Identification of different larval stages is of much importance in the successful operation of a hatchery. As the different larval stages need different types of food and water quality environment, it becomes necessary to identify and segregate them from other groups. Penaeid Shrimps pass through three larval stages before attaining the postlarval stage i.e., nauplius, zoea, and mysis.

Follow the points given below to identify the larval stages of shrimps.

Nauplius stages:

• The first larval stage is known as nauplius.
• The unsegmented body which is Pear shaped with 3 pairs of appendages.
• Moults every 3-5 hours and passes 6 substages (Nauplius 1 - Nauplius 6).
• They are positively phototactic, it swims actively towards weak source of light.
• No feeding due to absence of mouth and alimentary canal.
• After 2 days, the nauplius converts to protozoea by metamorphosis.
• They swim intermittently with their antennae and mandibles.

Protozoea stages:

The body of the zoea is more elongated than the nauplius. The body consists of the carapace, thorax and abdomen. They have functional digestive tract and filter feeders. They feed mainly on unicellular algae present in the medium.

It has 3 sub stages, Protozoea I, II and III. 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.

• PZ I - Broad head and narrow tail with forked end. Eyes sessile.
• PZ II - Eyes stalked. Rostrum developed.
• PZ III - Abdominal segments develop dorsomedian spines. Uropod bud appears near forked end of tail.

The stages lasts for 3-4 days and is succeeded by Mysis stage.

Mysis stages :

Mysis swims through flexing their abdomen with the head pointing downwards.

It also has 3 sub stages:

M I - No pleopod bud

M II - Pleopod bud appears

M III - Pleopod bud with 2 segments

This stage lasts for 3-4 days and then pass to I post larval stage.

Post Larva:

The postlarva resembles the adult shrimp and it can be distinguished from the mysis stage by the presence of setae on the pleopods which are functional. The postlarvae also prefer to cling to the solid surfaces and the walls of the tank. Substages of postlarvae are called as PL 1, PL 2, PL 3 etc., wherein the number indicates the number of days as postlarva. Postlarvae feed on zooplankton and other dead organic matter. They can tolerate lower salinities.

Litopenaeus vannamei is a decapod crustacean which is native to the Eastern Pacific coast from Sonora, Mexico in the North, through Central and South America as far South as Tumbes in Peru, in areas where water temperatures are normally >20°C throughout the year. Penaeus vannamei live in tropical marine habitats. It has been introduced widely around the world since the 1970s, but especially since 2000, as it has become the principle cultured shrimp species in Asia.

Litopenaeus vannamei (Boone, 1931)

Common name: Whiteleg shrimp

Colour: Coloration normally translucent white, but can change depending on substratum, feed and water turbidity.

Species identification: It has 7-10 teeth on the dorsal rostrum and 2-4 on the ventral side.

Taxonomy:

Order: Decapoda

      Suborder: Natantia

             Infraorder: Penaeidea

                  Superfamily: Penaeoidea

                          Family: Penaeidae

                                   Genus: Penaeus

                                           Species: P. vannamei

Habitat: Depth 0 to 72 m. Bottom mud. Marine (adults) and estuarine (juveniles)

Size: Maximum size 23 cm, with maximum Carapace Length of 9 cm. Maximum weight of female 120 g. Females commonly faster growing and larger than males.

Biology: Males become mature from 20 g and females from 28 g onwards at the age of 6–7 months. P. vannamei weighing 30–45 g will spawn 1,00,000–2,50,000 eggs of approximately 0.22 mm in diameter. Hatching occurs about 16 hours after spawning and fertilization. The first stage larvae, termed nauplii, swim intermittently and are positively phototactic. Nauplii do not feed, but live on their yolk reserves. The next larval stages (protozoea, mysis and early postlarvae respectively) remain planktonic for some time, eat phytoplankton and zooplankton, and are carried towards the shore by tidal currents. The postlarvae (PL) change their planktonic habit about 5 days after moulting into PL, move inshore and begin feeding on benthic detritus, worms, bivalves and crustaceans.