Introduction
Lionfish are beautiful animals that are nonetheless decimating our habitats around the Caribbean and Gulf of Mexico. Native to the Indian and Pacific Oceans, these fish were likely brought over to the East Coast of the US in the aquarium trade. Since then, they have proliferated like crazy, outeating and outcompeting many of the native fish. Their ravenous appetite has worried many experts, who are most concerned about possible food web interactions. The lionfish has venomous spines, making it very difficult for predators to eat them, and so their populations have increased largely unchecked.
The lionfish is what scientists may call a “textbook example” of an invasive species: eats everything in its path but not eaten itself. But, what if the lionfish has another factor helping it to be such a “successful” invader? A team of researchers at the University of Alabama wanted to figure that out, so they turned to what seems like an unlikely source: the fish’s slimy mucous coating.
For everyone who has ever touched a fish, you know that it feels slippery and often slimy. That is no accident. Over time, fish have developed this mucous coating over their bodies to help them fight off pathogens. Bacteria will live in the fish’s slime even when the fish is healthy because it provides nutrients and a space for them to grow. Previous studies have reported that these external microbes help fish with disease resistance just like internal microbes help many animals (including humans – this is why people eat probiotic yogurt) with digestion. The bacteria in the mucus stop pathogens directly, by taking up space so that the pathogens can’t grow, and indirectly, by releasing compounds that inhibit the pathogens. Because there are so many different pathogens, a fish needs a dense and diverse community of these “bugs” to increase the likelihood that the fish will be protected from disease.
The researchers designed a study to look at the bacterial communities on lionfish in their native and invasive ranges. They wanted to find out what diseases the lionfish were immune from due to the presence of the bacteria, and if that was different in their home range and in their invasive range.
Methods
To accomplish that, the researchers had a glamorous task ahead of them – they had to collect some fish slime. They sent out divers to collect lionfish in the Caribbean (invasive range) and in the Indo-Pacific (native range). Those divers collected the lionfish, then swabbed their skin with a q-tip to pick up the slime. Then they brought the slimy q-tips back to the lab, where they isolated the bacteria and grew it on petri dishes. Once they had colonies of bacteria, they needed to grow colonies of disease. They grew petri dishes of six common fish diseases, then introduced small disks of the fish bacteria. From there, they could see clearly if the fish bacteria stopped the growth of the pathogens – after the incubation period, there would be rings around the fish slime where the pathogens didn’t grow (Figure 2).
Results and Importance
They found that 47.7% of the bacteria in fish slime from the lionfish in their native range stopped disease growth, compared to 31.4% of bacteria in fish slime from the fish in the invasive range. Those results make a lot of sense – the lionfish should be the best adapted to handle diseases when they’re in their home range. However, 31.4% is still a large percentage, and implies that the lionfish is still very efficient in fighting off disease in its new, invasive range. For comparison, the researchers also isolated the slime from a native Caribbean squirrelfish, and that slime bacteria only fought off 7.7% of the six common fish pathogens.
They also looked at the DNA of the bacteria to determine if the bacterial communities in the slime were made up of lots of different types, or if all the bacteria were closely related to each other. The antibacterial properties of these slime bacteria are extremely variable in their effectiveness, so the lionfish benefits by having more diverse bacterial groups living on it. The results from this study indicate that the bacteria in the slime are in fact genetically diverse, helping the lionfish avoid diseases more often than other fish. Combined with the bacterial growth results, the researchers concluded that the bacterial communities within the lionfish slime may be yet another factor (in addition to its voracious appetite and predator defenses) in helping it be such a successful invader. Guess we won’t be getting rid of these guys too easily!
Engage: Many invasive species start out as pets released into the wild. Encourage your friends and family not to release any pets (aquatic or otherwise) into the wild!
Hi and welcome to oceanbites! I recently finished my master’s degree at URI, focusing on lobsters and how they respond metabolically to ocean acidification projections. I did my undergrad at Boston University and majored in English and Marine Sciences – a weird combination, but a scientist also has to be a good writer! When I’m not researching, I’m cooking or going for a run or kicking butt at trivia competitions. Check me out on Twitter @glassysquid for more ocean and climate change related conversation!