The incredible vastness of the virosphere is hard to overstate. While several thousand kinds of virus have been studied in detail, scientists say we haven't even scratched the surface. There could be trillions of species overall, some think.
Against all this viral enormity, it's reassuring to remind ourselves of two encouraging constants, both of which are explored in a new study led by marine ecologist Jennifer Welsh from the Royal Netherlands Institute for Sea Research (NIOZ).
In short: not every virus infects every living thing, and some animals actually prey upon viruses, in a sense, by removing them from the environment. Despite this valuable and under-appreciated public service, much of what underlies the phenomenon remains a mystery.
"Viruses are the most abundant biological entities in marine environments, however, despite its potential ecological implications, little is known about virus removal by ambient non-host organisms," Welsh and her team explain in a new paper.
In a series of experiments in the lab, the researchers examined how a range of these non-host marine organisms fared at removing viral particles from their aquatic environment – either via active predation, or via passive mechanisms, such as filter feeders and organisms that create physical barriers between viral parasites and their hosts.
Of the 10 different animal species tested, crabs, cockles, oysters, and sponges turned out to be the most effective at reducing viral abundance.
"In our experiments, the sponges reduced the presence of viruses by up to 94 percent within three hours," Welsh explains, although after a full 24 hours, that figure reached even 98 percent virus removal.
"Another experiment showed that the uptake of viruses happens indeed very quickly and effectively. Even if we offered new viruses to the water every 20 minutes, the sponges remained tremendously effective in removing viruses."
In comparison to the sponges tested, crabs were the second most effective, reducing viral abundance by 90 percent over 24 hours, while cockles managed 43 percent, and oysters 12 percent.
Of course, these impressive results from lab experiments might not be equally successful in the wild, given the range of behavioural changes that can occur in bio-diverse aquatic environments, not to mention a host of other environmental variables at play under the sea.
"The situation there is much more complex, as many other animal species are present and influence one another," Welsh says.
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