Larval survival characteristics and behaviour of the gill monogenean Discocotyle sagittata.

The survival characteristics of Discocotyle sagittata larvae are both age- and temperature-dependent. In laboratory studies at constant temperatures of 6, 10, 13, 18 and 22 degrees C, oncomiracidia had a maximum life-span of 96 h at 6 degrees C, declining with increasing temperature to 26 h at 22 degrees C. Larval swimming activity was also age dependent, and continued for a large proportion of survival time. The relationship between time at which larvae stop swimming as a proportion of total survival time was statistically significant between temperatures. Proportionally, oncomiracidia swam for longer periods at lower temperatures. 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 after emergence (within the minimum of 2 h studied), and feed on blood from the gill capillaries within 2 h post-exposure. The temperature-dependent characteristics of the larval stage are likely to have important implications for seasonal changes in parasite transmission.