so many authors have studied clinostomum parasites , for example , the few recent studies like ( 14 ) are well documented .
however , record on the occurrence of co - infection among three clinostomum species is unknown .
the present study is the first record of co  infection of three clinostomum species and report on the parasites of opi lake .
29 35 e is a tropical freshwater lake located in the valley of river uhere , northeast of nsukka , nigeria .
the lake is about 300 meters from uhere river , the soil is porous and subject to severe erosion ( 5 ) .
however , the best knowledge about the lake was information on the systematic and basic biology of the species of organisms other than parasites found there .
for instance , the climate , hydrobiology , macro invertebrates , and vegetation of the dominant flora have been described ( 69 ) .
therefore , investigation into its parasites and description of co  infection by three clinostomum species was necessary .
species of tilapia zilii ( 10 ) were collected using multiple fishing gear ; cast nets , hook and line , and seine nets ( 150 mm  200 mm ) monthly for twelve months from nov 2007  oct 2008 .
rank - abundance and species diversity of the parasites were determined using quantitative analysis of shannon - wienner index ( 14 ) .
three metacercariae of clinostomatids were discovered to be the parasites associated with the t. zilii of opi lake .
the parasites include : clinostomum tilapiae ukoli , 1966 , c. complanatum rudolphi , 1819 , and euclinostomum heterostomum rudolphi , 1809 .
c. complanatum ( 9.4 % ) , e. heterostomum ( 10.4 % ) , and c. tilapiae ( 4.8 % ) with mean intensity ; c. complanatum ( 4.2 ) , c. tilapiae ( 4.6 ) , and e. heterostomum ( 2.1 ) ( table 13 ) .
out of 392 t. zilii examined , 38 were infected with c. complanatum ; c. tilapiae 19 , and 41 hosts had e. heterostomum .
c. complanatum and c. tilapiae were equally abundant ( pi 0.26 ) , while e. heterostomum was most abundant ( pi 0.48 ) ( fig .
1 ) . three major microhabitats  buccal cavity , eye , and skin were seriously affected .
in addition , the infection of the buccal cavity had the highest infection than the other two microhabitats of the eye and skin . implications of their presence caused pronounced inflammation as well as roughening of the skin by the encysting metacercariae .
out of 85 e. heterostomum only 15 were not excysted , 57 c. tilapiae and 112 of c. complanatum were encysted respectively .
these excysted forms caused serious damages to the infected fish as they burrowed through the organs of the host .
damages to the skin , blindness and bumps on the skin could affect the palatability and marketability of the infected fish as well as the acceptance of fish as the primary source of animal protein .
for instance , nigeria with population of over 120 million individuals is the largest consumer of fish in africa and fish serve as primary source of animal protein to this huge population .
the crowding population of opi around the lake is also prone to the inflammation of the naso - pharynx known as pharyngitis .
this is because ingestion of under cooked fish with clinostomum infection results in the attachment of the trematode on the pharynx .
spatial adaptation of theses clinostomatids in the various microhabitats of t. zilii indicate selection for relatively better adaptiveness , and host location .
consequently , this resulted in a trade - off among excysted individuals , encysted individuals , damages to organs by scavenging large population size of the parasites in the host .
adaptation is a heritable trait that either spread because of natural selection or has been maintained by selection to the present or currently spreading relative to alternative traits because of natural selection . in all such cases ,
the trait in question has conferred and continues to confer or is just beginning to confer higher genetic or reproductive success on c. complanatum with highest population than the other two species of the parasites . even with more encysted forms ,
adsorption of nutrients such as free proteins , amino acids and transaminases was readily very successful ( 15 ) .
fitness is a measure of an individual s reproductive or genetic success , so that  fitness benefit refers to the positive effect of a trait on the number of surviving offspring produced by an individual or the number of genes it contributes to the next generation whereas  fitness cost refers to the damaging effects of the trait on these measures of individual genetic success ( 16 ) . although , encysted larvae can feed from the host , excysted larvae could be more voracious feeder .
these suggestive pressures on natural selection would favor e. heterostomum in t. zilii during the cause of time and possible domination in the fish hosts in this freshwater lake . also , because the parasites do not depend on stored food of the host ; the nutrients they can obtain from their intermediate hosts are sustainable .
it might last throughout the lives of the host fish due to difficulty in locating their definitive hosts , piscivorous birds such as egreta egreta ( 17 ) .
ethical issues ( including plagiarism , informed consent , misconduct , data fabrication and/or falsification , double publication and/or submission , redundancy , etc ) have been completely observed by the authors .
 the present study investigated the effects of first record of co - infection of three clinostomum sp . ; clinostomum complanatum ( rudolphi , 1819 ) , c. tilapiae ( ukoli , 1966 ) , and euclinostomum hetereostomum ( 1809 ) in tilapia zilii . 
 there was differential parasitic effects resulting in selection for relatively better adaptiveness to host s microhabitats , more population size , and frequent host location of these parasites during the one year survey ( nov 2007oct 2008 ) in opi lake , nigeria . 
 prevalence of 9.4 % was recorded in c. complanatum , 10.4 % in e. heterostomum and 4.8% in c. tilapiae . 
 the parasites were recovered from three major microhabitats of buccal cavity , skin and eye . 