There’s something moving towards you in the water. You strain to get a better look but all you see are small white specks like dust drifting about. It’s the moment when you realize that it’s not dust that you also realize your life will never be the same.
The use of a microscope confirms it: the small white specks are circles that have whip-like tails, swimming not at random, but with purpose. In fact, these 1-mm-long wormlike creatures are known as cercariae and represent just one of the stages in the life cycle of a parasitic marine flatworm called a trematode. These exotic creatures are all the more mysterious because they spend almost their entire lives inside other animals; the cercaria stage is the free-swimming one where the worm must emerge from its initial snail host to find a new host to infect. In fact, so little is known about the species that we investigated the past week that it does not even have a species name, only the working name “angry fraggle”(large)!
Various trematode species found on Vancouver Island. How little we know about them is reflected in the lack of scientific names assigned. (Photo credits: Colin MacLeod, PhD)
Given how we value free will, one of the most fascinating aspects of parasites is their ability to modify hosts’ behaviours. For example, when similar parasitic flatworms invade cockles, they can hamper their ability to burrow into the sand which increases the chance they will be eaten by seabirds. This may be bad for the host, but it is beneficial to the parasite since they will then adopt the bird as their final host where they can mature and reproduce. (See diagram below) Interestingly, this is also a boon for a wide variety of marine life that can encrust the cockles’ exposed shells and thrive, increasing biodiversity in otherwise relatively barren mud flats.
An example of a trematode life cycle. (Photo credits: https://the-episiarch.deviantart.com/art/Digenean-trematode-life-cycle-197095162)
In the lab, we exposed cercariae to various kinds of seawater with modifications to reflect human impacts on the environment, for instance including heavy metals, raising the water temperature, and increasing the acidity of the water. Most notably, we found that heavy metal pollution did not affect their swimming speed, while increasing the nutrient content of the water, much like agricultural fertilizer runoff would, decreased their speed and increasing temperature made them swim faster. In fact, the most any individual swam in three minutes was 199 mm, or nearly 20 times their body length!
As the free-swimming stage, the cercariae are the ones that will be directly exposed to and acted on by the environment. Like the salmon we have here on the West Coast, they must swim enormous distances to find home again so they can reproduce. Seeing how they will respond to a climate change and human pollution could help predict how their hosts might respond to the changing world and how that will shape the diversity and health of our oceans. Simple yet cute, repulsive yet fascinating, these worms have certainly reminded me, like Master Yoda, that “size matters not” when considering impact.
For more cool information about wacky host-manipulating parasites, check out this site: