LARVAL SURVIVAL CHARACTERISTICS AND BEHAVIOR OF THE GILL MONOGENEAN DISCOCOTYLE SAGITTATA

The survival characteristics of Discocotyle sagittata larvae are both age- and temperature-dependent. In laboratory studies at constant temp eratures of 6, 10, 13, 18 and 22 degrees C, oncomiracidia had a maximu m life-span of 96 h at 6 degrees C, declining with increasing temperat ure to 26 h at 22 degrees C. Larval swimming activity was also age dep endent, and continued for a large proportion of survival time. The rel ationship between time at which larvae stop swimming as a proportion o f total survival time was statistically significant between temperatur es. Proportionally, oncomiracidia swam for longer periods at lower tem peratures. As oncomiracidia age, they become progressively less active , spending less time in the water column. In contrast to observations recorded in the literature, D. sagittata larvae are infective soon aft er emergence (within the minimum of 2 h studied), and feed on blood fr om the gill capillaries within 2 h post-exposure. The temperature-depe ndent characteristics of the larval stage are likely to have important implications for seasonal changes in parasite transmission.