Optimal foraging theory as a predictor of chemically mediated food selection by suspension-feeding copepods

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1 Limnol. Oceanogr., 34(l), 1989, , by the American Society of Limnology and Oceanography, ITIC. Optimal foraging theory as a predictor of chemically mediated food selection by suspension-feeding copepods William R. DeMottl Department of Physiological Ecology, Max Planck Institute of Limnology, Plijn, F.R.G. Abstract Chemically mediated food selection was studied by offering copepods pairs of particles which differed in nutritional quality or size. Eudiaptomus spp. fed in varying concentrations of algae and polystyrene spheres, high-quality algae of different sizes, toxic and nontoxic algae, digestible and digestion-resistant algae, live and dead algae, and dead algae with and without attached bacteria. Experiments with polystyrene spheres showed that the rejection of nonfood particles did not interfere with feeding on relatively scarce algae. Particle selection in mixtures of algae exhibited good agreement with predictions of optimal foraging theory. Selectivity did not change with food concentration for mixtures of high-quality algae or mixtures of toxic and nontoxic algae. Copepods showed a weak but consistent preference for the larger of a pair of high-quality algae, whereas a toxic blue-green bacterium was eaten at a very low rate whether offered alone or in combination with an edible alga. Selection against low-quality nontoxic particles (e.g. digestion-resistant algae), however, was sensitive to food concentration. Discrimination against low-quality particles was strong when high-quality food was abundant and weak when high-quality food was scarce. In agreement with theory, diet selection by Eudiaptomus spp. is strongly influenced by both particle quality and the abundance of alternative foods. Optimal foraging theory proposes that evolution will favor behaviors that maximize net nutritional gains. Optimal diet models make predictions about food selection within homogeneous patches. Most tests of optimal diet models have used carnivores or granivores, organisms whose prey are high in nutritional quality (reviewed by Stephens and Krebs 1986). For these organisms, diet selection is usually a compromise between small prey that can be rapidly captured and consumed and large prey that require longer handling times but provide more energy per item (e.g. Werner 1977; Elner and Hughes 1978). Theoretical models for suspension-feeding zooplankton, however, have assumed that handling time is negligible and that gut I Present address: Department of Biological Sciences, Indiana University-Purdue University at Fort Wayne, Acknowledgments This research was supported by a fellowship from the Max Planck Society for the Advancement of Science. Preparation of the manuscript was supported by a faculty grant from Indiana-Purdue University at Fort Wayne. I am grateful to W. Lampert and my coworkers in PlSn for assistance and advice. I also thank W. C. Kerfoot, M. Vanni, H. A. Vanderploeg, and several reviewers for critical comments on earlier versions of the manuscript. 140 processing places upper limits on ingestion rates (Lehman 1976; Lam and Frost 1976; Hughes 1980; Taghon 198 1). It follows from these assumptions that nutritional benefit can be maximized by food selection based on particle quality rather than particle size. For example, Lehman (1976) developed a model to predict how a filter feeder should select between particles that vary in nutritional quality. His model was designed specifically for algae that differ in digestibility, yet the model can be extended to include other low-quality particles, such as detritus, inert matter, and even toxic algae. The model presumes that the optimal filter feeder first captures particles and then rejects or ingests individual items after assessing their nutritional quality. The highest. quality food should always be ingested; whether lowquality items are eaten depends on the costs of rejection and on the abundance of higher quality foods. In common with other optimal diet models, Lehman s (1976) model pre#dicts that discrimination against low ranking foods should be strong when preferred foods are abundant and weak when preferred foods are scarce. Until recently, it was generally assumed that crustacean zooplankton collected food by mechanically sieving water through their filterlike feeding setae (Boyd 1976; Nival

