Compensatory or ‘catch-up’ growth following prolonged periods of food shortages is known to exist in many free-living animals. It is generally assumed that growth rates under normal circumstances are below maximum because elevated rates of growth are costly. The present paper gives experimental evidence that such compensatory growth mechanisms also exist in parasitic species. We explored the effect of periodic host unavailability on survival, infectivity and growth of the fish ectoparasite Argulus coregoni. Survival and infectivity of A. coregoni metanauplii deprived of a host for selected time periods were age dependent, which indicates that all metanauplii carry similar energy resources for host seeking. Following the periods off-host, metanauplii were allowed to settle on rainbow trout and were length measured until they reached gravidity. During early development on fish, body length of attached A. coregoni was negatively correlated with off-host period indicating a mechanism that creates size variance in an attached parasite cohort originally containing equal amounts of resources. However, over time the size differences between parasites became less pronounced and eventually parasites that were kept off-host for longest periods of time reached the length of those individuals that had been allowed to infect a host sooner. A. coregoni thus appears to compensate for delayed growth resulting from an extended host searching period by elevated growth rates, although we show that such accelerated growth incurred a cost, through decreased life-expectancy.

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°C, oncomiracidia had a maximum life-span of 96 h at 6°C, declining with increasing temperature to 26 h at 22°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.