You can swim, but you can’t hide – Sheltering behaviour in small-bodied reef fish following the death of corals

Picture your living room for a second – a comfortable couch, your favourite armchair and a nice cup of tea on the table. Imagine for a minute that one day you enter this favourite room, and something has changed – the whole room has been painted white. The chair, the sofa, the rug, all white! Although it still performs the same function, it feels fundamentally different? Most importantly, you feel different about the room, don’t you? It’s just not the same anymore!

Now, let’s snap back to real life and consider this reality for millions of fish on a coral reef during a coral bleaching event. These bleaching events, like those that occurred along the Great Barrier Reef for the past few summers, and other disturbances like outbreaks of the crown-of-thorns starfish, are becoming more and more common on the world’s coral reefs. For many small fish, their comfortable living room is a single coral colony where they shelter from predators between the branches of the coral.

When corals die, they quickly become overgrown by algae and settling organisms, such as sponges, which reduce the space available between branches. Eventually, the coral skeleton begins to crumble, and the structure provided by the corals is lost completely. 

It’s long been the belief that for small-bodied reef fishes, the physical structure that the branches provide is the key to survival. When the shelter space is lost, fish no longer have room to hide and are at higher risk from predators.

I wanted to explore this in detail, to figure out whether this loss of shelter space really was the driver behind fish declines following coral death or “mortality.” To do this, I conducted an experiment where I would scare small common reef fish, lemon damselfish (Pomacentrus moluccensis), and record their behaviour. I tested their sheltering behaviour on a series of coral colonies, from “live” to “completely dead and overgrown by algae.”

I expected that a majority of fish would hide between the branches when the coral colony was alive, and that less fish would have room to shelter as the branches became overgrown with algae and sponges.

Instead what I found was that most fish stopped hiding immediately following the death of “their” coral. At this stage, no space has been lost between the branches, so the fish would still have plenty of space to hide if they wanted to, but they are just not choosing to do so anymore.

What this suggests is that to the damselfishes, the living coral is vital in other ways than just providing structure.  

In the paper, I hypothesise that the live coral tissue sends out a chemical cue that informs the fish that “hey, this is a great place to hide,” and when this signal is lost, the fish no longer recognise the coral as a suitable habitat.

The study highlights the complexity of the relationship between living corals and reef inhabitants, and provides an insight into the processes that occur following disturbance events on coral reefs.

The paper “Loss of live coral compromises predator-avoidance behaviour in coral reef damselfish,” is published in Scientific Reports and now available online.