The genus Macrobrachium contains more than 120 species and occurs in tropical and subtropical regions worldwide (VALENT, 1987). They are freshwater shrimp family Palaemonidae and order Decapoda (RAFINESQUE, 1815; LATREILLE, 1802). In Brazil there are 18 species, classified until now distributed throughout the Amazon Basin (MELO 2003). Among these, Macrobrachium amazonicum (HELLER, 1862) popularly known as shrimp quiet or shrimp cinnamon, widely used in shrimp farming. Crustaceans have sensory structures located on the cephalothorax, which allow then to receive the stimulus means to locate and capture food (BARNES, 1998). The compound eyes are present in all classes of crustacean. Thus, this research hypothesis was that in Macrobrachium amazonicum, these eyes are kind of overlay reflective, where the dioptric apparatus and rhabdom extend in layers and
are separated by a clear non-pigmented area. In this work we have the following objectives: To evaluate the morphological aspects of the eye of Macrobrachium amazonicum scanning electron microscopy; characterize the morphology of the photoreceptor cells; describe the morphological structures of the eye of M. amazonicum; characterize the morphometric relationships between the eye and other structures of M. amazonicum. The animals were acquired in the district of ‘Mosqueiro’ in the rainy season, in March 2009 and March 2010, with fishermen in Santa Barbara city, the metropolitan area of Belém, and transported to the laboratory in polystyrene boxes, being held in quarantine in a vessel containing a solution of potassium permanganate at 1.3 mg / L (CARNEIRO et al., 2005). The animals were fixed in Davidson Karnovisky then the eyes of each animal were carefully sectioned and placed in plastic. Relations were performed in biometric analysis of variance with α =
0.05, was performed with Bio Stats 5.0 for the lengths of the side of the eye and
anterior-posterior of the four morphotypes, and SigmaPlot 11.0 and simple linear regression for the variables eye total and cephalothorax. It was observed that the average size of the cephalothorax, the body, the eye is respectively: 1.03 mm, 70.62 mm and 4.52 mm, and the average weight of shrimp was 7.97 g. The structures of these maximum values recorded were 31.95 mm for the cephalothorax; 100.10 mm body size, 6.80 mm for the size of the eye and 20.54 g for the weight of the shrimp. Histological analysis identified the following structures (Fig. 13:14 AB): cornea, crystalline cone, pigment distal shaft of the cone, clear zone, cuticle, retina, rabdoma, optic nerve fibers and laminates. With respect to scanning electron microscopy, were selected to observe the main structures and especially the arrangement of ommatidia square cross section (Figs. 16 AB). The morphological and morphometric analysis (by histology and scanning electron microscopy) showed characteristics of Macrobrachium amazonicum adults from the metropolitan region of Belém. These findings include a reflective optical superposition eyes fitted with the perception of light stimuli.
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