This contribution if from Ellie Bors, a PhD student in the Joint Program in Oceanography between the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution. Her research topics include rapid range expansion on populations at the genomic level, climate-driven distributional shifts, invasive species, and deep-sea biology. You can learn more about her work and interests on her personal website.
Our contributor, Ellie Bors, a PhD student studying lionfish invasions
Lionfish are being served up hot on tables throughout the Caribbean and farther afield. But are lionfish speared by divers in Florida the same as those filleted in Aruba? This question is complicated. The answer depends on natural selection, random genetic changes (called genetic drift), and ocean currents.
Lionfish (profiled by invasivore here) were reported in Southern Florida in the late 1980s. Around 2000 the population exploded and spread up the US East Coast and then south into the Caribbean Sea and Gulf of Mexico. Along the way, lionfish have decimated native reef fish populations and unfortunately, no native predators seem excited to eat them. You can read more about the invasion online with the NOAA National Ocean Service lionfish fact sheet and check the invasion’s progress on this USGS website. My dissertation research focuses on how the process of invading a new region might alter lionfish genetics.
At the center of any species’ survival toolbox—dictating their ability to live certain places, eat certain prey items, and reproduce—are genes, or segments of DNA in present in every cell that help determine what that individual cell does and how it interacts with other cells in the organism. This is why I and other biologists obsess about genetic diversity, or the full array of genes that can influence species’ traits. Generally, higher levels of diversity allow populations of a given species to adapt to regional differences in environment, or to new and changing environments. For example, if a population has diversity in the genes that make individuals resistant to a disease, some individuals are likely to survive an epidemic, allowing that population to persist into the future. However, if all individuals have the same genes that leave them susceptible to the disease, the whole population could die off. Two of the forces that impact genetic diversity in all species are natural selection and genetic drift.
You’ve probably heard about natural selection—the idea that there are pressures in the environment that cause certain versions of a species (faster vs. slower; shorter vs. taller; seaweed-colored vs. sand-colored) to be more successful. Typically, selection is pushing genetic diversity in a specific direction. A group of scientists in Australia have shown how natural selection during invasion has caused invasive cane toads to have longer legs, move in straighter lines, and invade faster. This supports the idea that the act of invading actually makes toads at the edge of the invasion better at invading—a dangerous feedback loop!
The other side of the evolutionary coin is genetic drift. Genetic drift is the mischievous counterpart to selection. While selection acts to favor a specific gene at any given time, drift is random. Genetic drift happens because no population on the planet is infinite, and so the genetic makeup of that population will, necessarily, have some random fluctuations each generation. Imagine you have a bag of jellybeans with two flavors. If you pick out twenty jellybeans, you probably won’t get exactly ten of each flavor. If you do it again and again, you’re likely to have a slightly different proportion each time. That’s the same as random fluctuations in the genetic makeup of a population caused by drift.
Now put those twenty jelly beans on a table exactly one foot from the bag. Then randomly take ten of the twenty and put those jellybeans another foot away, and then randomly take five of those and put them yet another foot away. Your final jellybean population is now three feet from the bag and you’ve simulated a mini range expansion! It’s possible that the five jellybeans that are three feet from the bag are all the same flavor, showing that you’ve completely eliminated half of the jellybean diversity. That’s a big problem for the jellybean population, especially if it’s your favorite flavor. The same thing happens with an invading species.
So, what if your jellybeans were in the ocean and they were actually lionfish? Everything in the ocean happens in a fluid context. Currents act as conduits for dispersing animals’ eggs and larvae. In the case of an invasion, the currents could launch the invaders quickly between sites. For lionfish, we can see this when juvenile lionfish show up in New England during the summer (photographed preserved in the lab below). These fish do not survive the winter (thankfully!!) but they are carried to the shores of Long Island and Cape Cod in eddies known as warm core rings that pinch off of the Gulf Stream and come into shore. For lionfish, it’s possible that the complex currents of the Caribbean Sea and Gulf of Mexico could diminish some of the signals of genetic drift that we might expect to see. But it’s also possible that these currents could drive stronger genetic drift by connecting some islands and isolating others. On the US East Coast we think we see certain patterns of genetic diversity because of the strong Gulf Stream shooting the invaders up the coast.
Juvenile lionfish from the invasion edge
With both natural selection and genetic drift at play, and in a swirling sea of currents, it becomes a tricky job to predict how the invasion will impact the evolution of lionfish. But by looking into the patterns of genetic diversity, we will better understand which of the three forces is the most important in a marine invasion like this one.
So next time you contemplate devouring an invasive lionfish, think about where it came from and the three main forces that have shaped its identity. And stay tuned for the results of ongoing lionfish genetics research!