Corals combine forces

They look peaceful, but these colourful sea creatures demonstrate deadly teamwork

Always staying in the same spot, wafting gently in the current, corals are often mistaken for plants. Their calcified bodies form stunning reefs, but while they’re still alive these stationary invertebrates need to eat just like any other animal.

Most corals catch zooplankton – tiny drifting animals – with their tentacles, and many have photosynthetic algae called zooxanthellae living within the safety of their tissues in exchange for some of the energy they produce.

Smashing the illusion of corals as peaceful ocean decorations, scientists have discovered the dark secret of one species. Astroides calycularis lives on rocks in caves in the Mediterranean Sea, where it’s easily spotted thanks to its bright orange colouration.

Ocean currents sometimes pull other animals into the caves, including Pelagia noctiluca – the mauve stinger. This jellyfish is the culprit behind many of the painful stings experienced by swimmers and beachgoers in the Mediterranean, as its tentacles contain stinging cells with a powerful toxin. It’s far too big to be caught by a single coral polyp, but meets its match in a determined and hungry Astroides calycularis community.

When a mauve stinger gets swept under an overhang of rock, its pulsing swimming movement brings it into contact with the orange coral. Acting like one organism with multiple mouths, the polyps attack the four long arms around the jellyfish’s mouth and begin to eat them. The more the victim moves in its efforts to escape, the more it drags past waiting polyps, and within a few minutes its other arms and bell are being chewed too.

While the thought of mobs of predatory corals might sound creepy, the scientists believe their discovery is a real eye-opener. If we’re still learning new things about species that don’t even go anywhere, how many surprises must there still be in the oceans?