Strong mussels

Filter-feeders can reduce transmission of trematode parasites

Most bivalves (mussels, clams, scallops, etc.) are filter feeders, meaning they obtain nutrients by filtering their food out of the water. They use a siphon to suck in water and use tiny hairs (cilia) to trap food particles before siphoning the water back into the environment. Some species can be prodigious feeders: a clam the size of your thumbnail can filter 6 L of water each hour.

The focus of this weeks’ blog is on a recent study that tested whether the natural feeding behaviour of the duck mussel (Anodonta anatine), a common freshwater species found in northern Europe and Asia, could help reduce the number of trematode parasites in water.

The trematode parasite that was used in this study is the eye fluke (Diplostomum pseudopathaceum). Infected freshwater snails (i.e. Lymnea stagnalis) can produce tens of thousands of free swimming larval cercariae which then infect fish. The parasite can infect a broad range of fish species; the parasite migrates to the eye and develop into metacercariae. The life cycle is completed when a bird consumes an infected fish and the parasites develop into adults and reproduce sexually in the intestine (Figure 1).

Figure 1. Life cycle of Diplostomum parasites.

Gopko and colleagues first tested whether mussels would reduce the number of free swimming cercariae in aquarium. They measured the number of free swimming cercariae after 2 hours in an aquarium with mussels. Originally, there were 6000 cercariae/L in each container. Mussels were left in the containers for 2 hours and the remaining cercariae numbers were estimated from 3 5-ml aliquots. In all cases, mussels significantly reduced the amount of free swimming larvae over 2 hours (on average 4- fold decrease). It is not known if this is due to ingestion or if the mussels were mechanically damaging the cercariae.

Next, the authors looked to see if the mussels could affect transmission to rainbow trout. Trout, with and without mussels were placed into tanks with three different densities of cercariae. At all three densities; there was lower infection rates of eye flukes on fish in tanks with mussels present (Figure 2).

Figure 2. Declines in infection intensity of fish in the presence of mussels (Anodonta) across high, medium, and low densities of a trematode cercariae. Controls are fish in a tank without mussels present. More details can be found from the paper.
from Freshwater mussels (Anodonta anatina) reduce transmission of a common fish trematode (eye fluke, Diplostomum pseudospathaceum)


What does this mean for the future of bivalves as disease control agents?

These results support several other studies that have shown that various species of bivalves can filter out bacteria, viruses and even parasites in a range of aquatic environments. Fewer studies have demonstrated that filtering can inactivate parasites, reducing onward transmission.  The recent study described here supports this body of literature and expands the number of systems where bivalves can help reduce parasite transmission.

The impressive filtering abilities of bivalves has not gone unused over the years.  They have been used to measure bioaccumulation at local sites. The most famous example is the Mussel Watch Programme that started using mussels and oysters along coasts and lakes in the US in 1986 to monitor accumulation of pollutants. They are now used worldwide for monitoring metal and pollutant accumulation. Maybe next on the list is reducing disease?

This study, and others, point to a role for bivalves in controlling disease, especially amongst high density transmission settings like commercial fish farms. However, bivalves can also contribute to disease transmission; many species act as hosts for other pathogens and parasites. Before bivalves are considered for disease control, it will be important to understand the biology of the particular species and potential diseases it could carry or its potential to become invasive before being used for control of eye flukes or other diseases.

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