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Lions of the Caribbean

 The red lionfish hides in plain sight using stripes and fins that disrupt the b

The red lionfish hides in plain sight using stripes and fins that disrupt the body shape.

By Oona Lönnstedt & Mark McCormick

Despite the extravagent appearance of red lionfish, these voracious carnivores are virtually undetectable by small prey and are causing massive problems in the Caribbean. So why aren’t they taking over the Great Barrier Reef?

Most of us have seen or at least heard of the movie Pirates of the Caribbean. In this film, a rugged gang of plundering pirates roam the waters of the Caribbean Sea. Luckily, Captain Jack Sparrow comes along and manages to put a halt to the dangerous crew of undead pirates.

Today, the Caribbean is facing an even greater threat than pirates. It is dealing with a seemingly unstoppable predatory fish originally from Australian waters.

Lionfish are native to the Indian and Pacific Oceans, where they roam the waters of coral and rocky reefs. During the day you often find them under a ledge or in a cave, but at dusk these predators come out of their hiding spots and swim the reefs in search of food, like hungry ghosts in the night. In the Pacific Ocean they are a natural and rather uncommon member of the coral reef communities, but in other parts of the world the story is very different.

The red lionfish was accidentally introduced into the Caribbean almost 30 years ago and, like cane toads in Australia, it is wreaking havoc in its new environment by eating its way through the ecosystem. The predatory success of lionfish in the Caribbean has long been a mystery to ecologists as populations in Australian waters are far more sustainable. Our research suggests that lionfish use a handy trick to be undetectable by their prey, becoming ultimate feeding machines in areas where they’re not wanted.

Lionfish have a very distinctive and rather extravagant appearance. Their look differs from most other fish in that they have long, extended fin spines covered in flaps of skin that flow around them like long banners. In addition to this, lionfish have several branched and fleshy protrusions around the head. They also have striking colour patterns in bands of red, white and orange broken up by bright white spots with mottled red and white fins. Studies suggest that this rather odd appearance has evolved to confuse and lure unassuming prey fish into their close vicinity.

Not only is the general outline of these fish confusing and strange, they also have a unique protective system: the adult fish have long, grooved fin spines with a venom gland in the tip of each groove. If a predatory fish or diver touches these spines, the gland breaks and a dangerous and sometimes even deadly venom is pushed into the offender. The pain it causes is excruciating and an excellent way to ward off attackers. Because of this, very few (if any) predators want to eat an adult lionfish.

Even though lionfish have been studied extensively in invaded regions, few researchers have studied them in their native waters. This is precisely what led us to Lizard Island, a small tropical island on the northern Great Barrier Reef. Here we wanted to investigate why lionfish are such seemingly excellent predators.

Normally, small reef fish are quite good at learning to avoid predators. When baby fish arrive at the reef after developing for a few weeks in the open ocean, they are met by a suite of hungry reef predators.

In order to survive the first few days after settling into their new habitat, fish have developed a neat trick called “associative learning”. Basically, the small prey fish learn who the dangerous predators are by coupling the smell of a wounded fish (bitten by a predator) with the smell or sight of that same predator. The next time they see that predator they know that it’s dangerous and that they should hide.

We set about finding several newly settled baby damselfish to use in our experiment, which is relatively easy as they settle in large numbers in the summer months.

However, finding lionfish on the Great Barrier Reef is not an easy task. They are nocturnal, and their cryptic colour and form makes them blend into the reef environment. When they are still and extend their pectoral fins they look just like a feather star and are easily missed by seeking eyes. With stubbornness and some serious willpower we set about diving for long hours, entering any caves we could find, looking in every nook and cranny in search of these mysterious creatures.

After many weeks of diving and searching we had enough lionfish to start our experiment. In addition to using red lionfish, we had also collected a smaller cousin of theirs, the zebra lionfish (which is more common and easier to find) and the very common rock cod.

We then taught baby damselfish that the different predators were a serious threat using associative learning. After teaching the little prey fish how dangerous these predators are, we examined how well they had learnt about the dangers the predator represented by examining their behavioural responses to them.

Surprisingly, our research found that red lionfish are largely invisible to the small fish that they eat. In fact, they are almost like ghosts as the small fish do not seem to be able to learn that they are a threat. They allow the lionfish to simply swim up to them, showing no signs of fear as the voracious predator approaches. This was very different from their response to the rock cod, which they learnt to hide from right away. Although they responded to zebra lionfish, it was not with the same intensity as to the rock cod. Clearly, there is something in the appearance of lionfish that label them as safe.

After we had looked at the fright responses of fish, we placed them together with each of the three predators in a mock reef environment. The research station is a great place as it has these really large flow-through sea water aquaria where you can build your own reef. Here you can control a variety of variables in a very natural environment, which is really useful tool for researchers.

The findings were baffling, as the lionfish had ludicrously high feeding rates and most of the young damselfish were gulped up in just a few hours. With the other predators the damselfish had better luck, as they had the good idea to hide in the coral as the predators approached. It was obvious that the predator training had been successful with the other species.

So what is it about red lionfish that makes them invisible to prey? No other species has been able to get around the sophisticated threat learning mechanisms that fish possess. One suggestion is that their unusual body features label them as a non-threat, and fish assume that they are just another part of the environment.

It’s quite amazing to think that Australia has a species naturally resident in its reefs that is such a ridiculously efficient predator. In the Caribbean they are a massive problem, some saying that these “Frankenfish” are the world’s worst marine invasion. So why aren’t they taking over the warm waters of Australia?

We believe that the integrity of the GBR is playing a big part in this, as this system hasn’t been put under the same level of human stress as many parts of the Caribbean. It’s plausible that the relatively intact nature of Australia’s tropical marine ecosystems puts a limit on important parts of the lionfish life cycle, such as larval and juvenile survival.

Why do we need to know this? The future of the world’s coral reefs depends on knowledge. Coral reef ecosystems all over the world are put under a huge amount of stress from various disturbances – from climate change to species invasions and pollution. The information scientists gather helps us to understand how the reef works, and studies like ours can hopefully help future protection of the reefs.

Oona Lönnstedt and Mark McCormick are from James Cook University, and carried out this research at Lizard Island Research Station.