceans. This fell to 63 percent when the carbon dioxide concentration was raised to the levels predicted for 2100 under the “business as usual” scenario modeled by the IPCC.

Even more starkly, the fish only spent about a third as much time within the tentacles in the high-CO2 environment. In Nagelkerken's tanks, this did not affect survival, but in the open ocean this failure to take refuge could leave fish exposed to predators.

“Shelter is not widely available in open water so juvenile fish rely on the jellyfish for protection against predators,” said co-author Dr Kylie Pitt of Griffith University. “As shelter providers, the jellyfish could play a role in enhancing the populations of these fish species. Changing ocean conditions are likely to have significant negative impacts on this relationship and therefore fish populations.”

Although the reason for the jellyfish aversion in high-CO2 environments is unclear, Nagelkerken told IFLScience the effects of acidity on the fishes' smell and hearing may make it harder for them to dodge the stinging tentacles. Unlike Nemo in the anemone, these juveniles are not immune to the stingers' dose, instead slipping between them.

Nagelkerken told IFLScience the relationship does not appear truly symbiotic. The jellyfish occasionally eat the fish darting among their stingers, but otherwise seem to gain no advantage. On the other hand, the fish get protection and even compete with their hosts for plankton as food. Nevertheless, he added, “we don't really know because symbiosis is so complex and this relationship has been poorly studied. Sometimes there turns out to be an advantage that was not recognized before, such as the recent discovery that crabs living in corals protect their homes against browsing fish.”

Many of the fish that depend on jellyfish protection when young are commercially important as adults.

In the open ocean, a jellyfish can be the safest place to be. Alexius Sutandio/Shutterstock

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