THE DISTRIBUTION OF THE METACERCARIAE OF DIPLOSTOMUM-PHOXINI IN THE BRAIN OF MINNOWS, PHOXINUS-PHOXINUS

Quantitative data are presented on the spatial distribution of metacer cariae of the digenean trematode Diplostomum phoxini (Faust, 1918) in the brains of minnows, Phoxinus phoxinus(linnaeus, 1758), from two Sco ttish populations. Sequential examination of serial histological secti ons revealed metacercariae to be unevenly distributed throughout the b rain, aggregating in specific regions including the cerebellum, the me dulla oblongata and the optic lobes. In addition, a number of metacerc ariae were found in the anterior part of the spinal cord. The inferior lobe of the cerebellum, pituitary, olfactory lobes and olfactory bulb s were largely free of metacercariae. Reasons for the uneven distribut ion of metacercariae within the brains of infected minnows are discuss ed, including the possibility that the parasite may have evolved to en hance its transmission to subsequent hosts by aggregating in regions k nown to be important in the control of the host's antipredator respons es.