2 Optimal foraging by copepods 141 and Nival 1976). Due to the lack of evidence for the handling and rejection of individual small particles, Lehman (1976) himself doubted whether his optimal diet model would be applicable to suspension-feeding zooplankton. Thus, attention focused on optimal ingestion rates and particle-size selection, leaving optimal diet theory untested. More recently, observations with highspeed cinematography have shown that Calanoid copepods can actively capture and manipulate individual particles (reviewed by Koehl 1984; Price in press). Moreover, food selection experiments have shown that calanoids can discriminate between algalflavored and unflavored artificial particles (Poulet and Marsot 1978; DeMott 1986, 1988), between algae and polystyrene spheres of similar size (Donaghay and Small 1979; Fernandez 1979; Huntley et al. 1983; DeMott 1988), and between live and dead algae of the same species (Starkweather and Bogdan 1980; Paffenhijfer and Van Sant 1986). Recent studies also demonstrate that marine calanoids discriminate against toxic dinoflagellates (Huntley et al. 1986) and that freshwater calanoids discriminate against noxious blue-greens (Fulton and Paerl 1987a). Thus, there is convincing evidence that copepods use chemical cues to assess the quality of individual food items-precisely the kind of ability required by optimal diet theory. Moreover, two studies provide preliminary evidence that discrimination against low-quality particles can be influenced by the abundance of higher quality foods. Paracalanus discriminated against a large diatom with low nitrogen content when a small, higher quality diatom was abundant (Paffenhijfer 1984) and Eudiaptomus spp. discriminated more strongly against algal-flavored microspheres when algae were abundant (DeMott 1988). None of these studies, however, posed explicit a priori predictions based on optimal foraging the- OrY- Here I examine the responses of Eudiaptomus spp. to mixtures of high- and lowquality particles. The first experiments use mixtures of algae and polystyrene spheres to test whether the rejection of inert particles interferes with feeding on algae. A lack of interference is considered support for the assumption that handling times and rejection costs are unimportant. A dual-label isotope technique (Lampert 1974; DeMott 1982) is used to quantify selection between pairs of algae that differ in size or nutritional quality. The effects of the concentration of food on selection behavior is tested by offering particles singly and in low- and highdensity mixtures. Methods Collection and maintenance of experimental animals-zooplankton were collected by vertical tows from Schiihsee, a mesotrophic lake in Pliin, F.R.G. The lake is inhabited by two morphologically and ecologically similar diaptomid copepods, Eudiaptomus gracilis and Eudiaptomus graciloides (Hofmann 1979). Eudiaptomus gracilis, the slightly larger species (cephalothorax length, mm), was more abundant. Similar morphology and overlap in body size prevented me from consistently separating the two species. Therefore, mixtures of these two species, including adults of both sexes and some C5 copepodites, were sorted under a dissecting microscope for use in laboratory feeding experiments. Additional experiments compared the feeding selectivity of adults and C5 copepodites of Eudiaptomus spp. with either C3 and C4 copepodites or Daphnia pulicaria in the same experimental beakers. Daphnia pulicuria was obtained from the culture collection of the Max Planck Institute of Limnology. Copepods were maintained in the laboratory in 2-liter jars with membrane-filtered (0.45 pm) lake water and fed a variety of cultured algae. Experiments were conducted from January through June Copepods collected during winter and early spring were gradually acclimated to increased temperatures over 2 d and then held at 15 C for a total of at least 5 but no more than 10 d before experiments. During late spring, zooplankton were collected 6-24 h before experiments and then kept at 15 C. Characteristics and culture of algae--a- ble 1 lists the sources and characteristics of the nine species of algae used in feeding experiments. Exponentially growing cultures

3 142 DeMott Table 1. The sources and characteristics of algae used in feeding experiments. Size is expressed as equivalent spherical diameter (ESD) and greatest linear dimension (length). - ESD Length Source* Food quality (w4 (w) Chlamydomonas reinhardi UT90 high 6 7 Chlamydomonas sphaeroides UG58.72 high Chlorella minutissima UG1.80 high 2 2 Scenedesmus acutus MPI high 5 12 Staurastrum punctulatum UG679-1 high Monoraphidium minutum UG243-1 high Microcystis aeruginosa MPZ toxic 5 5 Planktosphaeria gelatinosa UG262- la gelatinous Crucigenia tetrapedia UG218-3 gelatinous * UT-University of Texas Culture Collection; UG-University of Gijttingen Culture Collection; MPI-Max Planck Institute of Limnology. were prepared for experiments. Chlorella, Scenedesmus, and Monoraphidium were cultured in chemostats in a system similar to the one described by Lampert (1976); Staurastrum, Microcystis, Planktosphaeria, and Crucigenia were cultured in tubes with continuous bubbling, whereas both species of Chlamydomonas were cultured in Erlenmeyer flasks. A modified Chu 12 medium (Miiller 1972) was used for the nine green algae, and the medium of O Flaherty and Phinney (1970) for the sole blue-green, Microcystis. Cultures of Planktosphaeria were filtered through a 30-pm screen to remove large colonies. A Coulter counter (TA 11) was used to measure cell volumes and equivalent spherical diameters of the three irregularly shaped cells: Scenedesmus, Staurastrum, and Monoraphidium. Both Chlamydomonas species tended to shrink in the saline solution that is required for electronic particle counting and the Coulter counter did not properly measure the gelatinous sheaths of Crucigenia and Planktosphaeria. The diameters of these algae and Chlorella were measured with a microscope. Readily digested, nontoxic algae were classified as of high quality without regard to cell or colony size. Three algae were considered to be of low nutritional quality: Microcystis, Planktosphaeria, and Crucigenia. I used the same strain of Microcystis that Lampert (198 1) found to be toxic when ingested by D. pulicaria. Microcystis consisted primarily of single, spherical cells with occasional doublets. The gelatinous sheaths of Planktosphaeria and Crucigenia provide protection against digestion. Algae with ge- latinous sheaths are poorly assimilated (Porter 1976) and often survive gut passage (Porter 1975; Kerfoot et al. 1985). Experimental design -Two basic kinds of laboratory feeding experiments were conducted. The first involved mixtures of radioactively labeled algae and polystyrene spheres of the same size. The second examined selection between pairs of cultured algae (live or dead) that differed in size or nutritional quality. The ability of Calanoid copepods to reject polystyrene spheres (DeMott 1986, 1988; Paffenhijfer and Van Sant 1986) provides an opportunity to test whether the process of rejecting inert particles interferes with feeding on algae. Here the concentration of 14C-labeled Chlamydomonas reinhardi was kept constant while the concentration of 6-pm polystyrene microspheres was varied from 0 to lo4 spheres ml--l. Copepods were acclimated to unlabeled C. rcinhardi and then allowed to feed for 15 min in mixtures of microspheres and radioactively labeled algae. A zooplankton sandwich technique was used to determine feeding rates on microspheres and labeled algae for the same animals (DeMott 1988). Experiments in which one cell type was labeled with [14C]bicarbonate and the other with [32P]orthophosphate were used to quantify selection between algal foods. Preliminary feeding trials with mixtures of 14Cand 32P-labeled C. reinhardi revealed no evidence of bias due to isotope type (see Lampert for a similar test). As a further guard against potential biases, the iso- tope chosen to label each particle type (i.e. high- and low-quality particles) was varied between experiments. Since the amount of

4 Optimal foraging by copepods 143 labeled algae within animals is directly measured, radiotracer experiments can provide higher sensitivity and precision than cxperiments based on electronic or microscopic estimates of particle removal (Lampert 198 1; DeMott 1985). Other advantages of the dual-label technique include the ability to differentiate between particles of the same size, shape, and superficial appearance (e.g. live and dead algae of the same species; Starkweather and Bogdan 1980) and the possibility of comparisons between different developmental stages or species feeding within the same experimental vessels (e.g. DeMott and Kerfoot 1982; DeMott 1985). The basic design for dual-label experiments consisted of four treatments for each pair of particles: low quantities (0.125 ppm) of one particle alone, low quantities (0.125 ppm) of the alternative particle alone, mixtures of both particles in low but equal volumes (0.25 ppm total), and mixtures of both particles in high but equal volumes (2.0 ppm total; 1.0 ppm = lo6 pm3 ml- ). Volumes of and 1.O ppm represent about 25 and 200 pg C liter-l of high-quality algae. These food levels were chosen to bracket 100 pg C liter- -the incipient limiting (satiating) concentration estimated by Muck and Lampert ( 1984) for E. gracilis feeding on Scenedesmus. According to Lehman s (1976) optimal diet model, selection for particle quality should be strong at or above the incipient limiting concentration and weaker at lower food levels. Preparation of labeled particles- Methods for preparing radioactively labeled algae and for estimating their carbon contents and cell numbers followed DeMott (1988). In addition to experiments with pairs of algal species, the