"Will jellyfish take over the world?"

From Mongabay:

A ‘monoculture of jellyfish’ threatens the oceans as we know them.

It could be a plot of a (bad) science-fiction film: a man-made disaster creates spawns of millions upon millions of jellyfish which rapidly take over the ocean. Humans, starving for mahi-mahi and Chilean seabass, turn to jellyfish, which becomes the new tuna (after the tuna fishery has collapsed, of course). Fish sticks become jelly-sticks, and fish-and-chips becomes jelly-and-chips. The sci-fi film could end with the ominous image of a jellyfish evolving terrestrial limbs and pulling itself onto land—readying itself for a new conquest.

While this scenario sounds ridiculous, all of it—except the last sentence, of course—could conceivably come to pass. Dr. Anthony Richardson calls this the ‘jellyfish joyride’ and it is already happening in parts of the ocean: diverse fish populations are being replaced by jellyfish.

"Dense jellyfish aggregations can be a natural feature of healthy ocean ecosystems, but a clear picture is now emerging of more severe and frequent jellyfish outbreaks worldwide,” Richardson, from the University of Queensland and CSIRO Climate Adaptation Flagship, explains. "In recent years, jellyfish blooms have been recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and particularly in Far East coastal waters.”

Once jellyfish gain a foothold, Richardson says that if conditions are right they can establish a massive population at the expense of other ocean life: “the problem is that jellyfish might form an alternative ‘stable state’. What this means is that parts of the ocean might switch from being dominated by fish to being dominated by jellyfish.”

In a new study appearing in Trends in Ecology and Evolution, Richardson and colleagues explore the causes behind the jellyfish infestation and the need for swift, decisive action to stem the jellyfish take-over. Jellyfish explosions are linked directly to human actions, including over-fishing, the input of fertilizer and sewage into the ocean, and climate change.

Overfishing has removed fish from marine ecosystems at astounding rates. According to Richardson this has opened the door for jellyfish to take their place: “this is because small fish (e.g. anchovy, sardine, herring) appear to keep jellyfish in check by predation (on jellyfish when they are very small) and competition (for the same zooplankton food). So, once we remove fish, jellyfish can proliferate.”

As an example Richardson points to Namibia where "intense fishing has decimated sardine stocks and jellyfish have replaced them as the dominant species.”

Eutrophication is another human-caused change in the ocean that has likely contributed to jellyfish explosions. Eutrophication is an increase of nitrogen and phosphorous in the ocean, largely caused by fertilizer and waste runoff seeping into the oceans. This leads to algae blooms, which lower oxygen in the marine ecosystem creating so-called ‘dead zones’, which have been increasing dramatically around the world.

According to Richardson, these low-oxygen waters give jellyfish the advantage: “fish avoid low oxygen water but jellyfish, having lower oxygen demands, not only survive but can thrive in these conditions as there is less predation and competition from fish.”

Furthermore, Richardson and his colleague speculate that climate change may expand the traditional ranges of jellyfish at the expense of other marine species. “As water warms, tropical species are moving towards the Poles. This has been documented on land and in the sea. Many venomous jellyfish species are tropical (e.g. box jellyfish and irukandji) and…could move south into more densely populated subtropical and temperate regions,” Richardson says.

As an example the paper points to box jellyfish and the incredibly small irukandi in Australia. These fatal species often cause beach closures in their native northeast Australia, and there is a concern that as the water warms they will make their way to more populous southern Australia.

Once jellyfish appear en masse in an ecosystem they can make it very difficult for fish to stage a come-back. By feeding on fish eggs and larvae in addition to competing with fish population for zooplankton, the jellyfish successfully “suppress fish from returning to their normal population numbers,” says Richardson. “One can thus think of two alternate states with each being stable: one dominated by fish and the other by jellyfish. Unfortunately, when there is a jellyfish dominated state then this does not support the higher trophic levels of other fish, marine mammals, and seabirds.” In other words an ecosystem that loses fish also loses the species that depend on fish for survival.

The study describes this state as a “monoculture of jellyfish”: an apt analogy since the situation shares similarities with other monocultures. When the rich biodiversity of tropical forests is replaced by a plantation growing a single species of tree, an area of rich variety becomes a desert in terms of biodiversity, as do ocean ecosystems when jellyfish become the dominant species...

Certainly all of these recommendations would aid marine biodiversity and ocean productivity in other ways in addition to stemming the jellyfish take-over. If not tackled, a future ocean of jellyfish could have dire economic, social, and, of course, ecological repercussions.

While jellyfish are edible, it is doubtful that they could serve as rich—or as diverse—a food source as marine fish. Richardson, who has tried jellyfish says “the best types are slightly crunchy. Not a strong taste and usually had with a sauce. Excellent diet food, as it has virtually no calories!”