The behavior of parasitic flatworms in vivo: what is the role of the brain?
The ecological interactions that contribute to successful host-parasite relationships are complex and involve all levels of biotic organization between the participants. At the level of parasites living within their hosts, it is felt that the parasite's environment is predictable because of host mechanisms maintaining biochemical and physiological homeostasis. It is hypothesized that fixed behavior patterns in the parasites will evolve under these specialized conditions. Current thinking on fixed behaviors in invertebrates holds that they are generated by specialized neural circuits in the brain. Therefore, it can be expected that the brains of parasitic flatworms will have important roles in the control of the organisms' behaviors. However, in the tapeworm Hymenolepis diminuta, complex fixed patterns of behavior, associated with locomotion and migration, are not affected by the removal of the worm's brain. This suggests peripheral, and not central, control of fixed behaviors. In Fasciola hepatica, at least 6 distinct fixed patterns of behavior are responsible for guiding the worm to its final habitat in the liver. Giant neurons and other phylogenetically advanced features develop in the adult worm's brain after the expression of the sequence of distinct migration behaviors. Yet, there is no apparent new locomotory behavior, corresponding to the new advanced brain, as the parasite assumes its placid life-style as a hematophage in the bile duct. Removal of the adult brain of this parasite also does not appear to affect worm locomotory activity. Thus, the regulation and control of locomotion may not be the only important roles for the brains of parasitic flatworms. It is suggested that neuroethological approaches may hold the key to understanding the biology of these parasites.
Sukhdeo
M
M
Department of Animal Sciences, Rutgers University, New Brunswick, New Jersey 08903.
D000818:Animals; D001921:Brain / Q000502:physiology; D005210:Fasciola hepatica / Q000502:physiology; D006790:Host-Parasite Interactions; D006926:Hymenolepis / Q000502:physiology; D010986:Platyhelminths / Q000502:physiology*