dual-label technique was used to test for discrimination between live and dead algae of the same species. Dead algae were considered models of detritus. Staurastrum and Scenedesmus were chosen for these experiments because their robust cell walls prevent changes in cell size or shape after death. Since the live and dead algae were identical in size and shape, selective feeding can be attributed to chemical differences. Two days before an experiment, cultures of labeled and unlabeled algae were killed by heating to about 80 C and then treated in one of two ways. In some experiments, cultures were opened, centrifuged, and resuspended in fresh medium soon after cooling. Within 2 d, bacteria were abundant and dead cells tended to form clumps. Clumps were broken down into single cells by brief (m 30 s) sonication immediately before experimental trials. In other experiments, cultures were heated and then kept sealed until just before the experiment. Bac- teria were not observed in these cultures, and algal cells did not tend to form clumps. Measurements on live and heat-killed cells revealed that dead cells lost about half of their radioactive label (32P or * 4C) and about half of their organic carbon within 2 d. Similar treatment of marine diatoms resulted in the loss of 30-50% of cell carbon and organic nitrogen (Paffenhafer and Van Sant 1986). Experimental protocol- In each experi- ment animals were placed in 500-ml glass bottles with membrane-filtered lake water (0.45 pm) and allowed to acclimate to experimental concentrations of unlabeled par- ticles for 2-6 h. E&h bottle received -2O- 30 copepods. To, begin a feeding trial, I transferred animals to a beaker containing a 500-ml suspension of labeled particles and allowed them to feed for 10 or 15 minperiods shorter than the time for gut passage (Muck and Lampert 1984). Animals were then anesthetized with carbonated water and sorted into scintillation vials within 5-l 5 min. Rapid sorting without fixation pre- vents isotope leakage (DeMott 1985; Lampert and Taylor 1985). Typically, two scintillation vials with 10-l 5 copepods each were processed for each beaker and there were three or four replicate beakers per treatment. Clearance rates were calculated by comparing the specific activities (dpm ind.- ) of animals with the specific activities (dpm ml-l) of suspended particles. Methods for dual-isotope scintillation counting and calculation of feeding rates followed Lampert and Taylor ( 198 5). Data analysis -Food selection was quantified by the selectivity coefficient cy (Chesson 1983). This index is equivalent to the W coefficient recommended by Vanderploeg and Scavia (1979) for expressing food selection by zooplankton. My experiments

5 144 DeMott h A -i Jz T -ci.c.3 z b.2 rii! E 5.l G a, G Microspheres (1 03/ml ) Fig. 1. The effect of varying concentrations of 6-vrn spheres on the clearance rate of Eudiuptomus spp. on 6-pm Chlamydomonas reinhardi. Symbols represent means (+ 1 SE) for three replicate beakers for low (O-800 cells ml-l) and moderate (a-3,000 cells ml-l) densities of algae. Least-squares regressions (dashed lines) from both low (r2 = 0.02) and moderate (r = 0.003) densities are not significant Diameter (ym) Fig. 2. Selection between high-quality algae of different sizes. Dual-label experiments paired Chlamydomonas reinhardi (6 pm; open bars) with either Chlamydomonas sphaeroides (12 pm; stippled bar) or Chlorelln minutissima (2 pm; solid bar). Eudiaptomus spp. fed in equal-volume mixtures (0.25 ppm total). Bars are means &SE for four replicate beakers. tested for selection only between pairs of food types; depletion of food particles during experiments was negligible due to short duration and moderate densities of experimental animals. In this simple case, cy is the ratio of the clearance rate on one food to the sum of the clearance rates on both food types. The index ranges from 0 to 1.O, with a value of indicating nonselective feeding. By convention, CY was calculated as the contribution of the lower quality particle to the diet. Thus, in experiments with particles of differing quality, an a value CO.50 indicates a preference for the higher quality food. Noting that cx has an ap.proximately normal distribution when sample sizes are not too small and values are not too close to 0 or 1, Chesson (1983) recommended parametric statistics for testing hypotheses involving a. As a precaution, I conducted statistical testing with untransformed values of LY and after subjecting them to arcsine transformation (Sokal and Rohlf 198 1). In the few instances in which data transformation affected the significance level, the more conservative level is reported. Statistical testing was two-step process. First, a t-test or oneway ANOVA was used to determine whether selectivity varied with food concentra- tion. When selectivity did not vary with food concentration ( invariant selectivity ), values of cx were pooled among density treatments and t-test determined whether feeding was selective or nonselective (a different from 0.50). F-tests for homogeneity of variances preceded all t-tests. All statistical tests are based on comparisons between fully replicated treatments. Results Rejection of inert particles-by counting the numbers of polystyrene spheres within copepod guts, I verified that Eudiaptomus spp. is very efficient at rejecting inert particles. On the average, copepods ingested only 0.5, 0.9, or 2.5 spheres ind.