Control mechanisms of photosynthetic epibionts on zooplankton: an experimental approach

Andrea Bertolo,Marco A Rodríguez,Gérard Lacroix

doi:10.1890/es14-00451.1

Abstract

Several top‐down and bottom‐up forces have been put forward to explain variable infestation rates of zooplankton by epibionts. Among top‐down forces, fish predation affects epibiont prevalence on zooplanktonic organisms, either by eliminating more conspicuous, heavily burdened individuals, or by reducing population size of zooplankton hosts, with consequences for substrate availability for epibionts. However, detailed experimental‐based information on the effects of top‐down forces is still lacking. Among bottom‐up forces, light can potentially control populations of photosynthetic epibionts. Therefore, both changes in light penetration in the water column and the vertical position of hosts in the water column could affect the photic conditions in which epibionts live and could thus control their population growth. We tested experimentally the hypothesis that both light limitation and fish predation affect epibiont burden on zooplankton. Moreover, we also tested the hypothesis that zooplanktivorous fish affect the prevalence and burden of the epibiotic alga Colacium sp. (Euglenida) on zooplankton not only by direct predation, but also by affecting the vertical distribution of zooplankton. We analyzed Colacium burden on two zooplankton genera that responded differently to the presence of zooplanktivorous fish by altering their daytime vertical distributions, thus exposing photosynthetic epibionts to different light conditions. Colacium burden on the two zooplankton genera was also compared between enclosures with different degrees of light limitation. Our results suggest that (1) ambient light limitation has the potential to reduce the burden of photosynthetic epibionts on zooplankton in natural conditions, and (2) zooplankton behavior (e.g., daytime refuge use to escape fish predation) can reduce the burden by exposing photosynthetic epibionts to suboptimal light conditions.

Highlights

Epibiosis is a form of inter-specific interaction that is commonly found in aquatic ecosystems (e.g., Cattaneo and Kalff 1980, Bickel et al 2012).It could result in parasitism, mutualism or commensalism (e.g., Decaestecker et al 2005) and could have important direct communitywide effects, such as the collapse of host populations due to shading (Sand-Jensen 1977)
Whereas for Ceriodaphnia this relationship seems to be weak (CI including the zero value), the evidence is much stronger for Diaphanosoma, whose coefficients for body length differ from zero (Tables 2 and 3)
Ceriodaphnia epibiont burden did not respond to the variations in enclosure depth (Table 2: coefficients including the ‘‘enclosure size’’ factor)

Summary

Introduction

Epibiosis is a form of inter-specific interaction that is commonly found in aquatic ecosystems (e.g., Cattaneo and Kalff 1980, Bickel et al 2012) It could result in parasitism, mutualism or commensalism (e.g., Decaestecker et al 2005) and could have important direct communitywide effects, such as the collapse of host populations due to shading (Sand-Jensen 1977). For example, alter host vulnerability to predators which can, in turn, have an impact on food-web dynamics (Wahl et al 1997, Wahl 2008a) Despite these potential effects, epibiosis has attracted relatively little interest in ecology and much of our knowledge on this form of parasitism still stems from case studies (e.g., Hanamura 2000, Regali-Seleghim and Godinho 2004). Our knowledge of the effects of epibionts on hosts is expanding (e.g., Stirnadel and Ebert 1997, Barea-Arco et al 2001), very little is known about the effects of hosts (e.g., in terms of abundance or behavior) on epibiont populations

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