الفهرس 
Introduction and historical reviewOnly 14 pages are availabe for public view
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Abstract
In Egypt, there are mainly four Tilapia species naturally distributed in the River Nile and its branches. These species include the mouth brooding genera; Oreochromis (0. niloticus & 0. aureus) and Sarotherodon (S. galilaeus) and substrate spawning genus; Tilapia (T. zillii). Striking similarities and overlapping of morphological characteristics have been recoded among these Tilapia species. So, classical methods as well as molecular biological techniques have been applied to differentiate between these species. As the normal cycloid scales have general morphological characteristics, they can be helpful in the identification of fishes. For that reason, the Atomic Force Microscopy (AFM) was used to scan the anterior regions with their rostral rims, radial grooves and inter-radial circuli of the normal scales of the studied species. Also, the focus regions as well as the posterior regions with pigments were scanned by the same technique. The images of 35 p.m square of the inter-radial circuli were obtained and analyzed using SPIP program to obtain the 3D images and roughness charts for the scale of each studied species. Furthermore, the lateral line scales were scanned and the lateral line canals with their anterior and posterior openings were studied. The analyzed AFM images indicated that, most of the roughness parameters recorded the maximum values in T zi//ii scale and minimum values in 0. niloticus scale. These results indicate that, T. zi//ii scale is the roughest scale but, 0. niloticus scale is the smoothest one. The general inter-specific osteometric differences between the four Tilapia species were reflected qualitatively by X-ray photographs. The photographs verify the more closely relationship between 0. niloticus and 0. aureus, furthermore, the less degree of similarity was reflected between S. galilaeus, T. zillii and the previous species of genus Oreochromis. During the present work, attention has been directed toward the types and arrangement of lower jaw and lower pharyngeal teeth in the four Tilapia species. The results obtained for lower jaw teeth support the monophylogenetic relationship in which, all studied species have a bicuspid teeth. But the results of lower pharyngeal teeth confirm the pholyphylogenetic relationship between the studied three genera, sarotherodon (unicuspid teeth). Tilapia (Tricuspid teeth) and Oreochromis which contains two monophylogenetic species, 0. niloticus and 0. aureus, each of them has bicuspid lower pharyngeal teeth. In brief, the classical methods such as, osteometric analysis can successfully differentiate genera. But at the species level, there is great perplexity in identification studies. Thus, advances in molecular biology techniques have enabled specific identification and genetic differentiation between fish species. The polymerase chain reaction (PCR) has been useful in resolving the complex taxonomy and identification process of the species. Nearly complete sequences (1916-1986bp) of nuclear small subunit ribosomal RNA (n18S rRNA) genes from different studied Tilapia species were obtained. Then, the analysis of DNA nucleotide