Biologists examine shrimp breeding

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by Mary Lisa Russell

Raising shrimp or other aquatic animals under controlled conditions -- aquaculture -- is an integral part of the research being conducted by biologist Dr. Brian Tsukimura and his colleagues.

Although an estimated 150 million pounds of marine shrimp are harvested in the U.S. each year, more than 450 million pounds - equivalent to about $3 billion in value - are imported to satisfy market demand.

In the early 1980s, major improvements and new techniques allowed for rapid advances in shrimp farming. But aquaculture failed to live up to its economic promise in its first phases, largely due to insufficient information about controlling the time the shrimp bear eggs, says Tsukimura, assistant professor of biology.

Tsukimura and his colleague Ray Pooyan have been researching the shrimp's reproductive cycle, trying to determine if they can control how and when the shrimp breed. "This would mean that shrimp farmers would not have to wait for the normal spawning cycle anymore to produce more shrimp," said Tsukimura.

Pooyan said that the ridgeback shrimp - the species used in his research - are reproductive four to five months out of the year. "It depends on the species of shrimp and the region they come from, but the ridgebacks are reproductive between late May and early November."

"Current practices use wild caught shrimp that are in the native spawning cycle, and production is limited to only those times when shrimp are in their standard breeding season," said Tsukimura.

Crabs, lobsters, shrimps and other crustaceans harvested from the sea don't reproduce well in captivity. Female shrimp hormones are adversely affected, and larvae do not readily mature.

"Yolk is the major constituent of an egg and provides the nutrients necessary for the developing embryo. Therefore, you have to identify the site of yolk protein synthesis to be able to produce viable eggs," said Pooyan, biology lecturer who was the first researcher to pinpoint the yolk protein production site in the ridgeback shrimp. "Once you figure out where yolk protein is synthesized, the next step is to determine what stimulates its production."

Until recently, researchers had not resolved the site of yolk protein production. However, using molecular biology techniques, Pooyan found that, in the ridgeback shrimp, yolk protein is synthesized in the hepatopancreas (shrimp liver) and ovary.

The Tsukimura team developed a technique that measures the yolk protein concentration in the shrimp blood. Pooyan said measuring the amounts of the yolk protein as well as its fluctuations is extremely important. This technique may provide the means to identify chemical mediators that stimulate yolk protein production.

Farmed shrimp production has truly skyrocketed, rising from just 26,000 metric tons of production in the 1970s to 100,000 metric tons in the early 1980s to more than 700,000 metric tons in 1995. The development of aquaculture, especially in South America (Ecuador) and Asia (China and Thailand), has substantially increased imports of shrimp.

The Food and Aquaculture Organization of the United Nations has projected that aquaculture production must double in the next 15 years in order to meet global seafood demands.

Pooyan said the importance of his research is two-fold. First, there is very little molecular data available on crustacean (shrimp) reproduction and his research serves as a stepping-stone. Second, although molecular research on shrimp reproduction is in its infancy, it may help the aquaculture industry by increasing the production of viable eggs.

The Tsukimura research team agrees there is still a lot of work to do studying shrimp reproduction. "The next step is to identify potential chemical mediators that can stimulate yolk protein production," said Pooyan.

Tsukimura said biology graduate student Josh Oldenkamp, who came to Fresno State from Pennsylvania State University, will conduct the experiments.