Chaoborus predation and sex-specific mortality in a copepod

The susceptibility to Chaoborus predation in males and in females with and without eggs of the freshwater copepod Eudiaptomus gracilis was studied in held enclosures and by videotaping and analyzing the predation cycle in the laboratory. Female mortality exceeded male mortality both in the field and in the laboratory. Chaoborus predation may therefore contribute to skewed adult sex ratios in diaptomid populations. Clutch size and the frequency of females carrying eggs was higher in predator-exposed field populations, but this may have been an indirect effect of a higher phytoplankton standing stock. There was no mortality difference between females with and without eggs in the laboratory. The attack rate did not differ between prey types, and all captured prey were ingested. The lower male mortality was due to a significantly lower efficiency in strikes directed against them. Analysis of the attack events revealed that swimming angle of prey in relation to the Chaoborus larvae affected the probability of being captured. Differences in escape ability, size, and morphology may also contribute to the sex-specific mortality.