Predators of the Oyster Garden

Oyster drills and mud snails number among the most common predators. Characterized by heavy shells and slow movements, their voracious appetites more than make up for other deficiencies. After clinging to an oyster, they secrete a chemical that little by little softens the oyster shell, followed by an abrasive action that erodes the shell, weakens it, and eventually bores a hole through to the oyster. The snail or drill then inserts a proboscis through the hole, feeds on, extracts, and kills the oyster

Crabs kill oysters, especially spat and juveniles, by crushing the shell and eating out the meat. Blue crabs crush small oysters and chip the lips of larger oysters to extract the meat. Stone crabs crush even large, market size oysters. However, they do so at a much slower rate than when they are crushing small, juvenile oysters. While stone crabs are intolerant of low salinity water, blue crabs tolerate a much greater range of salinity such that there likely will be some type of crab population in predation at any site that supports oyster culture. Crabs are a very mobile species. They can and will enter trays, spat bags, chubs, and cages as juveniles, feed off whatever is available, and grow to substantial size, noshing on small fishes and oysters, especially small seed as they go.

Flatworms, also called oyster leech and Styloci, are thin, flat, elliptical, and extremely fecund. In one observation, a flatworm laid 39,000 eggs in a 48-hour period.
They grow rapidly, mature sexually in about two months, and live for about a year. They flourish in a salinity greater than 15 parts per thousand but can tolerate salinity as low as six parts. Flatworms enter an oyster through a partially gaping mouth and generally prefer small (less than 6 mm) oysters. Though they do not generally attack large oysters, in one lab experiment, three flatworms killed and ate an adult oyster in a week. They prefer barnacles, but can cause extensive mortality in crowded mariculture conditions.

Control of flatworms is not difficult. Either a fresh water or highly saline water bath will immediately eliminate the flatworms, however, neither will protect against reinfestation when the oysters are returned to their habitat. Cages with large mesh (5/8”) will allow small fish to enter the cage and eat the flatworms. Fish, including cownosed rays, summer flounder, than 8mm. Small toadfish may sometimes find their way into a cage of oysters, but the rays, flounder, sheepshead and drum rarely make it through the mesh. When they do, they rarely are a significant problem. Toadfish, for example, prefer crabs and thus reduce the overall population of predators. Therefore, care should be exercised in controlling fish populations because they may also exercise a positive influence on the overall culture operation.

Barnacles are a competitor that can become so invasive in a set of cultured oysters that they have to be nubbed off the oyster shell by hand to make the oysters marketable. They tend to reside in the upper portion of the water column and settle on subtidal chub oysters more than intertidal cages. Barnacles can be controlled to a degree by giving the oysters a heavy salt water bath as described in the following section on mud worms. Sponges are among the most common intertidal pests of oysters. They attack the shell by very slowly boring holes using both a mechanical and chemical etching action. In some older oysters they may bore completely through an oyster and kill it. However, most likely the damage to the oysters will be in the outer shell, producing unsightly holes and reducing the marketability of the oysters. If the oyster shell is penetrated completely, the oyster quickly will lay down another layer to prevent contact between the oyster and sponge. In severe cases, the oyster will not rebuild its shell quickly enough to protect against the sponge, and the sponge can form adhesions to the oyster tissue, weaken it,reduce its ability to reproduce, and eventually kill the oyster.

There are a variety of species that tolerate a wide range of salinity, so it is unlikely a crop of cultured oysters can avoid the habitat. Sponges seem to favor mature bottom grown oysters. Cultured oysters, however, are not immune from a sponge infestation, but usually grow to maturity before the sponges have the opportunity to significantly deteriorate the shell. Giving mature seed a salt bath and drying before final plant-out will deter most boring sponge. Mud Worms (polydora) lay encapsulated eggs that are attached to the inner surface of a tube on the outside shell of the oyster. The eggs develop in four to eight days depending on the temperature, settle on the outer surface of the shell, make their way to the shell lip of the oyster and immediately bore into the shell using a chemical agent.

Mud worms do not directly attack the tissues of the oyster but force it to rebuild the inner shell over and over. As it does so, the oyster produces an
unsightly black and yellow blister on the inner shell. The ultimate result is an oyster with a educed value on the half-shell market. At best, oysters with mud blisters could be mixed with bottom crop oysters and sold by the bushel. When mud worms attack, the oyster puts its energy into shell maintenance,
thereby reducing the energy it puts into growth and reproduction. Poor health and a
brittle shell lead to a susceptibility to disease and predator attacks.

An effective control for mud worms is a simple saturation of the juvenile seed in a heavy salt solution of three parts water to one part salt. Usually the seed is given a fresh water rinse followed by 10- to 15-minute saturation in the salt solution and at least 15 minutes of drying. This procedure has proven effective for a variety of problems that tend to devalue the oyster crop and should be a regular part of seed maintenance performed during the late fall, winter, or early spring of the first year

Sea Squirts (tunicates) thrive in high salinity waters and can become very destructive during extended periods of drought, especially in waters that are normally high salinity. Sea squirts attach themselves to oysters, oyster chubs, oyster trays and oyster cages, and can become so densely packed that they virtually choke out the oysters and rob them of all available food.

Sea squirts can be controlled by physically removing them from cages and through regular maintenance with a fresh water wash and heavy salt water bath followed by a period of drying. Cages and other intertidal devices that get daily drying should remain relatively free of tunicates.

Overspatting occurs when small spat to attach to growing or mature oysters. While the oysters that attach pose a minor problem of competition, their cumulative effect plus the effect of barnacles, sea squirts and other competitors, can lead to stunted growth, a weakened crop, and susceptibility to disease.

Oysters that become heavily overspatted have to be cleaned up by nubbing and washing at harvest time just as those with heavy overgrowths of barnacles. Some control of overspatting can be obtained with a late fall or early winter salt bath and drying. The heavy salt solution will kill most of the small spat.