Microecology and Virulence of Some Pathogens in Gill Biotope of the Catfish Host, Clarias gariepinus (Burchell, 1822): Does Organism Body Size Matter?

This study investigated the diverse metazoan pathogens inhabiting the gill biotope of the African sharptooth catfish, Clarias gariepinus. The pathogens identified include the viviparous gyrodactylid monogeneans Macrogyrodactylus clarii and Gyrodactylus rysavyi, the copepod Ergasillus sieboldi, and oviparous monogeneans from the genus Quadriacanthus. Among these, E. sieboldi was the largest, measuring 2500μm (2200-2700) in length, followed by M. clarii at 1950μm (1900-2000) and G. rysavyi at 1500μm (1350-1600). In contrast, Quadriacanthus was the smallest, measuring 500μm (430-560), while encysted metacercaria were 200μm (180-220) long. Notable variability in spatial distribution and population sizes of these pathogens suggests that factors beyond space and food supply may influence their occurrence. The study revealed significant species-specific differences in morphological features, feeding behaviors, attachment organ design, and reproductive strategies. Monogeneans primarily feed on gill epithelium, have simple life cycles, and utilize haptoral attachments, while E. sieboldi consumes blood from gill capillaries, has a complex twelve-stage life cycle, and employs specialized segmented antennae. Spatially, Quadriacanthus prefers the distal areas, while E. sieboldi and M. clarii inhabit proximal regions, with Gyrodactylus favoring the borders of the gill filaments. Two primary factors influencing this distribution are the specific demands of each parasite species and their interactions. Additionally, unlike Quadriacanthus and Macrogyrodactylus, Gyrodactylus and E. sieboldi exhibit skillful swimming behaviors, though all monogeneans exhibit leech-like movement. Lastly, the impact of encapsulated digenean metacercarial cysts on the gill's resident organisms remains to be explored.