Monday, July 18, 2016

Giant river prawn - Macrobrachium rosenbergii

Taxonomic and biological features:
Distinguishing characters
Macrobrachium rosenbergii is the largest Macrobrachium species. The maximum recorded size for males and females are 33 cm and 29 cm, respectively. The cephalothorax consists of 5 indistinct segments in the head and eight in the thoracic region. The abdomen has six distinct segmented movable terga. Each abdominal segment has a pair of biramous pleopods (swimming legs). Eyes compound. Cephalothorax with 2 pairs of antennae, 3 pairs of jaws, 3 pairs of maxillipids and 5 pairs of walking legs. The second and largest abdominal tergum laterally overlaps the first and the third. Dorsal surface of abdominal carapace smooth and rounded. Rostrum is slender and curved upwards. The rostrum extends beyond the antennal scale and has 11-14 dorsal teeth and 8-10 on the ventral side. Mature male prawns are considerably larger than the females and the second pair of chelipeds are much larger and thicker than in the female. The head of the male is also proportionately larger and the abdomen is narrower. The left and right chellipeds of M. rosenbergii are equal in size, unlike some other Macrobrachium species. In adult males they become extremely long and reach well beyond the tip of the rostrum. The genital pores of the males are situated at the base of fifth walking leg and in females at the base of the third walking legs. The tip of the telson extends beyond the posterior telson spines (Jayachandran, 2001).

M. rosenbergii is nocturnal, bottom dwelling and sluggish in nature and is territorial. During the day they remain half buried in sediments and prefer shallow, detritus rich and vegetated areas.

The spawning season of M. rosenbergii is protracted and varies in different regions in its distributional range in the Indo-west pacific region. In temperate regions, the breeding season coincides with the summer, while in tropical regions it is generally restricted to the onset of the rainy season (Valenti, 1984). Spawning occurs two or more times per season (Ling and Merican, 1961).

Successful mating occurs between soft shelled females and hard shelled males. The mating process is described in detail by Ismael and New (2000) and Karplus et al. (2000). Gravid females migrate downstream into estuaries, where the eggs hatch as free swimming larvae. The larvae pass through 11 zoeal stages before reaching the post larval stage (Uno and Kwon, 1969).

Fecundity of M. rosenbergii varies considerably with the age, size and stage of maturity. According to New and Singholka (1982), the giant river prawn lays between 0.8 to 1.0 x104 eggs during one spawning. The eggs of M. rosenbergii are slightly elliptical with a long axis of 0.6-0.7 mm, and are bright orange in colour until 2 or 3 days before hatching, when they become grey-black (Ling, 1969; Manus et al., 2006).

The zoea are planktonic, swimming upside down within the water column and exhibit positive phototaxy. Larval development is temperature dependent. After metamorphosis, the PL assume a benthic lifestyle and begin to migrate upstream into freshwater. Post larvae resemble miniature adult prawns and swim with the dorsal side uppermost (New and Singholka, 1985).


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