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

Abstract

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 lm (most bacteria pass, but detritus is retained), 0.45 lm (standard) and 0.20 lm (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 fishrelated 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 lm that were performed under limiting algal food conditions or with species particularly able to benefit from bacterial food.

Selectivity by planktivorous fish at different prey densities, heterogeneities, and spatial scales

“…Each of the three predictions was confirmed by the analysis of data from experiments with young European roach, a typical size-selective predator-harvester, and a medium-sized cladoceran, Daphnia hyalina, that has been used as a prey in many earlier studies with roach (Gliwicz and Wrzosek 2008;Maszczyk and Bartosiewicz 2012), including experiments on prey-size selectivity (Gliwicz et al 2010(Gliwicz et al , 2012(Gliwicz et al , 2013. The choice of this prey species was based on the findings of preliminary experiments that demonstrated strong positive size selectivity in young roach for egg-carrying females of D. hyalina, as compared with weak selectivity for smaller adults of D. cucullata (possibly too small a difference in net gain between juveniles and adults) and negligible selectivity for large-bodied adults of D. magna, which could stem from the gape limitation of young roach.…”

Predator chemical cues increase growth and alter development in nauplii of a marine copepod

“…Incubation of fish inevitably increases the level of nitrogen and bacteria in the water, which could potentially enhance growth conditions (Maszczyk & Bartosiewicz 2012). However, even for small copepod nauplii, bacteria are well below the optimal prey size range (Berggreen et al 1988).…”

The reaction distance of a planktivorous fish (Scardinius erythrophthalmus) and the evasiveness of its prey (Daphnia pulex × pulicaria) under different artificial light spectra

“…per day), we used juveniles (instead of adults), which were easier to obtain in a shorter time period. It has been demonstrated that juveniles also clearly respond either to light cues (Pijanowska et al 2006) and fish kairomones (Maszczyk and Bartosiewicz 2012).…”