Corals can be aggressive

@Siouxsiew

I’ve recently become quite fascinated with corals. My interest was sparked after reading a scientific study about a coral bleaching event that happened in Ka¯ ne’ohe Bay, Hawai’i, in 2015. Dr Ty Roach and colleagues were trying to understand why some corals bleached during the event and others didn’t, in the hopes of identifying corals that could be used to regenerate damaged reefs.

While I’ve always thought coral reefs were beautiful and I’d read and heard about how toxins and high-water temperatures can cause them to bleach, I’d never really thought much about what corals were or how bleaching happened.

It turns out that what we know of as coral is the hard skeleton of tiny marine animals, made from the calcium carbonate they secrete. The animals themselves live in compact communities made up of genetically identical individuals called polyps. Each polyp is just a few millimetres in diameter and a few centimetres in height. It also has a mouth and tentacles with stinging cells they can use to capture plankton and small fish.

In places like the Great Barrier Reef, corals form a symbiotic partnership with photosynthetic algae. The algae supply the coral with much of their energy and nutrients and also give the coral their bright colour. Bleaching happens when stressors like high water temperatures and toxins cause the tiny animals to expel their colourful symbiotic algae.

Another study has shown they aren’t completely defenceless. Dione Deaker, a PhD student at the University of Sydney, and her supervisor Professor Maria Byrne and colleagues have found that coral can inflict damage on young Acanthaster planci that attack them. And sometimes that damage is even lethal.

A. planci is one of the largest starfish in the world. It’s more commonly known as the crown-of-thorns starfish for the venomous thorn-like spines that cover its upper surface.

A. planci are common off the coast of Australia and while they start their life feeding on algae, as young juveniles they switch to preying on coral. Alongside bleaching, outbreaks of

A. planci are a massive threat to the health of tropical coral reefs. To study what happens when young A. planci switch from feeding on algae to coral, the researchers reared 37 juvenile starfish in their lab and monitored their condition, growth, and survival. They found that coral polyps use the stinging cells on their tentacles to defend themselves against the young starfish, in some cases causing massive damage to their arms that for four poor starfish was lethal.

The starfish even had a reflex response to being stung, with their arms recoiling and twisting if they encountered a polyp.

What I find interesting is that the damage caused to the starfish’s arms delayed the starfish’s growth into adulthood. Which is good news for the coral as it extended the time the starfish spent feeding on algae instead.