 hatchery spawning by thermal shock of the American oyster. Broodstock are selected by hatchery managers based on characteristics deemed valuable, including disease resistance, fast growth rates, and tolerance to poor water quality variables. Once selected for spawning, broodstock must be cleaned prior to being placed in the individual spawning units. First, hatchery personnel must remove barnacles, oyster spat, or mussels which may be found growing on the outside of the oyster. Broodstock are then scrubbed in seawater using a stiff brush to remove bryozoans, algae, worms, and mud. Broodstock are then transferred into a freshwater bath for several minutes to drive out any remaining organisms. Only now is the selected broodstock ready to be spawned. Each selected oyster is placed in a separate spawning unit supplied with ambient temperature filtered seawater. The lids prevent the cross-contamination of gametes between the units. With all of the broodstock in place, the hatchery personnel check and adjust the flow of the ambient temperature filtered seawater to each spawning unit. After several minutes in the spawning units, the broodstock will begin to open and filter the seawater. Once the broodstock open and begin to filter, the hatchery personnel will introduce filtered seawater that has been heated several degrees Celsius above the ambient temperature. This thermal shock induces the oysters to spawn. In their natural habitat, oysters spawn together in a synchronized event to maximize fertilization success. As a result, the presence of sperm in the water can stimulate spawning behavior in other oysters. Hatchery managers can utilize this characteristic by adding microwave-killed oyster sperm to the spawning units. At this point, hatchery personnel do not know if an oyster is a male or a female. Only when the spawning begins are they able to tell. The female releases eggs in bursts through the bill of the shell, while the male releases sperm in a steady stream from the side of the shell. After a male oyster has spawned, hatchery personnel collect the sperm in a beaker until it is time to fertilize the eggs. Sperm from several males is mixed prior to being added to the eggs. This helps maintain the genetic diversity and to avoid bottlenecks. After a female has spawned, hatchery personnel collect the eggs and filter them into a larger container. The mesh size of the filter is large enough to allow the eggs to pass while trapping any debris that may be in the water. Once aggregated, hatchery personnel add seawater to allow the eggs to hydrate. When the eggs are ready to be fertilized, a small sample of the collected sperm is taken using a pipette. The sperm are then introduced to the hydrated eggs and the mixture is gently agitated. An air stone is added to the container of fertilized eggs to ensure the dissolved oxygen levels are maintained. After fertilization, hatchery personnel check the fertilization rate and perform the first larval counts using a sedgwick rafter slide and a microscope. Here is an oyster egg prior to fertilization. About five minutes after fertilization, the nucleus will disappear. About 20 minutes after fertilization, polar body extrusion will occur. The next several photographs are of cell cleavage events. At four hours post fertilization, the blastula stage is reached. At four and a half hours post fertilization, the gastrolysis stage is reached. With the information from the initial larval counts, hatchery personnel are able to estimate the number of larvae produced during the spawn. This will allow them to stock appropriate numbers of larvae into the larger rearing tanks and feed at the correct levels. Hatchery personnel will continue to take counts and adjust feeding rates accordingly as the larvae continue to develop over the next several days. At five hours post fertilization, a trocophor is formed. At 24 hours post fertilization, a dehinged velager is formed. At two days post fertilization, the velager ranges from 60 to 70 microns. These next slides show the rapid growth of the velager. At six days post fertilization, the velager has grown to 160 to 180 microns. At nine days post fertilization, the velager has grown to 250 to 275 microns. After 10 to 12 days post fertilization, an eyed larva is formed. At 11 to 14 days post fertilization, a peta velager has formed at a size of 300 to 320 microns. At this stage, the larvae are ready to set on a hard substrate such as oyster shell or microculture.