That principle is true for humans as well as in the water-filtering services of the humble bivalves we call oysters—according to recent research involving the University of Maryland.1,2
Dr. Matthew Gray, an ecologist at the University of Maryland Center for Environmental Science, studied the water-filtering abilities of oysters. He showed how real-world conditions can handicap the ability of oysters to improve water quality, compared to oysters in idealized experimental conditions.1,2
Oysters feed on algae and other organic particles by pumping water through their gills. [Dr.] Gray said research he and others have done shows that an oyster’s filtration rate depends on a lot of environmental factors.1
Living in and below the tidal zone in coastal shores, oysters must be flexible to survive.
Oysters commonly reside in the shallow intertidal and subtidal zones where they are exposed to a wide range of environmental conditions … Although well adapted to the dynamic environmental conditions found in estuaries, oyster feeding rates and particle sorting efficiencies are sensitive to a variety of biological factors such as live algae composition and concentration…suspended organics and detritus…[as well as] environmental factors such as salinity, oxygen, pH, and nutrient concentrations….2
In other words, as they eat, oysters filter the waters that they live in. If those waters contain ideal conditions—such as temperature, salinity, dissolved oxygen levels, etc.—then the oysters can filter lots of water, every day.
However, if oysters live in less-than-ideal conditions, then they filter less water, and so add less improvement to their habitat.1 This is shown by quantitatively comparing experimental results from laboratory tests to observational data of what oysters do in the wild.
ICR has previously reported on bivalve-produced benefits of water quality improvement—removing picoplanktonic algae biomass from coastal waters of the Chesapeake Bay—to prevent “dead zones.”
Another example is found in Chesapeake Bay, which is burdened with excess nitrogen and organic nutrients that people release into its tributaries. The nitrogen compounds fuel picoplankton, which comprise ~15% of bay phytoplankton biomass during the summer. If left unchecked, their growth would lead to algal blooms that would block sunlight from submergent aquatic plants, leading to oxygen-depleted “dead zones.”3
The habitat heroes, who provide clean-up services to estuarial coast-waters, are oysters and mussels.
Thankfully, oyster reefs, bolstered by attached mussels, filter huge volumes of the bay’s water and consume the otherwise unrestrained picoplankton. This filtering ultimately benefits the dissolved oxygen and accessible underwater sunlight needs of the interactive Chesapeake Bay ecosystem.3
Laboratory experiments can facilitate interesting investigations, and may even yield informative data, but how much of those artificial scenarios are representative of real-world activities? With shellfish like oysters and mussels, as with other animals, field studies allow for data observed in the wild that is much more relevant (and realistic) for understanding how such creatures really live.4
But even heroes need a little help from friends—or from environmental conditions. That’s why oysters—if they are living in not-so-optimal conditions—don’t “clean up” estuarial waters as efficiently as they could if their living conditions were qualitatively improved.
Oysters are filter feeders that can help clean up the Chesapeake Bay, right? Many have seen the various web videos showing a dozen or so bivalves clearing a murky fish tank in just an hour. But are they such ecological superheroes that each one can siphon 50 gallons of water in a day? … 50 gallons of water in a day … “is about the near maximum rate at which the Eastern oyster will filter under laboratory conditions at optimum temperatures and very high-quality diets,” said Gray, whose specialty is the study of oysters, clams and mussels.1
But what about bivalves feeding in normal (natural) conditions? What about oysters that live in real bay-water? The water temperature makes a difference on how actively oysters filter-feed.
In reality, Gray said, under average conditions in the wild…“They don’t feed much at very low temperatures and get stressed out at super high temperatures,” he explained. They tend to be hungriest when the water is in a 10-degree range from the high 60s to high 70s Fahrenheit. Consequently, he pointed out, Bay oysters don’t eat or filter water year-round—not when a frigid winter sets in or when summer turns blistering.1
Also, what about the water’s saltiness? If the oyster-bed is subjected to a drop in salinity the oysters lose their appetite for filter-feeding.1 Likewise, what about water turbidity—clarity versus cloudiness?
Turbidity also can make a difference. While oysters can clear up cloudy water, Gray said that “if there’s a lot of sediment and dirt in the water column, they’ll spend more time sorting that than just ingesting it. And if it’s really, really bad they’ll just stop feeding. They’ll close up.”1
Also, the amount and type of available food makes a difference. Some algae are delicious and nutritious, yet other algae are undesirable or even harmful. Even oysters have taste sensitivities!
Furthermore, a relaxed oyster will filter-feed more than a frightened oyster. So, if predatory crabs are lurking nearby, oysters will defend themselves as bivalves do, by “clamming up” (closing their shells together), which necessarily interrupts an opportunity to filter-feed.1 Better to eat less plankton than to be eaten by a carnivorous crustacean!
Meanwhile, there is a lesson for us humans: notice that oysters nevertheless filter-feed at a less-than-optimal level—which is better than nothing! So they do contribute some improvement to water quality in their estuarial waters, even when doing so in less-than-ideal habitat conditions.1,3
Of course, ideal circumstances usually exist only in imaginations. Yet the lack of ideal circumstances is never an excuse to avoid doing what is feasible with whatever is available.5
May God give us wisdom and courage, to use whatever resources we now have, for that which is truly good.
Stage image: Oysters filter-feeding in the Chesapeake Bay.
Stage image credit: Dave Harp. Copyright © 2020. Adapted for use in accordance with federal copyright (fair use doctrine) law. Usage by ICR does not imply endorsement of copyright holders.
References
1. Wheeler, T. B. Pumped-up performance: Oysters’ filtering feat overstated. Fisheries News. Posted on BayJournal.com May 20, 2020, accessed June 2, 2020.
2. Quoting from Wang, L., J. Song, et al. 2020. Adaptive Feeding in the American Oyster Crassostrea virginica: Complex Impacts of Pulsatile Flow during Pseudofecal Ejection Events. Limnology and Oceanography. doi: 10.1002/lno.11433. See also Gray, M. W., P. zu Ermgassen, et al. 2019. Spatially Explicit Estimates of In Situ Filtration by Native Oysters to Augment Ecosystem Services during Restoration. Estuaries and Coasts. 42: 792-805.
3. Quoting Johnson, J. J. S. 2019. Termite Skyscrapers Hidden in Plain View. Acts & Facts. 48(4): 21. See also Gedan, K. B., L. Kellogg, and D. L. Breitburg, 2014. Accounting for Multiple Foundation Species in Oyster Reef Restoration Benefits. Restoration Ecology. 22 (4): 517-524; Pipkin, W. 2018. Freshwater Bivalves Flexing their Muscles as Water Filterers. Chesapeake Bay Journal. 28(7): 1.
4. Johnson, J. J. S. 2015. Crayfish, Caribou, and Scientific Evidence in the Wild. Acts & Facts. 44(6): 20.
5. Consider the example of the Old Testament champion named Shamgar, whose example is a true lesson about doing—with God’s grace—whatever we can, with whatever we have. Not having ideal weaponry Shamgar nonetheless used what he had, a farm tool (ox-goad), to accomplish a great military feat for God and God’s people. (Compare Judges 3:31 with 1 Samuel 13:19-22.) God blessed Shamgar’s resourcefulness and courage—by giving Shamgar (and his people) a providential deliverance.
*Dr. Johnson is Associate Professor of Apologetics and Chief Academic Officer at the Institute for Creation Research.
