Macrobrachium dayanum (Henderson, 1893)

M. dayanum take 5 to 6 months to attain full maturity. In this species it was observed change in colouration of body during maturity. In young condition the body colour is slight grayish with scanty deposition of greenish coloured j3 carotene pigments. As the growth approaches towards maturity, the body colour changes to dark grey with full deposition of greenish coloured carotenoid pigments in the body surface. Towards old stages, the species is black in colour and it is a very diagnostic character for this species. These types of frequent changes of body colouration are not observed in other collected species of Macrobrachium of Assam except M. assamensis. This species attains maturity towards late monsoon and lays eggs towards early part of winter. Larval stages completed during late part of the winter and becomes adult before the spring.

Maximum size = 93 mm.

Colouration: In young condition light greenish colour is more towards dorsal part of the body. Adult shows light to dark ash colour. In adult, the body shows heavy deposition of green, pink to light black coloured pigments. Rostrum: Dorsally in the rostrum, all spines are not equidistant from each other but ventrally, specially the second, third, fourth and fifth are equidistant. Moreover, dorsally the rostrum shows slight convexity from the base. The concavity starts just immediately after 3rd spine and finally upturn towards apex. Ventrally the spines develop from the concavity apart. Rostral Formula (RF): 7 - 1 1 / 5 - 9 . Carapace: It is smooth and highly pigmented with frequent change of colour. Antro-laterally on either side of the rostrum, towards the base of the eye stalk, one pair of small pointed spine is present. One pair of well-developed stalked compounded eye. The length of the eye stalk: ranges from 2.5 - 3.3 mm (2.75 ± 0.5123 mm) and diameter is 0.5 - 1.2 mm (0.85 ± 0.3548 mm). Antennules: Pre Coxa > Coxa ^ basis. Laterally towards the left side the coax bears fine hair like structure but the basis show s hair like out growth laterally towards both side. Antenna: Coxa half of basis. Basis with single pointed spine. It showed uniform distribution of large number of very fine, spherical, red coloured pigments. Leaf - like expodite with uniform distribution of setae towards periphery. Antero-laterally 2/3 rd of the expodite towards inner side from the base shows hair like structure without setae. Coxa < Basis. First pareopod: Coxa = basis, ischium^ meras, Carpus dactylus. The ischium and fingers are provided with left setae, while on the rest of the legs, hairs are sparsely distributed. Second pareopod: coax < basis, ischium = meras, carpus dactylus. The ischium, meras and carpus are rod like and laterally compressed. Non-chelate legs: Coxa = basis, ischium < meras, carpus Dactylus. Except meras and carpus, all parts are hairy. Finger: In female 2 - 3 conical teeth with 7 - 9 minute spines. In male fingers: three equal sized conical teeth having three minute spines like processes. They are [generally omnivorous in nature.

It is abundantly found in the standing water Ijke beel, big swampy water bodies, ponds, tank with full of plankton and aquatic vegetation. Decayed fallen leaves, soft vegetation and plankton are the favourite food. They also never hesitate to eat the earthworm and the larva of many crustaceans and insects. Some species are also collected species from many small rivers of sandy loamy soil bed plus remnants of soft aquatic vegetation and plankton. Rice band, oil cake are also the favourite food because of surface column dwelling habit.

In this species, the eyes are larger in size and very less in number in compare to other running water prawn. During the development of eggs, the gradual change in colour from light green to green and finally to light black are noticed. In M. dayanum, the total number of eggs counted in both the ovaries are ranged from 1200 - 2000.

Notes

In M. dayanum from spring to monsoon, the price ranges from Rs. 120.00 to 150.00 per kg and during winter due to availability of the species, the price goes down to Rs. 80.00 to Rs. 100.00 per kg.