The Cichlid Fishes of Lake Malawi, Africa

Abstract of Publication

Msiska, O.V. 2002. The histology of mature gonads of Oreochromis (Nyasalapia) karongae (Trewavas). African Journal of Ecology 40 (2): 164-171.  

Histology of gonads of Oreochromis karongae was undertaken to study internal cell characteristics during maturation. This study was necessitated by low spawning output of the fish species. Several oocyte stages, ranging from primary forms to vitellogenesis, suggest that the maturation was generally succesfully attained in the fish ponds. Pre-vitellogenesis oocytes (oogonia to perinuclear stage) and more advanced vitellogenesis (primary vesicle to tertiary yolk vesicle) oocyte stages were all found in the same gonads. However, there were some discontinuities observed during stages 3 and 4, suggesting selective maturation. Failure of gonads to mature normally is attributed to an ecological crunch that was in a previous study associated with environmental factors. Atretic oocytes were also recorded in the same gonads, a sign that some oocytes failed to mature normally. This indicates insufficient stimuli for normal gonad development. Several stages of spermatogenesis (spermatocytes, spermatid and spermatozoa) were also found in the same gonads. Selective recrudescence was more pronounced in O. karongae because generally less oocytes attained final maturation stages compared to Oreochromis niloticus and other tilapias. This could be the main reason for low natural breeding that has been observed in both wild and captive stocks, and led to the abandonment of its use in aquaculture. This study corroborates findings of previous studies that depended solely on external gonad characteristics. Histology provides conclusive evidence from internal cell characteristics that other techniques are unable to show.




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