This article originally appeared on Undark.
The world’s aquatic habitats they are a heady mix of pollutants. It is estimated that 14 million tons of plastic enters the ocean as litter each year. Farther inland, more than 40 percent of the world’s rivers contain a human pharmacopoeia, including antidepressants and pain relievers. Heavy metals like mercury from industrial waste can also show up. And agricultural fertilizer can seep out of the soil into rivers and eventually find its way into the ocean.
There are an estimated 20,000 species of fish in the world, and possibly many more. They and many other organisms living in “contaminated systems are contaminated with a cocktail of chemicals,” said Michael Bertram, a behavioral ecologist at the Swedish University of Agricultural Studies.
Bertram and other researchers are increasingly discovering that these compounds can alter the behavior of fish. In some experiments, pollutants appear to alter the way fish socialize, either by exposing them to psychoactive drugs or by altering their natural development, which can change the way they swim together and mate. Others seem to make fish take more risks, which, in the wild, could increase the chances that predators will kill them unceremoniously.
The effects of contamination, according to researchers working in the field, still have many unknowns. This is partly due to the large number of variables in real ecosystems, which can limit scientists’ ability to infer how pollutants affect fish in the wild, said Quentin Petitjean, a postdoctoral researcher in environmental sciences at the Institut Sophia. Agrobiotech in France, and co-author of a 2020 paper that reviewed the existing literature on pollution and fish behavior. “In nature, fish and other organisms are exposed to a large number of stressors,” he said.
Still, these altered behaviors could have a big impact, according to Bertram. Like many living things, fish are important parts of their ecosystems, and changing their behavior could hinder or alter their functions in unexpected ways. For example, a study suggests that various chemical contaminants and microplastics may affect the boldness of fish species of prey. Although the authors note that this is unlikely to lead to population collapse, these “subtle behavior modifications” could reduce the biomass of the fish, alter their size, and ultimately harm predators as well. This effect alone, they add, “may be a hidden mechanism behind changes in ecosystem structure, in both freshwater and marine ecosystems.”
B.but humans have a fun way to show your appreciation. One example: people regularly dump psychoactive substances, which then find their way into aquatic ecosystems. In 2021, Bertram and a team of researchers published a paper investigating how a common antidepressant, fluoxetine, better known under the brand name Prozac, affected the propensity of guppies to swim in schools or in groups. Over two years, the team exposed groups of guppies to different concentrations of fluoxetine: a low concentration (commonly seen in nature), a high concentration (representative of an extremely polluted ecosystem), and no fluoxetine.
In the high exposure concentration, the guppies appeared to be more sociable and spent more time in schools. However, this was only the case in male-female pairs, not when the fish swam alone. Previous research by Bertram and his colleagues shows that the drug increases the amount of time male guppies spend chasing females. “Being intensely courted” by males, Bertram said, females will preferentially choose the largest school to distract them and “avoid this incessant mating behavior.”
While drugs like Prozac are designed to change brain function, there are other, perhaps less obvious ways that contamination can change behavior. For example, contaminants can alter the microbiome, the collection of microscopic organisms such as fungi and bacteria that exist on or within an organism. In humans, disruptions to microbial life have been linked to disorders such as autism spectrum disorder, dementia, or even simply cognitive decline. Research published in 2022 suggests that fish brains may also depend on the menagerie of tiny organisms.
In the study, the researchers worked with two groups of zebrafish embryos that they had rendered germ-free, functionally stripping them of microbes. In the containers containing a group of embryos, the team immediately introduced water from a tank with adult zebrafish to provide a microbiome for the disinfected population. After a week, they did the same with the other group.
After another week, the researchers ran a series of experiments, putting two fish from the same group in neighboring tanks to see if they would swim alongside each other, a previously identified schooling behavior.
Fish deprived of an early-life microbiome spent much less time doing this behavior than those in the control group. Of the 54 control fish, almost 80% spent their time near the divide between the tanks, compared to about 65% of the 67 in the other group. Exposure to microbes early in life is important for the development of social behavior, said Judith Eisen, a neuroscientist and one of the paper’s authors.
