Under natural conditions, 50-90% of fresh water fishes harbor at least one species of parasites (Sieszko, 1975;Daniel, 1978). Parasitism is much more common and diversified in the wild than in the farms, ponds and hatcheries. Infections occur not only due to over crowding but also due to environmental stress. As a result, fish exposed to virulent pathogens under environmental stress such as temperature, sewage, metabolic waste products of fishes, pollution and pesticides 23 ABSTRACTThe study was undertaken to the determine species diversity of fishes, the level of parasitic infections and the relationship between infection and length/weight of fishes at the Lamingo dam, Plateau State, Nigeria. The work was conducted between May-August, 2008, using drag net and the catch transferred into plastic container and and transported to the laboratory where dissection was carried and examined microscopically. Three classes of parasites were recovered from both Clarias gariepinus and Tilapia zilli. A total of 399 parasites were recovered comprising 188 nematodes, 131 cestodes and 80 trematodes. A significant difference was observed in the parasite burden of the two fishes.

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Most humans carry mites in the hair follicles of their skin for their entire lives. Follicular mites are the only metazoans tha continuously live on humans. We propose that Demodex folliculorum (Acari) represents a transitional stage from a host-injuring obligate parasite to an obligate symbiont. Here, we describe the profound impact of this transition on the genome and physiology of the mite. Genome sequencing revealed that the permanent host association of D. folliculorum led to an extensive genome reduction through relaxed selection and genetic drift, resulting in the smallest number of protein-coding genes yet identified among panarthropods. Confocal microscopy revealed that this gene loss coincided with an extreme reduction in the number of cells. Single uninucleate muscle cells are sufficient to operate each of the three segments that form each walking leg. While it has been assumed that the reduction of the cell number in parasites starts early in development, we identified a greater total number of cells in the last developmental stage (nymph) than in the terminal adult stage, suggesting that reduction starts at the adult or ultimate stage of development. This is the first evolutionary step in an arthropod species adopting a reductive, parasitic or endosymbiotic lifestyle. Somatic nuclei show underreplication at the diploid stage. Novel eye structures or photoreceptors as well as a unique human host melatonin-guided day/night rhythm are proposed for the first time. The loss of DNA repair genes coupled with extreme endogamy might have set this mite species on an evolutionary dead-end trajectory.

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