Environmental effects on transmission of Discocotyle sagittata (Monogenea): egg production and development.

Egg production by the freshwater gill monogenean Discocotyle sagittata infecting rainbow trout (Oncorhynchus mykiss) was monitored at 5, 13 and 18 degrees C; eggs were incubated at 6, 10, 13, 15, 16, 18, 22 and 26 degrees C and the hatching times determined. Mean per capita output at 13 degrees C ranged from 0.8 eggs/worm/day (e/w/d) to 18.3 e/w/d for 25 hosts (burdens 1-17 worms/host). Average egg production rates for worms in single burdens varied from 3 e/d to 14.6 e/d at 13 degrees C, mean 7.6 e/d (+/- 0.63) (n = 8). Egg production was temperature dependent, increasing from a mean of 1.5 e/w/d at 5 degrees C to 12.0 e/w/d at 18 degrees C. Developmental rate was also temperature dependent: larvae began hatching after 84 days at 6 degrees C, 28 days at 13 degrees C, and 20 days at 18 degrees C. Eggs laid within the same 24 h period hatched over 3-7 days at 13-18 degrees C; this range was extended at decreasing temperatures. Egg viability was generally high: between 13 and 18 degrees C a mean of 92% of larvae successfully emerged. However, viability declined at the highest and lowest temperatures (23% hatching at 6 degrees C, 55% at 8 degrees C and 47% at 26 degrees C). The implications for seasonal parasite transmission are discussed.