Ciliate epibionts associated with crustacean zooplankton in german lakes: distribution, motility, and bacterivory

Abstract

Ciliate epibionts associated with crustacean zooplankton are widespread in aquatic systems, but their ecological roles are little known. We studied the occurrence of ciliate epibionts on crustacean zooplankton in nine German lakes with different limnological features during the summer of 2011. We also measured the detachment and re-attachment rates of the ciliates, changes in their motility, and the feeding rates of attached vs. detached ciliate epibionts. Epibionts were found in all lakes sampled except an acidic lake with large humic inputs. Epibiont prevalence was as high as 80.96% on the cladoceran Daphnia cucullata, 67.17% on the cladoceran Diaphanosoma brachyurum, and 46.67% on the calanoid copepod Eudiaptomus gracilis. Both cladoceran groups typically had less than 10 epibionts per individual, while the epibiont load on E. gracilis ranged from 1 to >30 epibionts per individual. After the death of the zooplankton host, the peritrich ciliate epibiont Epistylis sp. detached in an exponential fashion with a half-life of 5 min, and 98% detached within 30 min, leaving behind the stalks used for attachment. Immediately after detachment, the ciliates were immotile, but 62% became motile within 60 min. When a new host was present, only 27% reattached after 120 min. The average measured ingestion rate and clearance rate of Epistylis were 11,745 bacteria ciliate(-1) h(-1) and 24.33 μl ciliate(-1) h(-1), respectively. Despite their high feeding rates, relatively low epibiont abundances were observed in the field, which suggests either diversion of energy to stalk formation, high metabolic loss by the epibionts, or high mortality among the epibiont populations.

Keywords: Epistylis; bacterivory; ciliate epibionts; crustacean zooplankton; epibiont motility.

Figure 1

(A) The general study region in northeast Germany is outlined by the small square box, which is enlarged in (B). Three Lakes sampled are denoted by black dots.

Figure 2

Examples of the peritrich ciliate epibiont Epistylis sp. attached to the calanoid copepod Eudiaptomus gracilis (A,B) and the residual stalk (C) left attached to the dead copepod after detachment (D). Peritrich ciliates of the family Opercularidae, attached to a cyclopoid copepod (E).

Figure 3

Prevalence of ciliate epibionts (percent of the zooplankton population with one or more ciliate epibionts; mean ± SD, n = 3) on common zooplankters Eudiaptomus gracilis, Daphnia cucullata, Diaphanosoma brachyurum, and Cyclopoid copepods in each lake. 0 = no epibionts were found on that zooplankton group, X = zooplankton group was not present in that lake, nd = no data.

Figure 4

Frequency distribution of epibiont load (number of epibionts per individual, mean ± SD, n = 3) on E. gracilis, D. cucullata, D. brachyurum, and cyclopoid copepods. Epibiont load is presented only for lakes where epibionts were found on >5% of the zooplankton populations.

Figure 5

Percentage of epibionts remaining on zooplankton host, E. gracilis after host death (A), motility of epibionts detached from copepod carcasses (B), and re-attachment of free-swimming ciliates to new copepod hosts (C). All data points are mean values ± SD (n = 10).

Figure 6

Estimated ingestion rates (black bars, left axis) and clearance rates (white bars, right axis) of the peritrich ciliate Epistylis sp. when attached to the calanoid copepod Eudiaptomus gracilis collected from Lake Stechlin and Lake Dagow (mean ± SD, n = 5).