THE INFECTIVITY, GROWTH, AND VIRULENCE OF THE CESTODE SCHISTOCEPHALUS-SOLIDUS IN ITS FIRST INTERMEDIATE HOST, THE COPEPOD MACROCYCLOPS-ALBIDUS

In an experiment to study the infectivity, growth and virulence of Sch istocephalus solidus in their first intermediate host, copepods of the species Macrocyclops albidus were kept singly and exposed to up to 9 coracidia. Eleven or 14 days post-infection (p.i.) the presence and gr owth of the cestode larvae relative to survival, growth and reproducti on of their host was determined. As expected, the probability of a cop epod becoming infected increased with increasing numbers of parasites administered. However, the chances of a single coracidium establishing in a copepod also increased with increasing numbers of coracidia admi nistered, which indicates that the parasites profit from a dilution ef fect of the host's defence. Copepod size or developmental stage had no significant effect on the infection, but 14 days p.i., constraining e ffects of copepod size on the growth of the parasites were apparent. M oreover, procercoids in multiple infections grew smaller and developed their cercomer at a smaller size than those in single infections. No significant effect of the parasite on host mortality was found within the observation period. However, growth between the 5th copepodid stag e and adult stage was negatively affected by infection. An infection w ith S. solidus was also strongly linked with host reproduction: infect ed females were more likely to bear an egg sac at the end of the exper iment than non-infected ones. These egg sacs, however, contained fewer eggs.