How Oysters Grow Their Way to Your Plate

From their natural or man-made homes, these bivalves benefit people and the environment

Article February 7, 2020 By: Holly Binns & Joseph Gordon Read time:

Projects: Conserving Marine Life in the United States

This blog was updated on Feb. 13, 2019 to correct a photo caption.

Consuming an oyster on the half shell is about as quick and easy as eating gets—a stark contrast to what it often takes to get that mollusk to the dining table.

Oyster reefs are declining worldwide and face threats ranging from polluted and warming water to dredging, erosion, and diseases. So oyster growers, scientists, conservationists, and others are finding ways to build new reefs and help oysters reproduce. Healthy reefs not only deliver more oysters but also convey environmental benefits. Oysters filter water as they eat primarily algae. Reefs stabilize shorelines and provide habitat for many marine animals, including commercially and recreationally valuable fish species.

Here are eight things to know about oysters and how they grow:

Let’s talk spat. To reproduce, oysters produce larvae, which then drift for two to three weeks. Many are eaten by small predators or otherwise die; others settle on a surface—ideally an existing oyster reef—where they’ll spend the rest of their lives. Once attached to any surface, the larvae are called spat.
The housing shortage. In many places, larvae have nowhere to attach. Globally, including along the U.S. coasts, about 85 percent of reefs have been lost, and in some places they have become locally extinct. In response, in some areas, governments, researchers, conservation groups, and others have started making reefs using cultch—material such as oyster shells or crushed limestone and concrete. Restoration efforts may also include adding spat to reefs when building them. This technique has been widely used along the Atlantic and Gulf coasts to help the species and reefs recover, primarily to meet consumer demand for oysters.

Oysters are OK with manufactured housing. Sometimes people use rocks or precast concrete structures to build large, tall, and durable condolike reefs for oysters. These structures, which may incorporate recycled shell from restaurants and shucking houses or leftover concrete from construction or demolished dams, also provide homes for other marine life, such as crabs and fish. Reef builders, who include government agencies, commercial growers, academics, and conservation groups, tend to build these reefs well below the low tide line (that is, in subtidal areas) so they are less likely to be buried by sediment or degraded by waves. Oyster larvae are attracted naturally to these structures, and reef builders can also use hoses to spray oyster seeds or spat on them to get a colony going.

Sometimes youngsters need boarding school. An emerging technique is to grow spat on cultch in a secure environment, such as a laboratory or hatchery, before bringing that cultch to a reef. This can help build reefs in areas where there isn’t sufficient existing shell material or hard substrate to provide settlement habitat for larvae or where there isn’t enough wild larvae in the water to colonize the reef. Controlled spat-growing gives young oysters the best chance of survival.

Oysters aren’t fussy. Many attach to and grow on any hard substrate, such as oyster reefs, limestone outcroppings, seawalls, and various debris. Oyster farmers even grow them in racks, bags, and cages suspended above the seafloor. Growers then retrieve mature oysters from these locations at low tide. Oysters can also grow in tanks from broodstock—a group of mature individuals that supply larvae.
Oysters may need “me” time. Sometimes an oyster reef needs to be left alone to recover from overharvesting, disease, pollution, dredging, trawling, changing ocean conditions, or other threats. Some states designate areas in which harvest is prohibited all or part of the time to give oysters a better opportunity to breed—and even spread beyond the conservation area.
Healthy reefs help people. Placing reefs in shallower waters or adjacent to marsh areas helps create or enhance “living shorelines” that limit erosion by stabilizing sediment and diffusing the destructive power of waves before they reach shore. Oysters grown in these projects and those that grow on seawalls naturally and on docks can contribute, sometimes significantly, to reproduction at natural reefs. Also, oysters are filter feeders, so they clean the water as they eat, improving habitat conditions for all marine life. The ability to clean water is so valuable that in certain locations, fishery managers and local governments have set aside areas in which oysters are left alone in large part just to help improve and restore water quality, among other ecosystem services.

A shell’s second life. Don’t throw away oyster shells—they can be reused in reefs. In some areas of the country, restaurants and other businesses have recycling programs to ensure the shells are reused in restoration projects rather than heading to landfills.

Increased use of sound practices to boost, restore, conserve, and grow oysters will provide bountiful supplies for people and leave enough in the water to offer environmental benefits, from improved wildlife habitat to water filtration. Oysters have many roles to play other than a menu item, but they need a little help to get their jobs done.

Holly Binns directs The Pew Charitable Trusts’ U.S. Conserving Marine Life program in the Gulf of Mexico and U.S. Caribbean; Joseph Gordon directs the program along the Atlantic coast.