AT FIRST PASS, “CORALS EATING JELLYFISH” SOUNDS LIKE AN OXYMORON. But newly published research confirms that for at least one variety of stony corals, the concept of corals eating jellyfish is indeed a reality.
Moreover, these tiny colonial animals appear to work together to capture and devour the much larger jellies. It’s the first described case of “protocooperation” among corals, say the authors of the article published in the journal Ecology.
NORMALLY CARNIVORES, ANYWAY
Coral polyps are actually predators, even if their approach to predation is somewhat passive. After all, their core approach to earning their livings is to extend their tentacles into the water column to capture zooplankton moving past on the currents.
It should be noted that reef-building corals get some 80 percent of their nutrition not from grabbing plankton but from photosynthetic algae embedded in their tissues. That’s how they build coral reefs. But even there, some reef-building corals fight with each other for space, or at least for access to solar energy, trying to crowd each other out. Sometime they whip tentacles across each other to try to kill competing polyps.
A MEDITERRANEAN SPECIES
In the case of corals eating jellyfish, the corals in question are a stony cup coral species called Astroides calycularis. They’re found largely in the Mediterranean Sea around Sicily, west to the Straits of Gibraltar. Usually orange, they’re most often are found in caves and under ledges. They lack the embedded algae that power reef-building and are dependent entirely on their plankton-catching skills.
In appearance and size, they’re somewhat similar to more familiar orange cup corals, now found worldwide. My own unscientific observation is that such ahermatypic (non-reef building) corals compensate with fleshier bodies and tentacles for enhanced plankton-filtering purposes.
The jellyfish in question are “mauve stingers” (Pelagia noctiluca), a species found worldwide that grows to about 4 in/10 cm in diameter. Along with anemones, hydroids and other stinging creatures, corals and jellies are all members of Phylum Cnideria (“nettles”), which means that predator and prey are distant cousins. Which doesn’t deter either of them from attacking.
With tentacles extending as much as 10 ft/3 m, P. noctiluca can inflict painful stings. Sources note that they are the most common source of jelly stings in the Mediterranean.
Although they can achieve locomotion by pulsating, P. noctiluca are also captive to ocean currents. Which can be their undoing.
“The feeding behavior begins when a P. noctiluca jellyfish becomes trapped under an overhang with abundant A. calycularis,” the researchers say in their report in Ecology. “The pulsating swimming of the jellyfish moves the bell repeatedly against the overhang ceiling. Here A. calycularis polyps first adhere to the bell after which several polyps rapidly engulf the oral arms of the jellyfish, a process lasting between 1 and 5 minutes.
“Single polyps are able to ingest the oral arm tips preventing the jellyfish from escaping, while other polyps collaborate in ingesting pieces of jellyfish arms and umbrella,” they say. They note that a similar dynamic plays out when jellies are pushed by waves or currents onto colonies established on vertical walls.
They add that several colonies seem to take part in coordinated attacks – a fact that seriously impressed them.
OTHER CORALS EATING JELLYFISH
Mushroom corals elsewhere in the world have been observed capturing and devouring moon jellies, and anemones in Indonesia digesting jellies. But those are larger animals, not cup corals whose bodies are measured in single-digit millimeters.
And the most remarkable phenomenon is their apparent synchronization. This is the first described case of “protocooperation” among corals, they say.
PRINCIPAL SOURCES: “Protocooperation among small polyps allows the coral Astroides calycularis to prey on large jellyfish,” Luigi Musco, Tomás Vega Fernández, Erik Caroselli, John Murray Roberts, Fabio Badalamenti, Ecology; “Protocooperation turns small corals into jellyfish killers,” Stazione Zoologica Anton Dohrn Napoli; “Predatory corals team up to feed on jellfish,” The University of Edinburgh News Room; “Pelagia noctiluca,” Encyclopedia of Life; “Watch Corals Form a ‘Wall of Mouths’ to Catch and Eat Jellyfish,” Christi Wilcox, National Geographic.