The researchers also looked at the fish’s brains using powerful microscopes. Normally, cells called microglia move from the gut to the brain early in a fish’s life, Eisen said, around the time their microbiome begins to develop. Fish that lived without microbiomes for a week, she and the team found, had less microglia in a particular region of the brain that has previously been linked to schooling behavior. In normal brains (including humans), these cells perform synaptic pruning, which removes the weakest or least used connections.
Of course, the germ-free state of those zebrafish, Eisen said, would not exist in nature. However, some human contaminants such as pesticides, microplastics, and metals such as cadmium appear to alter fish microbiomes. Given that schooling is often a protective behavior, a reduced response to schooling can cause problems in the wild. “If you don’t want to hang out with other fish, that could expose you to predation,” Eisen said.
Pollutants can affect behavior beyond banks and also saltwater ecosystems. In a 2020 study, researchers brought Ambon damselfish larvae into the lab and exposed some of them to microplastic beads. They then returned the fry to different stretches of the Great Barrier Reef—some degraded and others still healthy—and watched how they performed. The team also tagged the fish with tiny fluorescent tags and returned to the reef multiple times over three days to check their survival rate.
Fish that had been exposed to microplastics displayed more risky behavior and survived for a shorter time before being preyed on, according to the study. Almost all of the tagged fish that were exposed to microplastics and released onto reefs near dead died after about 50 hours. Meanwhile, about 70 percent of unexposed fish released near live reefs survived beyond 72 hours. According to the article, while the health of the reef was a factor in risk behavior, fish exposed to the plastics had a six times lower survival rate than those not exposed to the compounds.
According to Alexandra Gulizia, one of the authors of the article and Ph.D. student at James Cook University, more work is needed to investigate the components of plastics and how they affect fish. For example, bisphenol-A, more commonly known as BPA, is a common additive to make plastics more flexible. It also appears in natural habitats and research suggests it may decrease aggression in fish. Gulizia added: “I think we are only scratching the surface of the chemical impacts that microplastics have on fish and fish behaviour.”
How does this all play out? in nature it is difficult to assess. Eisen noted that other factors that could affect the microbiome include nutrients in the water, water temperature, diet, and salt concentration. Another, perhaps more direct, complication: The contaminants can appear simultaneously and in different amounts, Petitjean said. For example, a 2016 paper shows that 13 percent of 426 pollutants in European rivers have been shown to be neuroactive.
Another complication is simply that not all organisms will act in the same way, even within the same species. According to Eisen, model organisms such as zebrafish are chosen to represent a wide range of species, just as mice are often used to study human health in medical research. But changes in pollutants and other factors can differ from species to species. Bertram noted that using model organisms saves researchers the trouble of studying each species, but also that there should be more studies in different fish.
At first glance, some behavior changes may not even look too bad. Increased mating behavior, as in the case of guppies exposed to fluoxetine, might seem like a blessing for the species. However, one species that thrives on another tends to throw natural habitats out of balance, Bertram said. His previous work suggests that Prozac similarly increases the mating behavior of invasive eastern mosquitofish. This could help it thrive and outperform native species. Also, in some concentrations, cadmium can increase the activity of fish, potentially helping them find food. However, the more they eat, Petitjean said, the more exposed they could be to microplastics.
Given these circumstances, he added, experiments in the laboratory need to inject as much complexity as possible into their methods in order to best replicate real wild systems. Some research attempts this. Bertram’s work showed test guppies as a predatory or non-predatory fish of similar size before her experiments, while Gulizia and her team conducted parts of their experiment in the wild. Some studies also expose fish species to water drawn from the environment and accompanying contaminants.
Despite the unknowns, Bertram said changes in the way fish socialize, mate or find food are unlikely to be good. “At the end of the day,” he continued, “any change in the expression of natural behaviors will have negative and unintended consequences.”
This article was originally published on Undark. Read the original article.