Parasites help brine shrimp survive toxic waters

Being infected with a parasite is usually not good news. Some of the critters can make you sick, and some will eventually kill you. And studies have found that when an animal has to deal with both a parasite and pollutants such as toxic heavy metals, the stressors add up.

But that isn’t true for Artemia brine shrimp in Spain, a new study finds. Infection with cestodes — a type of parasitic flatworm also known as tapeworms — results in an increased ability to survive in waters laced with toxic arsenic.
Marta Sánchez of the Spanish National Research Council in Seville has been studying the role of parasites in ecosystems. She and her colleagues were curious about the brine shrimp because they are key players in the ecosystem; they are eaten by many water birds, including flamingos, and can ferry pollutants and parasites into the birds. “Infected [brine shrimp] are more susceptible to predation by birds,” Sánchez notes, which contributes to pollutant levels in the birds.

When brine shrimp become infected with parasites, they turn red. This makes them especially attractive to birds not only because they are easier to see but also because birds are on the lookout for the carotenoid pigments responsible for the color. These pigments not only give a bird’s feathers their colors but they are also necessary for a healthy bird. “As birds cannot synthesize these pigments and they are a scarce resource in nature, they selectively search for them in their diet,” Sánchez says.
The red color also makes it easy for scientists to pick out infected brine shrimp. Sánchez and her colleagues collected brine shrimp from southwestern Spain’s Odiel and Tinto estuary, which is tainted with arsenic and other heavy metals from current and past mining activities. In the lab, the researchers separated the parasite-infected and uninfected brine shrimp and then ran tests to see how well they survived in arsenic-laced waters.

As the concentration of arsenic in the water increased, so did the number of brine shrimp that died. But more brine shrimp that were infected with cestodes survived than uninfected ones, the team reports March 3 in PLOS Pathogens. Then, curious about the effects of climate change, the researchers repeated their experiment with warmer water. Again, the parasites appeared to confer some level of protection to the brine shrimp.
It may not be obvious, but causing a quick death is not a good strategy for a parasite. That’s because a parasite needs its host to stay alive long enough for the parasite to reproduce, leave and find a new host. If the host dies too quickly, then the parasite dies with it. So for cestodes, giving brine shrimp some help in surviving polluted waters may be in the parasites’ best interest.

The results suggests that the cestodes help change the way the brine shrimp deal with pollutants and the resulting stress. When the researchers compared infected and uninfected brine shrimp, they found differences in the antioxidant defenses that protect an organism against the damaging effects of reactive oxygen species. “Infected individuals were better than uninfected individuals at coping under polluted conditions,” Sánchez says. Also, infected brine shrimp had higher amounts of lipid droplets that are thought to sequester toxins.

The researchers can’t say whether this beneficial relationship is restricted to this particular estuary in Spain. “What we can say,” Sánchez says, “is that the red coloration associated with tapeworm infections is something we have observed in many sites in different countries. Hence, we expect our results do represent what would be recorded at other localities.”