Threat or treat: The role of fish exudates in the growth and life history of Daphnia

Abstract

In kairomone studies, it is often implicitly assumed that the only effect of predator exudates on their prey is to trigger anti-predator defenses. However, chemicals originating from fish activity may also fertilize the environment and enhance the growth of zooplankton prey by increasing bacterial food availability. It is necessary to separate these two effects in order to examine the adaptive significance of zooplankton anti-predation defenses and the ability of the prey to benefit from fish-related food. Here, we have employed differential filtration of media to permit assessment of these two effects on growth rate and life history adjustments in two Daphnia species that differ in body size and the ability to collect small particles (greater in small-bodied D. cucullata than in larger D. hyalina). Filters of three mesh sizes were employed: 2 μm (most bacteria pass, but detritus is retained), 0.45 μm (standard) and 0.20 μm (most bacteria and detritus retained). The concentration of kairomones in the fish medium was assumed to be unaffected by filtration. However, the abundance of bacteria in the fish and no-fish media was dependent on the filter mesh size. This influenced the food level and its impact on Daphnia growth rate and life history. The effect of fish-related bacterial food was greater under low than under high algal food conditions (0.1 and 1.0 mg C/L) and more significant in D. cucullata. Use of media with a diminished bacterial abundance revealed the effect of kairomones alone as (1) reduced growth rate, size at reproduction, number of pre-adult instars, age at maturation, level of the integrated egg and body lipids, egg volume, and (2) increased number of eggs per clutch vs. body length. These effects were equally strong in both species, despite differences in body size, and were greater at the higher algal food concentration. The findings of this study indicate the need to re-analyze and possibly revise the results of previous kairomone studies, especially those using media filtered through mesh sizes coarser than 0.2 μm that were performed under limiting algal food conditions or with species particularly able to benefit from bacterial food.