|
The penaeid life cycle includes several distinct
stages found in a variety of habitats. Juveniles often prefer
brackish waters of estuaries and coastal wetlands, while adults are
usually found off-shore at higher salinities and greater depths. Larval
stages inhabit plankton-rich surface waters off-shore, with an on-shore
migration as they develop. Mating and spermatophore transfer take place
just before spawning in open thelycum penaeids, but
usually days or weeks before spawning in closed
thelycum species. During spawning, eggs and sperm are simultaneously
released from the female while she is swimming. Fertilization is external,
and egg development occurs in the water column. The eggs will sink, but
within about 14 hours the eggs hatch and the nauplii, being strongly
phototropic, swim towards the surface (Bailey-Brock & Moss, 1992). These larvae will pass through three distinct stages, naupliar, protozoeal (or zoeal) and mysis, before metamorphosing into postlarval shrimp. Their diet ranges from the hereditary yolk sack, during the early naupliar stage, to phytoplankton (microscopic plant organisms) and then to zooplankton (microscopic animals). Finally, at mysis stage and beyond, the shrimp is able to eat a wide variety of organisms, including Artemia (brine shrimp). During this period, the larvae drift with the currents. A small percent of them are swept into the bays and estuaries by the currents. Here, the postlarvae remain, through their juvenile months, until they too mature and seek the offshore spawning grounds. It has been estimated that only 1 percent of those spawned in nature actually reach the adult stage. For growth to occur shrimp must go through several moulting cycles (Treece & Yates, 1988). Schematic overview of penaeid life cycle Table with overview of different stages in the development of shrimp larvae |