Based on their mode of life, penaeid shrimps can be grouped into wandering and burrowing groups.
Species belonging to this category have pronounced age groups, from dense schools and are always on the move and active day and night. They prefer turbid waters with soft mud bottom, burrowing slightly or not at all into the substrates. A typical wandering species, P. orientalis in the Yellow Sea migrates southward for over wintering and northward in spring to its spawning ground. The distance between the areas for overwintering and spawning is approximately 700–1300 km. (Fig.14).
Fig. 12. Layout of medium scale hatchery.
Fig. 13. Layout of small scale hatchery
Fig. 14. Migratory pattern of wandering species of shrimp.
Other species belonging to this group are P. setiferus of the Atlantic coast, P. merguiensis and P. indicus of the Southeast Asian region. These tropical species however, do not need overwintering. P. merguiensis and P. indicus generally spawn at a depth of 7–30 meters in offshore waters, usually near the nursery ground. The larvae spend part of their life cycle in nursery ground until adolescent stage when they start migrating to deeper and more saline waters. Since P. merguiensis, P. indicus and other wandering type of penaeid shrimps from dense schools, fishermen can easily collect large number of spawners during peak seasons.
This group prefers certain habitats with sandy bottom. They do not have pronounced age groups. They show marked nocturnal activity, burrowing into the bottom substratum during the day and emerging at night to search for food. This diurnal activity is closely associated with changes in light intensity but may also be due to other factors. Examples of this group are P. japonicus in the Japanese coast, P. dourarum off Florida and P. monodon in Southeast Asia.
P. monodon spawners are found in offshore waters on sandy bottom at depths of 20–40 meters. The larvae move towards the coast, enter estuaries and mangrove swamps which serve as nursery ground. They then migrate to deeper water when they become adolescent and finally move to spawning grounds upon becoming adults. The pattern of migration is illustrated in Fig. 15.
Penaeid shrimps generally spawn in offshore waters. Each gravid female lays as many as half to approximately three quarter of a million eggs per spawning. After hatching, the early larval stages viz: nauplius, protozoea, mysis and early postlarvae remain planktonic for sometime and are carried towards the shore by tidal currents.
The first larval stage is known as nauplius (Fig. 16). The unsegmented body which is pyriform in shape possesses three pairs of appendages. The nauplius undergoes six moltings within 50 hours into a protozoea. (Fig. 17). The body becomes elongated with a distinct cephalothorax. The early protozoea stage has a pair of protruded compound eyes, the next stage is characterized by the presence of a rostrum and the late protozoea stage has a pair of uropods. After 4–6 days, the protozoea finally metamorphoses into a mysis. (Fig. 18 a-b). At this stage, the larvae assumes the form of a juvenile shrimp at which the pleopods are to develop. At this stage, tiny protrusion known as pleobases are seen on the ventral side of the abdominal segments. The next stage is marked by the devlopment of first segment of the pleopods development. The mysis remain drifting in the water column until they metamorphose into post larvae within 10–12 days.
Fig. 15. Migratory pattern burrowing species of shrimps.
Fig. 16. NAUPLIUS STAGES
|a. Nauplius||I||b. Nauplius||II|
|c. Nauplius||III||d. Nauplius||IV|
|e. Nauplius||V||f. Nauplius||VI|
Fig. 17. PROTOZOEAL STAGES
a. Protozoea 1
b. Protozoea 2
c. Protozoea 3
Fig. 18 A & B. Mysis and post larval stages
a. MYSIS 1
b. MYSIS 2
c. MYSIS 3
d. POSTLARVAE 1
At post larval stages (Fig. 18b), the pleopods become fully developed and functional. The animals grow very fast in terms of size and are able to swim freely although early postlarvae are still planktonic in offshore waters. At a body size of 0.8–1 cm in body length, they enter estuaries and inshore shore waters where they first adopt a benthic existence. The shrimps spend their juvenile, adolescent and sub-adult stages in estuarine waters and then gradually move toward deeper water as they grow and eventually returning to offshore water when they attain sexual maturity. The developmental stages of P. monodon in relation to mode of life is shown in Table 6, 7, and 8.