Is that salt marsh healthy? To answer this question, a biologist at the U.S. Geological Survey (USGS) and colleagues at the University of California (UC) are cracking open common marsh snails and counting parasitic worms. Their claim: the more parasites, the healthier the marsh.
While the parasite hypothesis may conflict with conventional ideas about infectious disease and human health (malaria, for example, is caused by a parasite), the worms the scientists are investigating aren't just any type of parasite.
For one thing, these worms, known as trematodes, must sequentially infect certain hosts to complete their lifecycle. Snails to crabs to birds might be a typical sequence for one species, snails to fish to birds for another. These trematodes also stand apart from other parasites in that they cause negligible disease for their highest trophic-level hosts, commonly birds. The worms' lifecycle thus typically begins in a snail and ends in a bird, with the intermediate host animals being the primary variables among worm species.
Intrigued by the prospects of developing a new tool for monitoring changes in wetland ecology, the National Oceanic and Atmospheric Administration (NOAA)'s California Sea Grant recently awarded support to parasite mavens Armand Kuris of the Department of Ecology, Evolution and Marine Biology at UC Santa Barbara and Kevin Lafferty of the USGS Biological Resources Discipline to collect California horn snails from more than 30 coastal salt marshes between Marin County and Imperial Beach at the United States-Mexican border.
"The horn snail is a mobile data recorder," Lafferty said. "It's a hub for more than 20 trematode species." If any one requisite intermediate host is missing, the parasite cannot reproduce and so will be underrepresented in the resident snail population, he explained.
A survey of the trematode population in resident snails thus becomes a clever means of reconstructing the food web in the area, because the trematodes reflect the predator-prey relationships that must be occurring to support their reproductive lifecycle.
"Trematodes require all of the pieces of the puzzle to complete their lifecycle," Lafferty said. "When we see a lot of parasites in an estuary, we know it's in good shape. For example, an estuary with high infection rates tells you that it's visited by many birds, and many types of birds."
The goal of the California Sea Grant project is to establish a baseline snail-trematode count in marshes, particularly those slated for restoration. By comparing worm statistics before and after a restoration project, which could include such activities as digging channels or removing nonnative plants, the biologists believe that wetlands managers will have a tool for gauging restoration success and its gaps.
If, for example, a certain trematode species is missing, it could indicate that its hosts lack appropriate habitats. "The trematode information provides a novel way to see what we need to alter to improve habitats," Kuris said.
The feasibility of the snail-as-datalogger idea was established at a case-study site at the Carpinteria salt marsh in Santa Barbara. There, the biologists showed that the trematode community did indeed become measurably more vibrant after restoration, because of an increase in the number of birds foraging on infected fish and benthic invertebrates.
To further validate the method, UCSB graduate student Ryan Hechinger conducted four 1-month-long bird surveys at the study site, using video cameras to capture images of as many birds as possible. The results proved encouraging: the video-based estimates of the bird community agree closely with those from the snail-trematode analysis. "The more birds there were at a site, the more parasites," Hechinger said. "The more types of birds, the more types of trematodes, just as we predicted."
Hechinger hopes to produce a sort of manual for resource managers that will explain how to collect snails, identify the trematodes inside them, and then translate those data into information about resident populations of birds, fishes, and benthic invertebrates.
"We think counting trematodes is an effective tool for assessing the biodiversity of salt marshes in California," Lafferty said. "We're interested in developing similar techniques for other ecosystems, such as coral reefs and kelp forests."
About the author: Article author Christina S. Johnson holds a bachelor's degree in mathematics from Washington University in St. Louis, Mo., and a master's degree in physical oceanography from Scripps Institution of Oceanography in La Jolla, Calif. She is currently a science writer at NOAA's California Sea Grant at Scripps Institution of Oceanography.
in this issue:
Biologists Count Parasites to Assess Health of Marsh