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White Shrimp

Description

white shrimp in bucketThe white shrimp, Penaeus (recently renamed Litopenaeus) setiferus, is a commercially important species along the East Coast of the United States. It is similar in appearance to two closely related species of commercially important shrimp, the brown and pink shrimp. White shrimp are rather large, sometimes reaching 25 cm (10 in) in length. They have dark brown antennae that are considerably longer than the shrimp’s body. Their horn, or rostrum, is long and thin, and three of their five pairs of walking legs end in weak pincers. The uropods (appendages making up the tail fan) are dark at the base and have a yellowish green edge. In brown shrimp, the uropods have dark green and red pigmentation, whereas those of pink shrimp are usually bluish. Shrimp have different kinds of appendages used for moving around: walking legs (periopods) are used for moving over short distances; swimming legs (pleopods), located in two rows under the abdomen, are used in swimming over long distances; and uropods are used in conjunction with strong abdominal contractions that propel the animal backwards. The latter movement, called the tail flex, allows a shrimp to escape predators quickly.

Habitat and Biology

White shrimp thrive on muddy bottoms of estuaries from New York to Florida, and in the Gulf of Mexico from the Ocklocknee River, Florida, to Campeche, Mexico. They are most abundant in areas with extensive estuarine marshes, such as those along the South Carolina coast. The greatest concentration of white shrimp is in the Mississippi River Delta in Louisiana.

White shrimp spawn offshore, and in South Carolina this begins in May and June, with a few spawning events occurring as late as July or early August. In the Gulf of Mexico, the spawning season extends from March to September/October. During mating, the male transfers a packet of sperm, or spermatophore, to the female, who attaches it to her underside. Eggs are fertilized externally as they are broadcast into the water. White shrimp eggs sink to the ocean floor, and after 12 to 24 hours they hatch into planktonic larvae which go through 10 larval stages and a postlarval stage that resembles the adult. The life history of white shrimp indicates that their population dynamics are strongly influenced by larval and post-larval recruitment patterns. Success of larval recruitment to estuaries and subsequent survival are of paramount importance, since larvae form the reservoir of individuals from which the new year-class is derived. The abundance of white shrimp postlarvae entering estuaries is being used as one index for prediction of adult catch.

About three weeks later, the postlarvae enter estuaries in currents generated by tides and wind and migrate upstream to nursery areas. In the vicinity of an estuary, postlarvae utilize selective tidal stream transport to migrate to estuarine nursery areas; that is, they stay close to the bottom during the outgoing tide and rise off the bottom during the incoming tide in order to be transported upstream. Recruitment of white shrimp to South Carolina estuaries occurs from May to September, with peak recruitment in late May and early June.

Once in the estuary, young shrimp move into tidal creeks, where they find ample supplies of food and protection from predators. During high tide, juvenile shrimp move to the marsh surface, where they are protected among the dense vegetation. There they can feed on a variety of organic material including small benthic worms, plant matter, and decaying animals. White shrimp are opportunistic feeders, consuming any food that is available, including other shrimp. At low tide, shrimp move off the marsh surface and are concentrated in the channels of tidal creeks. Shrimp abundance and distribution is dependent on rainfall and winter water temperatures. Adult and juvenile shrimp are most abundant in salinities ranging from 8 ppt to 15 ppt (25 - 40 % of oceanic salinity). During periods of heavy rain, individuals leave shallow tidal creeks and move into the deeper, more saline waters of the rivers and harbors of the estuary.

Shrimp remain in these nursery habitats until late spring or early summer, when they begin to move into larger creeks and rivers in preparation for their offshore spawning migration. During this migration, they move progressively down the estuary into more saline waters. White shrimp are also known to make coastwide migrations of considerable distance, moving south down the Atlantic coast during winter and returning in spring. Juvenile and adult shrimp are bottom-feeding omnivores; that is, they consume any type of organic material they come across on the sea floor. Diet varies as shrimp grow, with larger individuals becoming more predatory and consuming animals such as marine worms, larvae of other crustaceans, small fishes, and even other white shrimp.

The other two species of penaeid shrimp inhabiting South Carolina waters, brown shrimp (Penaeus aztecus; recently renamed Farfantepenaeus aztecus) and pink shrimp (F. duorarum; recently renamed Farfantepenaeus duorarum), have similar life histories to the white shrimp. Brown shrimp spawn in inlets and offshore during the fall. The nearshore distribution of younger stages (nauplii) and offshore distribution of older stages (zoea and post-larvae) show a widespread larval distribution pattern. It is unclear whether post-larvae that move offshore return to the estuaries. One suggestion is that post-larvae over-winter far offshore and return in the spring using wind and tidal transport mechanisms. Peak recruitment of brown shrimp to South Carolina estuaries occurs in February and March.

The spawning period of pink shrimp overlaps that of white shrimp and occurs during the spring and summer. Both white and brown shrimp prefer the muddy bottom habitats that are characteristic of South Carolina estuaries. Pink shrimp, on the other hand, favor a sandy/shell bottom. While the latter type of habitat occurs in South Carolina estuaries, it is not as widespread as muddy and sandy habitats. This may explain the lower abundance of pink shrimp relative to white and brown shrimp in South Carolina waters.

Species Significance

The white shrimp was the first of the penaeid shrimps to be commercially marketed for food. Penaeid shrimp constitute the most important commercial fishery in South Carolina, with landings of white shrimp and brown shrimp producing $12.2 million in 1996. Methods of harvest include commercial trawling, shrimp baiting, and cast netting.

The white shrimp resource in South Carolina waters is closely monitored, and its harvest is regulated by the South Carolina Department of Natural Resources. The annual abundance of shrimp is closely related to physical factors such as temperature, amount of rainfall, and meteorological events that influence recruitment to nursery areas. Drastic changes in environmental conditions can negatively impact white shrimp recruitment and survival. A serious threat to the population is loss of nursery areas due to filling, dredging and draining of critical marsh habitat. Also, land use changes in upland areas adjacent to marshes alter levels of freshwater runoff and create greater salinity variations that affect habitat suitability for white shrimp.

References

DeLancey, L. B., J. E. Jenkins, and J. D. Whitaker. 1994. Results of long-term, seasonal sampling for Penaeus postlarvae at Breach Inlet, South Carolina. Fisheries Bulletin 92(1):633-640.

Lam, C. F., J. D. Whitaker, and F. S. Lee. 1989. Model for white shrimp landings for the central coast of South Carolina. North American Journal of Fisheries Management 9:12-22.

Ruppert, E. and R. Fox. 1988. Seashore animals of the southeast. University of South Carolina Press, Columbia, SC.

Sick, L.V. 1970. Larval distribution of commercially important Penaeidae in North Carolina. Journal of the Elisha Mitchell Society 86:118-127.

Whitaker, J. D. 1995. Shrimp in South Carolina. Sea science educational series. South Carolina Department of Natural Resources, Charleston, SC.

Williams, A. B. 1984. Shrimps, lobsters and crabs of the Atlantic coast of the eastern United States, Maine to Florida. Smithsonian Institution Press, Washington, DC.



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