How Do Oysters Filter Water? Feeding by Straining Everything Through

The gills do two jobs at once, extracting oxygen and capturing food. They are covered in cilia, microscopic hair-like structures that beat in coordinated waves and drive a current of water in through the inhalant opening, across the gill surface and out again. Particles in that water are caught on a sheet of mucus covering the gill, and further cilia move the loaded mucus along grooves towards the labial palps beside the mouth. The palps then sort: acceptable particles are passed to the mouth and the rest are rejected and ejected as pseudofaeces, material that was never eaten but has been bound in mucus and deposited on the bottom. That sorting matters ecologically, because it means an oyster removes suspended sediment from the water column and deposits it on the seabed regardless of whether it eats it, which clarifies the water and transfers nutrients from the water to the sediment, where they become available to other organisms.

The effects of a large filtering population compound and are measurable:

• •Clearer water, since suspended particles are removed, which allows light to penetrate deeper
• •Recovery of submerged vegetation, particularly seagrass, which needs that light and which is itself a nursery habitat and a carbon store
• •Nutrient removal, since nitrogen incorporated into oyster tissue and shell is removed from the system when they are harvested, and deposited material fuels bacterial processes that convert nitrate to nitrogen gas
• •Reduced algal blooms, since phytoplankton is exactly what the oysters are eating
• •Physical habitat, since oysters cement together into reefs whose complex surface shelters fish, crabs and invertebrates at densities far above bare bottom
• •Coastal protection, since a reef dissipates wave energy and stabilises shoreline sediment

Oyster populations collapsed comprehensively across the northern hemisphere, with assessments finding that the great majority of historical oyster reefs worldwide have been lost, which is among the steepest declines recorded for any habitat. Chesapeake Bay is the best documented case: historical accounts describe reefs rising to the surface and presenting a hazard to navigation, and estimates suggest the population was once capable of filtering the entire volume of the bay within days, against a period now measured in much longer intervals. The causes were sequential rather than single. Overharvesting removed the animals, and dredging destroyed the reef structure itself, which is the more consequential damage since a flat bottom gives spat nowhere to settle. Sedimentation from land clearance smothered what remained. Disease, in the Atlantic case two parasites that spread in the twentieth century, killed a large proportion of survivors. Pollution and reduced water quality completed it, producing a reinforcing loop in which fewer filter feeders meant worse water which supported fewer filter feeders.

Restoration projects now operate in several countries, and the methods reflect what was destroyed. Because oyster larvae settle preferentially on oyster shell, projects collect and cure shell from restaurants and processors and return it to the water as cultch, providing the hard substrate that dredging removed. Hatchery-reared spat is set onto that shell. Sanctuary areas closed to harvest allow reefs to build. Native species restoration in Europe, principally the flat oyster, is at an earlier stage than the American work. The awkward complication is that the same filtration that cleans water concentrates whatever is in it, so oysters accumulate bacteria, viruses including norovirus, biotoxins from harmful algae and heavy metals, which is why shellfish harvesting waters are classified and monitored, why depuration in clean water before sale is standard practice, and why restored reefs in polluted water are valued for their ecological function while being closed to harvest entirely.

An oyster drives water across its gills with beating cilia, traps particles in mucus and sorts them at the mouth, rejecting much of it as bound pseudofaeces that settle on the bottom, which clarifies water regardless of what is eaten. Reefs improve light penetration, remove nitrogen, shelter fish and absorb wave energy. Most oyster reefs worldwide have been lost to harvesting, dredging, sedimentation and disease. Restoration returns cured shell as substrate, and filtration concentrates pathogens, which is why harvesting waters are classified.