Macrobrachium amazonicum is the most commercially-explored fresh water shrimp in the Amazon, finding great acceptance in the consuming market. The objective of this work is to characterize the structure and the ultrastructure of the gonads of different M. amazonicum morfotipos in natural population. The shrimps (500 specimen) were collected in Furo das Marinhas (Mosqueiro – PA), by local fishermen and brought alive to the Laboratório, in an appropriate box containing local water. Fifty males were selected, weight, measured and killed. Gonads were collected and fragments were fixed in 2,5% glutaraldehyde 4% paraformaldehyde in sodium caccodylate buffer at 0,1M pH 7,2. For light microscopy the fragments were processed according to the standard histological technique for paraffin embedding. The sections were stained with Hematoxilin and Eosin. For transmission electron microscopy the gonads were post-fixed with 1% osmium tetroxide in sodium caccodylate buffer at 0,1M pH 7,2 and processed for embedding in Epon resin. Semithin sections were stained with toluidin blue. For scanning electron microscopy the fixed and post-fixed specimens were dehydrated, submitted to the critical CO2 drying point and then coated with gold and examined in electron microscope. Four morfotips in natural population were observed, in agreement with corporal dimensions and coloration. The morfotips were denominated as TC, CC, GC and GC’ according to Moraes-Riodades (2002). The biometric relation total/length cephalothorax and total/weight had presented negative allometrie.The testicles and are located in the cephalothorax. The testicle is constituted by seminiferous tubules; each tubule presents a very thin capsule of conjunctive tissue that divides the parenquime of the organ. The tubule is covered by flat cells and in its interior we observed a cell population of spermatogenic lineage in different maturation phases that were denominated as: spermatogônias, spermatocytes, spermatids and spermatozoids. The spermatogonias are the largest cells with cromatina organized in clots, located eccentrically in the tubule. The spermatocytes are round cells with cromatina in dense clots. The spermatids are small cells with great amounts of mitochondria. The spermatozoids are concave cells in the apical portion and convex cells in the distal portion, presenting a small spike assuming an aspect similar to a cup. In cross section, the vas deferens come covered by conjunctive tissue and they can present from a cubic to a cylindrical epithelium; they present in their interior an agglomeration of feeder cells located in the eccentric portion and spermatozoids located in the lumen of the ducts.The structural analysis of the M. amazonicum gonads, demonstated the of three distinct morfotips in the Furo das Marinhas.
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