Physiology of Finfish and Shellfish (2+1)

3.1.29. Life histories vary from the simple to the complex within the different groups of Crustacea

The life history of the three species of penaeid shrimps is generally similar, although the time of spawning varies with each species. Mating takes place in shallow offshore waters and involves the transfer of spermatophores from the male to the female shrimp. Actual spawning takes place in deeper offshore waters. The eggs are released and fertilized externally in the water. During spawning the last 3 pairs of periopods are held tightly together and are flapped vigorously. The unfertilized eggs are extruded from the paired ovipores located at the base of the third periopods. Simultaneously stored sperm are released from the thelycum. Both are ejected with considerable force where the eggs appear as a greenish, and the sperm as a whitish, cloud exiting from beneath the females as she swims. The sperm are non-motile and must come into contact by passive collision with the surface of the egg at the moment of ejection. It is believed that the vigorous flapping of the female’s pereiopods creates an intense vortex that increases the likelihood of sperm and egg coming into contact. A female may lay between one half to one million eggs at a single spawning. Within 24 hours, the tiny eggs hatch into a microscopic larva known as a nauplius. The nauplius stage is followed by the protozoeal, mysis, and postlarval stages (Figure 1). Passage from the nauplius to the postlarval stage takes several weeks. All of the developmental stages are found in the offshore plankton. The larvae are capable of little horizontal, directional movement and are unable to swim independently of the water currents. They are totally reliant upon favorable currents to transport them to inshore waters. The migration from offshore waters to coastal bays occurs during the last planktonic stage and shrimp enter estuarine nursery grounds as postlarvae. Once they move into brackish waters, the postlarvae abandon their planktonic way of life and become part of the benthic community.

Postlarval and juvenile shrimp occupy the shallow, brackish waters of the Sound where they feed and grow. Growth of the young is rapid when waters are warm (above 20.0C). Young shrimp remain in the estuary until they approach maturity. Adult shrimp migrate offshore to spawn, and the cycle is repeated.