Feeding response of a benthic copepod to ciliate prey type, prey concentration and habitat complexity

Abstract

1. Protozoans are important consumers within microbial food webs and, in turn, they represent potential prey for small metazoans. However, feeding interactions within these food webs are rarely characterised and this is especially true for freshwater sediments. 2. We aimed to quantify the feeding links between a freshwater meiofaunal copepod and ciliates in two laboratory experiments. The first experiment addressed the response of Eucyclops serrulatus towards ciliate density and type (two ciliate species of the same genus differing in terms of body size). A second experiment assessed the effect of habitat structure on feeding rates by introducing different structural complexity into the feeding arena. In contrast to the first experiment, which was run only for one time period, this experiment also tested three different total feeding times (4, 7 and 9 h). 3. Eucyclops serrulatus exhibited high ingestion rates, with 3-69 ciliates copepod )1 h )1 consumed depending on food concentration, food type and habitat complexity. Copepods exhibited a preference for the smaller ciliate when total ciliate concentration was low, but selected both ciliates equally when food concentrations were medium or high. However, at very high food concentration, Eucyclops preferred the larger ciliate (which was 1 ⁄ 3 of its own body size), suggesting that the longer handling times of the larger prey are rewarding when the large prey is present in high numbers. In terms of total numbers consumed, copepods fed on more small ciliates, but in terms of carbon units both ciliates were selected equally when total prey concentration was low or medium. However, copepods derived more carbon from the larger prey at high and very high prey concentrations (up to 0.7 lgC out of a maximum of 1.1 lgC copepod )1 h )1 ). Habitat complexity influenced the feeding of copepods when it was observed over time. 4. The copepod-ciliate link is well known from the pelagic zone of both marine and freshwater habitats. We have shown its potential importance within the benthos, where it can be influenced by food identity, food quantity and possibly by habitat complexity.

Selective predation by a harpacticoid copepod on ciliates in phytotelmata: a laboratory experiment

“…This allowed E. serrulatus to maximize its nutritional benefit in spite of the longer handling times of the larger prey. The ingestion rates observed for the adult harpacticoid in the present study (Table 4) are within the range measured by Rieper (1985) for a marine harpacticoid (11-65 ciliates copepod -1 h -1 ) and that reported by Reiss & Schmid-Araya (2011) for E. serrulatus (3-69 ciliates copepod -1 h -1 ).…”

“…but not against the platyhelminth Stenostomum sphagnetorum. Although the possible effectiveness of Paramecium trichocysts against harpacticoids remains to be investigated, it is well known that at least some cyclopoid species do feed on Paramecium (Reiss & Schmid-Araya, 2011). Also, some cyclopoid species of similar size to our copepod can be reared in mass culture on a diet containing Paramecium caudatum (Suarez et al, 1992).…”

“…It is a well known fact that the preference for a given prey may be affected by the relative abundance of alternative prey. Reiss & Schmid-Araya (2011) demonstrated that the adult Eucyclops serrulatus selected for the larger ciliate from the two species available only when the large species was present in high numbers. This allowed E. serrulatus to maximize its nutritional benefit in spite of the longer handling times of the larger prey.…”

“…Tukey's test was used to estimate the statistical significance of the differences between treatment pairs. The ingested ciliate biomass was estimated following the method described by Reiss & Schmid-Araya (2011).…”

“…In contrast to the plankton, the relationship between benthic copepods and ciliates has been much less studied (Reiss & Schmid-Araya, 2011;Cnudde, 2013). In particular, little is known about harpacticoid feeding in freshwater habitats (Sarvala, 1998).…”