Parasites are organisms that live off of other organisms, generally harming them in the process. But this isn’t always the case. A new study found that rather than weakening their host, brine shrimp parasites actually help it cope with arsenic contamination in the water.

A male brine shrimp. Photo by djpmapleferryman.

The brine shrimp is a rather impressive creature. It has remained mostly unchanged since the Triassic period, over 200 million years ago. They have an unusual ability to survive in extremely salty waters (hence the name), up to up to 25% salinity. It also has the ability to produce dormant eggs, known as cysts and is highly resilient to a number of toxins. Artemia (the scientific name of the small crustacean) is especially resilient to arsenic, and biologists wanted to figure out how this works, and how having parasites affects this toxicity.

Marta Sánchez and Andy Green, both from the Estación Biológica de Doñana in Seville, Spain collected water and shrimps from a highly polluted estuary in Spain and determined their infection status with tapeworm larvae. During the first collection in April 2014, 98% of the shrimp were infected, about half of them with only one species of parasite. One month later, in May 2014, again 98% of Artemia were infected, most of them with more than one tapeworm species.

They then moved on and tested the samples for toxicity with arsenic. They were expecting to see a vulnerability in shrimp parasites, but they were surprised to observe the opposite. Infected shrimp were consistently more resistant to pollution than non infected ones. Parasite infection was also associated with significant changes in oxidative stress markers. This was equally valid for samples collected in the early and late spring, where the water was 4C hotter.

“[The study] provides the first empirical evidence that parasites can increase resistance to metal or metalloid pollution, rather than decrease it. It is also the first study to consider the influence of temperature change onparasite-pollutant interactions.” They add that the results “contradict the pre-existing view that pollution and parasitesare stressors that both have negative effects on the health of free living organisms”, and suggest that additional studies in other host-parasite systems are warranted to evaluate the broader relevance of these findings.