Be glad you’re not a snail – tiny parasites could be munching on your genitals.
Or, say, a fish, whose brain is being controlled by parasites, forcing it to float up to the surface and become a target for hungry birds.
Or the unlucky bird that eats the fish, dissolving the brain cysts in which these parasites live and allowing free parasites to roam inside its body.
Believe it or not, these parasites could actually be helping the snail, fish and bird rather than hurting them.
Parasitic trematodes, or roundworms, that follow this lifecycle in the salt marshes of California and beyond are abundant, yet little is known about what function, if any, these parasites are serving.
In hopes of answering this question, UCSB’s Marine Science Institute was recently awarded a grant of $2.2 million from the National Science Foundation.
The parasite study is being conducted by three principal co-investigators, as well as a team of graduate students assisted by four or five undergraduates each. According to Kevin Lafferty, a former professor of parasitology and invertebrate zoology at UCSB and one of the principal researchers on this project, the goal is to figure out the role that parasites play in ecosystems. Are the parasites affecting the lifecycles in these habitats? And, what is the correlation between parasites and a healthy ecosystem?
Despite the prevalence of parasites in the world – there are more parasitic species than there are all other free-living species combined – not much research has been done on exactly what these creatures are doing. To answer this question, researchers on the project are studying species of trematodes in salt marshes of Santa Barbara and surrounding areas such as Carpinteria and Morro Bay.
“In the salt marshes which serve as a model for any other ecosystem … it’s clear that parasites can be playing a pretty big role,” Lafferty said.
Because field experiments provide a more realistic and relevant picture of the ecosystem than a lab could, almost all of the studies thus far have been conducted in these natural habitats. The grant will allow researchers to carry out their experiments in regions outside of California, for longer periods of time and with the help of many more students.
Snails
In salt marshes, the trematode has a complex lifecycle traveling from birds to snails to fish and then back to birds again. This starts when eggs of the parasite come out in the feces of the bird and end up on the mudflat. At this point, either a snail can eat the egg or the parasite can hatch out into the mud, swim around, find a snail and penetrate through the snail’s foot. An infected snail can live an apparently normal life for around 15 years.
What the parasite does, however, is consume every bit of reproductive tissue in the snail, so that what looks and acts like a snail produces parasitic offspring. If parasites are common in that particular ecosystem, about half of the snails will be infected.
“The nematodes have a great opportunity to impact the host population because only half of the snails are reproducing,” Lafferty said.
Without actually killing the snails, parasites effectively reduce their population density in the mudflats. Once in the snail, the trematode matures into a free-swimming stage and these independent parasites soon become the dominant organisms in the water.
“Parasites take the energy that the snail would normally be using to produce baby snails and they put it out into the water column where a whole different type of food web is supported,” Lafferty says. These free-swimmers are a major source of food, and in this way benefit many types of animals.
Fish
Once in the water, the parasite drops its tail and prepares to infect its next host, the fish. The trematodes migrate in through the gills of passing fish and eventually end up on the brain of the animal where they form cysts. In the fish, the parasite cannot reach full maturity, so it waits for the fish to be eaten by a bird, which is its next and final host.
“The cool thing that happens in the fish is that the parasites aren’t patient waiters,” Lafferty said. “They are sitting on the brain and they release what we think is probably some sort of chemical that alters the behavior of the fish, and makes the fish swim up to the surface and turn on its side.”
The fish have bright sides as compared to the rest of their bodies, and this turning causes a flash of color under the water, making the fish more easily seen by birds.
Dr. Lafferty and his researchers proved this by placing infected and uninfected fish in small areas where it was easy for birds to prey on them. They watched the fish, and noticed that the infected ones behaved differently than the others.
“The parasitized ones were 30 times more likely to be eaten,” he said. “The uninfected guys hardly ever got eaten.”
Birds
When a bird eats the fish, the acid in the bird’s stomach dissolves the cysts containing the parasites. In the bird, the parasite can reach maturity and lays eggs that are released in the bird’s feces. The lifecycle is now complete.
If any one of the parasite’s three hosts – the bird, the snail or the fish – is missing in an ecosystem, the parasite cannot replicate. With all the affects that humans are having on nature today, it is easy to imagine this happening. The researchers believe that the healthier and more intact an ecosystem, the more parasites there will be living in it.
Another hypothesis of the study, based on this information, is that parasites can be used as indicators of ecosystem health.
“We can essentially pick up a pocketful of snails, bring them back to the laboratory, see how many are parasitized and that should give us a good understanding of how that wetland is functioning,” Lafferty said.
The theory has, in part, been proven through research in the Carpinteria Salt Marsh, a UCSB Natural Reserve. Snails from an area of this marsh that had been degraded carried about half as many parasites as snails in a healthy part of the marsh. In addition to this, once the degraded area had been restored, the number of parasites in that area rose to the same levels as the number in the intact part of the marsh.
“Degradation and restoration of an area changes the parasite population in a manner consistent with the prediction that parasites are associated with healthy ecosystems,” Lafferty said.
The research the scientists are conducting is a lot less about proving things than it is discovering things, Lafferty said. They are interested in learning more about how the parasites are able to affect the behavior of the fish, if the parasites affect just this particular species of fish, and what effect the parasites have on the functioning of the ecosystem.
According to Lafferty, the biggest questions are whether or not the parasites play a role in the larger scale of the ecosystem and whether or not they increase the functioning of the ecosystem.
Without parasites affecting the behavior of the fish in the salt marshes, it might be more difficult for birds to catch these fish. Because birds are of concern to conservationists, it is important that the wetlands support healthy numbers of birds.
“The irony is that to support healthy bird populations, maybe [the birds] need to be infected with parasites,” Lafferty said.
Research on this project will continue over the next few years. Dr. Lafferty will be heading the project along with fellow UCSB professor Armand Kuris and Princeton professor Andrew P. Dobson. The three professors will be assisted by post doctorates, graduate students and undergraduates.
