Individual feeding variability of protozoan and crustacean zooplankton analyzed with flow cytometry’

Feeding of protozoan and crustacean zooplankton was analyzed by flow cytometry (FCM), resulting in direct counts of particles ingested by each individual. Ciliates (Cyclidium sp.) were allowed to feed on Microcystis aeruginosa and analyzed directly on a flow cytometer for cyanobacterial autofluorescence coincident with each ciliate. Numbers of Microcystis cells ingested (e.g. 0, 1, 2, 3, or 4 ciliate-‘) were estimated from the total fluorescence intensity of each ciliate. Crustaceans (Daphnia magna and Bosmina tubicen) were fed fluorescent beads or Microcystis, sonicated to release their gut contents, and the resulting suspension from each individual was quantitatively analyzed by FCM. These methods allow estimation of several aspects of feeding behavior, including particle selectivity, mechanisms of particle capture, gut volume of small organisms, feeding and clearance rates, and individual variation of the preceding parameters. Flow cytometry is more rapid than direct counts by microscope and more accurate than many indirect methods. Feeding habits of aquatic animals can be analyzed by direct or indirect methods (see Peters 1984). Direct methods, such as gut content analysis, yield precise results for individual animals, but are laborious and time-consuming, often prohibitively so for small organisms such as zooplankton (e.g. Gerritsen and Porter 1982). This situation often leads to restriction of sample size and replication. Indirect methods involve counting prey items in the water column before and after exposure to feeding animals (Peters 1984). These methods generally use several feeding animals per experimental sample and automated counting methods, making them rapid and economical, but precluding information on individual variation. They may also introduce errors unrelated to feeding due, for example, to growth 1 This research was supported by NSF grants BSR 841585 1 (J.G.), BSR 84-07928 (K.G.P.), and the University of Georgia Program for Biotechnology. C. M. Yentsch and an anonymous reviewer commented on the manuscript. Lake Oglethorpe Limnological Association Contribution 30. and mortality of prey during the experiment. Variation among individuals is an important biological phenomenon (Mayr 19 8 2); observations of only population mean or aggregate behaviors lose potentially relevant information. For instance, nonselective feeding by all members of a population may produce similar aggregate behavior as varying selection among individuals within the population. Individual specialization on different food species is fundamentally different from nonselective behavior and can be an important factor in the autecology of animals. The introduction of flow cytometry (FCM) to the aquatic sciences has led to the development of techniques to rapidly count and sort phytoplankton (e.g. Trask et al. 1982; Yentsch et al. 1983; Wood et al. 1985) and to the improvement of indirect feeding analysis (Cucci et al. 1985; Stoecker et al. 1986). We report here analysis of feeding by individual zooplankton and protozooplankton using direct counts of ingested material on a flow cytometer. These direct counts are more rapid and require less sample processing than microscope direct counts. Large numbers of samples can be run to yield information on individual variability in feeding habits. Fluorescent microspheres have become increasingly popular in research on planktonic trophic interactions. Beads are not food items, and organisms may not respond to them in the same way as to food. The microspheres are useful, however, in that they are easily counted, well defined, and can be manipulated to elucidate feeding mechanisms (e.g. Gerritsen and Porter 1982; Hessen 19 8 5). In our investigations here we used both fluorescent beads and living cyanobacteria, Microcystis aeruginosa, as food for zooplankton. The ciliate Cyclidium sp. was isolated from Lake Oglethorpe, Georgia, and maintained in a 0.1% Cerophyll medium (w/v)