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¹ Central Institute of Freshwater Aquaculture (I.C.A.R), Aquaculture Production and Environment Division, Cryobiology Laboratory, Kausalyaganga, Bhubaneswar-751002, Orissa, India Viewed : 4154 - Downloaded : 10113 The aim of the study was to investigate the physical difference in biochemical characteristics of spermatological and seminal plasma and also ultrastructure of spermatozoa in six carp species, viz. catla, Catla catla, rohu, Labeo rohita, kalbasu, Labeo calbasu, mrigal, Cirrhinus mrigala, silver carp, Hypophthalmichthys molitrix and grass carp, Ctenopharyngodon idella. Semen yield from different carps ranged from 6.6 to 8.9-ml/kg body weight at the peak-spawning season. Sperm density (spermatocrit) showed a range of 69-81% among the six carps. Similarly, the sperm count varied between 2.6 x 1010/ml to 3.5 x 10¹⁰/ml in six species of carps. The motility of carp spermatozoa ranged from 80 to 110 sec in different species. Maximum motility of 110 sec was observed in mrigal. The mean pH and osmolality ranges were 7.3-8.1 and 269- 289 mOsm/kg. The ion concentrations were: Na+ 106±1.2, 85.2±1.0, 81.8±3.6, 94±2.27, 64±1.2, 110 ± 0.81 mEq/L, K⁺ 25±1.2, 32.1±1.1, 48.6±2.4, 51±3.67, 25±0.8, 36 ±1.2 mEq/L, Cl- 246 ±2.0, 175±4.3, 174±5.2, 245±7.7, 226 ±0.8, 253±0.4 mEq/L for catla, rohu, mrigal, kalbasu, silver carp and grass carp, respectively. The other parameters of seminal plasma were: total protein; 0.2±0.006, 0.1±0.006, 0.4±0.06, 0.4±0.04, 0.6±0.08, 0.8±0.14 g/dl; cholesterol 14.1±0.8, 17.4±1.7, 22.8±1.3, 15.4±0.61, 14.0±0.7, 12.7±0.7 mg/dl and glucose 1.2±0.11, 1.4±0.07, 1.8±0.16, 1.2±0.12, 2.0±0.34, 2.0±0.35 mg/dl for catla, rohu, mrigal, kalbasu, silver carp and grass carp, respectively. Ultra structure by electron microscopy revealed that spermatozoon of carp is composed of head without any acrosomal complex with circular to elliptical nucleus and tail or flagellum with a midpiece consisting of mitochondrial ring. The size of the spermatozoa varied significantly among different carp species (P<0.05). All six carp flagellum has a typical 9+2 axoneme arrangement. Indian major carps, silver carp and grass carp are seasonal breeders and to make artificial propagation successful by utilizing the available brood fish in an efficient way, the present information on the normal physical and chemical characteristics of semen of these carps presented in this paper will eventually help in selecting good milters and devising improved protocols for cryopreservation and artificial propagation methods. Keywords : CASA, carp, ion composition, motility, osmolality, semen, ultrastructure