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Hundreds of thousands of crown-of-thorns starfish have invaded North Queensland, devastating reefs. iStock

Love connection: breakthrough fights crown-of-thorns starfish with pheromones

Crown-of-thorns starfish are one of the most aggressive reef-destroyers in the world. A single female can produce up to 120 million offspring in one spawning season, and these spiny invaders eat coral, weakening entire reef systems. They’re a serious problem in northern Queensland, and are likely to move south.

But after three years of work, my colleagues and I have made a discovery, published in Nature today, that could offer a whole new way to fight them: we have decoded the gene sequence for the crown-of-thorns’ pheromones, which prompt them to gather for mating.

The project was built on the premise that if we could tap into the communications systems of starfish, we could modify their behaviours, and then eventually set up a program to capture them.

The ultimate goal was to find a way to get the starfish to converge, so it’s possible to set traps and remove them from the reef. Currently, crown-of-thorns starfish are removed by divers, who either collect them by hand or inject them with toxic solutions. This is labour-intensive and deeply inefficient.

So how do we get them into one place? Well, we exploited their natural mating behaviour. Starfish, like a lot of other marine animals – including corals – release their eggs and sperm into the water, and fertilisation occurs externally. For starfish to do this successfully they need to form a tight cluster, so there’s a strong imperative gather in one spot, given the right stimulus.

Crown-of-thorn starfish grazing on healthy coral leaving behind dead white skeletons. Outbreaks of this starfish is one of the leading causes of coral reef destruction throughout the Indo-Pacific. Oceanwide Images

How do starfish communicate?

We thought if we could figure out how starfish know how to get together, we might be able to replicate it. To find out what was going on, we put a group of crown-of-thorns starfish in a large aquarium, and waited for them to aggregate. We then set up what’s called a choice experiment.

We used a Y-shaped maze, and put new starfish at the base of the Y. The two arms of the Y contained either fresh seawater, or water that had just passed over the aggregating starfish in the other aquarium.

As expected, fresh seawater had no effect. These starfish aren’t very active animals – they just sat there. But as soon as the water from the aquarium hit them, they became highly active and moved towards the source.

That told us immediately that the aggregating starfish had changed the chemistry of the seawater in a significant way.

The next step was to actually sequence the pheromone proteins in that seawater. We then mapped these sequences back to the genome, and identified the genes that encode the pheromones that are making the starfish do this.

The beauty of this whole process is that there’s a direct one-to-one relationship between the sequence of proteins that make up the pheromones, and the gene sequence. Because genes are a lot easier to analyse than proteins, we can then look at them in great detail, and use that information in future projects.

A crown-of-thorns starfish eating a brain coral. Australian Institute of Marine Science

Eco-friendly pest control

What’s particularly good about this result is that these pheromones are unique to the crown-of-thorns starfish. The genes that encode the proteins have evolved rapidly and recently, and aren’t shared by other species of starfish that we’ve looked at. It looks like each starfish has its own unique repertoire of pheromones.

This means that any attractants or bait we develop from this project will only be recognised by crown-of-thorns starfish, and won’t impact other species.

We look at this paper as phase one: the discovery of the communication pheromones. We’re now in phase two: trying to mimic those pheromones so we can develop baits for traps to remove the starfish from the reef before they reproduce.

Ultimately we’d like for fishers up and down the Queensland coast to be able to go out and fish them and make some money out of it. That could be through a bounty, or through developing some useful (or edible) product out of the starfish to sell.

We need a quicker way to remove crown-of-thorns starfish, and real incentive to get plenty of people involved. No-one knows how many there are around Australia, but there are some reefs in Queensland that have had hundreds of thousands, or even millions, removed by conservation projects. If we see those amounts on individual reefs, the true numbers across the Indo-Pacific ocean must be astronomical.

The final, most exciting aspect of this project is the possibility of wider applications. This approach hasn’t been used before in a marine environment, but it could potentially work for a wide range of invasive species. Pest organisms are a multibillion-dollar global problem – and this could mean we move beyond mitigating invasive species and actually start controlling them.

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