All over the world, people have been witnessing gigantic blooms of tens of thousands of jellyfish where once there were only a few. Fishers find them clogging their nets and costing them dearly. In Japan, giant jellyfish capable of reaching six feet across even capsized a boat that tried to bring them aboard. Where are these stinging menaces coming from and why are they everywhere?
Jellyfish explosions are often triggered by overfishing of small fish, like sardines and anchovies, which compete with jellyfish for food. Jellyfish are also more tolerant of low oxygen conditions than fishes, and so can survive in polluted “dead zones.” What’s more, once they become super-abundant, they snack on the larvae (babies) of fish, keeping fish numbers down. The result is an ecological flip-flop that is hard to reverse. Not surprisingly there is concern that when a huge jellyfish population moves in—there goes the neighborhood!
So, are jellyfish a dead end in the food chain? Fortunately, not always. Off the coast of southwestern Africa, where jellyfish have replaced sardines, at least one fish benefits. The five-inch long bearded goby spends its day on the seafloor in the oxygen-depleted and toxic mud where most fish, including their predators, cannot survive. While there, they graze on worms and mats of bacteria, but they can’t digest their food because they shut down the water flow over their gills to keep from getting poisoned. At night they come up to the surface for a breath of fresh air and a very jelly meal. The gobies are able to hide among the stinging tentacles of the jellyfish at the surface, which protect them from being eaten by larger fish and in addition provide about 60 percent of the gobies’ diet. This makes these gobies a keystone for the ecosystem; by chomping on jellyfish they create a link in the food chain between jellyfish and the seabirds, mammals and larger fish that eat the gobies.
While predators of the jellyfish are rare, they are not limited to the bearded goby. Over 120 species of fish and over 30 other kinds of animals feed at least occasionally on jellyfish. The heaviest fish, the ocean sunfish, and the largest marine turtle, the critically endangered leatherback, are jellyfish specialists. Mushroom corals have recently been seen feeding on jellies off the Red Sea coast of Eilat, Israel. They suck jellyfish over half their size through their mouths and digest them whole. During coral spawning in Panama, we spotted a crab feasting on a moon jelly.
As overfishing becomes the norm and jellyfish spread like weeds, one solution is to add humans to the list of major jellyfish predators. In fact, jellyfish have been eaten by the Chinese since 300 AD, and 425,000 tonnes (more than 900 million U.S. pounds) are already harvested each year, making their way into products from salads in Houston to ice creams in Japan. Perhaps we aren’t far away from packing peanut butter and jellyfish sandwiches for lunch (as long as you aren’t allergic to peanuts!).
Editors Note: This post was co-written with Amanda Feuerstein, program coordinator in the office of the Sant Chair for Marine Science. Dr. Nancy Knowlton is the Sant Chair for Marine Science at Smithsonian’s National Museum of Natural History. Their blog series is based on Dr. Knowlton's book Citizens of the Sea, which celebrates “the Wondrous Creatures from the Census of Marine Life.”