- during the 15-min feeding trials with 1,000, 3,000, or 10,000 spheres ml-l. Clearance rates on spheres were < 1% of clearance rates on algae, further indicating that most spheres were rejected. Yet there was no evidence that the process of rejecting spheres interfered with feeding on algae of the same size and shape. In separate experiments conducted at two concentrations of C. reinhardi, clearance rates on the alga were independent of the concentration of spheres over a range of O-lo4 spheres ml- (Fig. 1). Size-selective feeding-before consider-

6 Optimal foraging by copepods 145 I -ET N.S. a f~~~ b Chlamy. sph. Scenedesmus u = Monoraphidium.6 - Ea Microcystis I p-= d a w Chlamy. rein. Planktosphaeria n m Chlamy. sph. Planktosphaeria Single Paired Paired Low Low High Single Paired Paired low Low High Fig. 3. Discrimination between pairs of living algae. Eudiuptomus spp. was offered each particle type alone and in low- and high-density mixtures. Selectivity coefficients ~0.50 indicate a preference for the first type of food. Shifts in selectivity between low- and high-density mixtures were tested with t-tests. Values are means (-t 1 SE) for three or four replicate beakers per treatment. ing selection between particles of differing nutritional quality, it is useful to examine the influence of particle size on selection among high-quality foods. Clearance rates on spherical, high-quality algae with diameters of 2, 6, and 12,urn were strongly influenced by particle size (Fig. 2). The clearance rate on the intermediate-sized cell, C. reinhardi, was about half of the clearance rate on the larger cell, Chlamydomonas sphaeroides, but almost 10 times higher than the clearance rate on the smallest cell, Chlorella minutissima. A single basic experimental design was used to test the effects of food concentration on selectivity. In experiments with each of 10 different pairs of dual-labeled particles, copepods fed on each food alone and in lowand high-concentration mixtures (see methods). Copepods feeding on two high-quality algae showed a weak, but consistent preference for the larger species, C. sphaeroides, over the smaller species, Scenedesmus acutus (t-test on pooled a values, selective feed- ing, P < ; Fig 3a). A selectivity coefficient calculated from feeding rates in respective single-species suspensions was a good predictor of the selectivity observed in both low- and high-concentration mixtures (Fig. 3a). Discrimination between high- and lowquality algae-six series of experiments

7 146 DeMott Chlamydomonas Concentration [ppm) El l - oi Fig. 4. Relation between food concentration, ingestion rate, and selectivity for Eudiaptomus spp. feeding on Chlamydomonas reinhardi and Crucigenia tetrapedia. Open symbols are selectivity coefficients for copepods feeding in single-species suspensions (A) or equal-volume mixtures (0); O-ingestion rates on C. reinhardi. Each circle is the mean value for an experimental beaker. Copepods were more selective at high densities of food (ANOVA, P < 10e6). paired high-quality algae with living algae which were assumed, a priori, to be low in nutritional quality. Experiments with a highquality green alga, Monoraphidium, and a toxic blue-green, Microcystis, revealed strong, invariant selection against the toxic cells. Both Monoraphidium and Microcystis are small cells ( 5 pm long) with Monoraphidium smaller in volume than Microcystis. Eudiaptomus SPP., however, showed a strong preference for the smaller, highquality alga over the slightly larger toxic species (t-test on pooled LX values, selective feeding P < ; Fig. 3b). Microcystis was ingested at a very low rate when offered alone and constituted only about 15% of the diet when offered with equal amounts of Monoraphidium at low or high concentrations (Fig. 3b). Probably due to small size, Monoraphidium was ingested at much lower rates than Chlamydomonas or Scenedesmus (Fig. 3a and b). In contrast with the above results, Eudiaptomus spp. exhibited variable selectivity in all experiments pairing high-quality algae with digestion-resistant algae. As predicted by optimal diet