It was the 1970s. The first Earth Day had happened, and there were new federal laws and a federal agency to protect the environment. Despite these important advances, a legacy of pollution still lurked in streams, marshes and other waterways. Stimulated by reports of terrible effects of mercury pollution in Japan, marine biologist Judith S. Weis, her husband, biologist Peddrick Weis, and numerous graduate students at Rutgers University set out on what would become a three-decade journey to understand how mercury and other chemical pollution affects estuarine animals, including those we eat.
They found that some species can tolerate mercury pollution, but that wasn’t entirely good news.
Weis, who taught for four decades at Rutgers, told The Revelator about the evolution of their ground-breaking research, what initially stumped them, and why pollution can change animal behavior.
How did your experiment start?
We decided to look at effects of methylmercury — an especially toxic form of mercury — on the development mummichogs, a small marine killifish. These fish, a few inches long, live in tidal creeks of salt marshes.
Our initial experiments, done at a marine lab in Montauk, N.Y., treated fertilized eggs with different concentrations of methylmercury over their two weeks of development. When they were getting ready to hatch, we examined all the embryos and saw a surprisingly large variation in responses of embryos that had been in the same concentration.
Embryos showed a variety of deformities, including abnormal head-and eye development. There were also problems in heart and skeletal development, which also ran the gamut from mildly affected to severely messed up.
Seeing such a huge variation was puzzling. What could cause such differences in response to the same concentration?
We considered ditching the project since the results were incomprehensible but decided to try to figure out why.
For the next experiments, we separated eggs from different females into different containers to see if the females might produce eggs with different susceptibility. Bingo! The variation in responses was because each female consistently produced eggs with specific susceptibility. (The male didn’t matter.) Females that produced susceptible eggs had different genetic traits from those that produced tolerant eggs.
Where did you go next?
We wondered how fish from an environment that was polluted with mercury might respond, and went to the polluted Newark Bay, N.J., area. There had been a lot of heavy industry there for a century, long before any environmental laws prevented them from dumping their wastes in the marsh and the water, so the sediments in the bay and creeks were highly contaminated with mercury, lead, cadmium and many other pollutants.
We chose Piles Creek, a small dead-end creek that enters the Arthur Kill in Linden. The sediments in the creek were highly contaminated, and the level of mercury was particularly high.
When we repeated the same studies with fish from the creek, practically all produced embryos that showed only slightly abnormal development, an indication that the population was tolerant to methylmercury.
This was the first study showing evolution of pollution tolerance in an estuarine fish. One can imagine that this evolution would have happened quickly since there were already females in the clean site that produced tolerant embryos. However, we surprisingly found that larvae and adults weren’t tolerant to the mercury and furthermore showed signs of ill health, didn’t grow as well or live as long as fish from the clean site.
We investigated two other species in Piles Creek for methylmercury tolerance: grass shrimp and fiddler crab. Adults from Piles Creek and Long Island were examined for effects of methylmercury on limb regeneration and molting. In all cases, methylmercury slowed the rate of regeneration and delayed molting, but the Piles Creek crabs and shrimp were more tolerant — their regeneration and molting in methylmercury was not slowed down nearly as much as animals from the clean environment.
We found an interesting adaptation in fiddler crabs, especially from Piles Creek: They moved much of the mercury and lead from their internal organs into their shell (exoskeleton) shortly before molting it — a very efficient way of getting rid of contaminants quickly.
Is tolerance to pollution a good thing or a bad thing?
Well, it’s certainly good for the species that can achieve it, since it allows them to continue to live in a habitat that might otherwise be lethal.
Does that mean we can relax pollution laws? No! Not all species are able to evolve increased tolerance as these three did. Estuaries like Piles Creek and Newark Bay have fewer species than cleaner places. One commonly used measure of environmental health is biological diversity — how many different species live there. The more species, the healthier the environment. These three species found in Piles Creek are the survivors.
What other changes did you find?
Through an accidental observation by a graduate student that Piles Creek fish didn’t seem to catch shrimp well, we were able to find an explanation for their shorter life span and poor growth: abnormal behavior.
In lab experiments, unfed fish were put in tanks with grass shrimp (and a rock for hiding). Piles Creek killifish captured far fewer shrimp than the “clean” fish. If we put “clean fish” in tanks with Piles Creek food (shrimp) and sediments, within six weeks their prey capture ability declined to that of Piles Creek fish, showing that the environment is responsible for the impaired behavior. We examined stomach contents of fish from the field: The Piles Creek stomachs contained mostly detritus — decaying plant material — which was known to be non-nutritious for them. The poor ability to catch prey and their eating of non-nutritious detritus (“junk food”) could explain the poor growth and survival.
It’s not a big surprise that mercury would cause behavioral problems if you remember the Mad Hatter in Alice in Wonderland.
But that also had an effect on prey.
Grass shrimp in Piles Creek were overall larger, and more numerous, than shrimp from the “clean” site. Since their major predator, killifish, are ineffective predators and less abundant, Piles Creek shrimp experience reduced predation, so more of them can live a long happy life, resulting in larger size and greater population density. That’s an important finding because it shows the importance of “top down” effects — if your predator is affected worse than you from the pollution, you can benefit.
Piles Creek fish were also more vulnerable to predation. We examined how many fish were captured by blue crabs in the lab. Over two weeks crabs from a seafood store, kept in an aquarium with Piles Creek fish, captured far more of them than crabs kept with “clean” fish. The greater likelihood of Piles Creek fish to be captured and eaten could account for their shorter life span. Impaired prey capture and predator avoidance can result from being generally “slow,” which we confirmed by studying overall activity levels.
We also looked higher up the food chain and studied bluefish. They spawn in the ocean, and the juveniles move into estuaries in the spring to grow over the summer, then return to the ocean in the fall. We collected early juveniles from a clean site and raised them in large tanks, feeding them frozen killifish or menhaden from either clean or polluted estuaries. We found that initially both groups grew comparably, but those fed food from the polluted estuary gradually ate more slowly, ate less, swam more slowly, and grew less. By the fall, they were much smaller and lighter than those fed clean food. Many fish collected from the polluted site had empty stomachs — highly unusual for this species. This would put them at a disadvantage in the fall when they go back to the ocean.
We found a similar result in studying blue crabs. Those from the clean site caught more active prey than those from the polluted one, and “switching” their habitats changed their prey capture ability. Like the killifish, the crabs in the polluted environment ate a lot of detritus, surprising for a “carnivorous” crab. The behavior changes in these species show that the killifish impairments (reduced activity, poor prey capture) aren’t unique to them but are seen in other species, including ones that are commercially important.
What did we learn from all of this?
In the years since these studies were performed, scientists have studied killifish from other polluted areas and have found tolerance to other pollutants such as PCBs and dioxin. Also “behavioral toxicology” has become a recognized field, studied mainly in the lab on animals exposed to selected concentrations of a chosen toxic chemical.
Our studies were with animals exposed naturally to the contaminants in their environment and focused on predator/prey behavior that is ecologically important. These real-world findings show that animals in nature can have their behaviors affected in ways that make their lives more difficult and shorter, and that altered behavior can change ecological relationships in the system.
Overall, in our work, it appears that the crustaceans are managing better than the fish.
We learned two major lessons through this: If data don’t make sense, don’t give up but try to figure out why, and accidental observations can lead to a new fruitful direction of research.
Previously in The Revelator: