Physiological analysis of cercarial behavior.

The behavior of trematode cercariae was made accessible to physiological analyses by splitting the continuous flow of behavior into separate units of behavior patterns. Such phases include active, passive, and resting phases in the intermittent swimming mode, attachment to the host, remaining on the host, directed creeping to suitable entry sites, and penetration phases including penetration movements, tail shedding, tegument transformation, and secretion of enzymes. Each of these phases may be stimulated by separate environmental and host signals. The pattern of the responses and the chemical, thermal, mechanical, and visual stimuli have been described in some detail but only a few studies have dealt with the question of how these responses are coordinated by receptors and nervous systems. Highly specific and sensitive chemoreceptors for host signals such as carbon dioxide, L-arginine, fatty acids, and glycoproteins have been defined from cercarial behavior, but they have not yet been allocated to morphological structures. Analyses of cercarial behavior of 6 schistosomatid and 4 fish-infecting species revealed that the individual parasite species show different behavioral patterns and respond to very different host signals, even though they infect congeneric hosts. The adaptive benefits of such behavioral diversity and complexity still are to be elucidated.