The ability of the nematode Anguillicola crassus to infect eel larvae (glass-eel stage) was tested. The results show that glass-eels fed on infected copepods, the natural intermediate host of the nematode, can be infected. Light microscopical
examination of the infected developing swimbladder tissue revealed that the infection results in a significant thickening
of the connective tissue. The basolateral labyrinth of gas gland cells is very much reduced in infected swimbladders, and
the distance of gas gland cells to blood capillaries is enlarged. Critical swimming speed, defined as the speed where the
larvae were no longer able to swim against the current, was similar in infected and uninfected animals. At intermediate
speeds (about 60–80% of critical swimming speed) infected eels showed a slightly higher swimming activity than control
animals. Resting oxygen consumption, measured as an index of metabolic activity, within the first 2 months of infection
was higher in control animals, which may be due to a reduced rate of activity in infected glass-eels. By 4–5 months after
the infection, however, it was significantly higher in infected animals. This may indicate that at this stage a higher activity
of the animals is required to compensate for the increase in body density, but swimming performance of infected and non-infected glass-eels was not significantly different. Oxygen consumption during swimming activity, measured in a swim
tunnel at 50% of maximal swimming speed, also was not affected. The results thus show that even glass-eels can be
infected with A. crassus, and this probably contributes to the rapid spread of the nematode in Europe. While aerobic
metabolism during swimming activity is not affected at this stage of infection, the swimbladder tissue shows severe
histological changes, which most likely will impair swimbladder function.
Use of metabolism and swimming activity to evaluate the sublethal toxicity of surfactant (LAS-C12) on Mugil platanus
