Density-dependent effects on Anguillicola crassus (Nematoda) within its European eel definitive host

Density-dependent effects on adult and larval stages of the introduced nema tode parasite Anguillicola crassus in the European eel definitive host were examined in the laboratory in naturally infected fish, and in field sample s. The effect of adult nematode subpopulations on larval development over t ime and the effect of increasing adult intensity on the gravid female subpo pulations were investigated. At high adult subpopulations the movement of l arvae from the swimbladder wall into the swimbladder lumen appears to be in hibited, and A. crassus larvae to be arrested in development in a density-d ependent manner. The number of gravid females per host reaches a constant l evel, and so the proportion of gravid female nematodes per adult subpopulat ion decreases relative to further increasing total adult nematode numbers. Both these mechanisms have the potential to regulate infrapopulations of A. crassus within the eel definitive host and thus the parasite suprapopulati on.