theory, Eudiaptomus spp. discriminated more strongly against low-quality algae at high food concentrations. For example, at low food densities, alone or in mixtures, Eudiaptomus spp. in- gested the large, gelatinous green Planktosphaeria at a higher rate than the small flagellate C. reinhardi (Fig. 3~). In high-density mixtures, however, the copepod preferred the small, high-quality species by a factor of about 311 over the larger, low-quality species (Fig. 3~). A similar shift in selectivity occurred in experiments pairing Planktosphaeria with a larger species of Chlamydomonas. Here, Eudiaptomus spp. showed a weak preference for C. sphaeroides at low concentrations and a strong preference (-6 : 1) at the high concentration (Fig. 3d). The size preference for large C. sphaeroides over small C. reinhardi noted earlier (Fig. 2) fully accounts for the differences in selectivity coefficients between these two series of experiments with Planktosphaeria. The relation between Functional response ancl selectivity was examined in experiments pairing C. reinhardi and a second gelatinous green alga, Crucigenia, over an expanded range of food concentrations (Fig. 4). When offered each food alone, the clearance rate on the larger, low-quality alga was significantly higher than on the high-quality flagellate ( ml ind.- h-l &SE vs t_o.o 12, t-test, P < 0.05). Eudiaptomus spp. fed nonselectively when offered both algae together at low concentrations, but became selective as the maximal ingestion rate was approached and exhibited strong (m 4 : 1) selection against Crucigenia at the two highest concentrations (Fig. 4). Daphniapulicaria feeding within the same beakers sh owed evidence of saturated feeding, yet fed nonselectively at all food concentrations (Table 2; t-test on pooled selectivity coefficients, N.S.). Additional experiments with Crucigenia and C. reinhardi tested whether altering their relative abundance would influence the selectivity of Eudiaptomus spp. A lo-fold increase in. the concentration of Crucigenia kept C. reinhardi unchanged but resulted in stronger selection against the more abundant, low-quality alga (Table 3, treatments 1 and 2, t-test on cy, P < 0.001). Increasing the total concentration of both food types, however, also produced increased discrimination against Crucigenia (Table 3, cf. treatments 2 and 3). Thus, increased discrimination against Crucigenia can be at-

8 Optimal foraging by copepods 147 Table 2. The clearance rates (ml ind.- h I) and selectivity (+ 1 SE) of Daphnia pulicaria feeding in cqualvolume mixtures of Crucigenia and Chlamydomonas reinhardi. A selectivity coefficient of 0.50 indicates nonselective feeding. Crucig. : Chlamy. (mm) 0.25 : : : : 5.0 Clearance rate Crucigenia Chlamydomonas a 1.8OkO kO f kO & ko &O _ LO to.039 tributed to increased absolute abundance rather than changes in the relative abundance of food types. Increasing the relative abundance of C. reinhardi resulted in somewhat weaker discrimination against Crucigenia (t-test comparing treatments 2 and 4, P < 0.005). Figure 5 compares the feeding behavior of C3 and C4 copepodites of Eudiaptomus spp. with adults and C5 copepodites in the same suspensions of C. reinhardi and Crucigenia. As expected, the smaller developmental stages exhibited lower clearance rates. Both groups, however, exhibited very similar shifts in selectivity with changes in food concentration. As in previous experiments, copepods exhibited stronger discrimination against low-quality algae at the high food concentration. Discrimination between live and dead algae-experiments with live and dead, sterile Staurastrum revealed remarkably strong discrimination between particles of the same size, shape, and superficial appearance. Whether offered as the sole food or together with living cells, clearance rates on dead, sterile Staurastrum were only l-2% of clearance rates on live cells (Fig. 6a; t-test on pooled values of CX, selective feeding P < lo+). A second experiment was conducted 2 d later, after the same culture of dead Staurastrum had become contaminated by bacteria. Here Eudiaptomus spp. ingested contaminated dead cells at rates - 10 times higher than measured for dead sterile cells 2 d earlier (Fig. 6a and b) but still showed a strong, invariant preference for living cells over dead cells (t-test on pooled values of a!, selective feeding P < 1 O-6). Experiments with Scenedesmus directly demonstrate that the presence of bacteria increases the palatability of dead cells, Eudiaptomus spp. preferred live Scenedesmus over dead, sterile Scenedesmus at both low and high concentrations (Fig. 7a). Unlike the trials with Staurastrum, however, Eudiaptomus spp. exhibited stronger discrimination against dead cells at the high concentration of food (Fig. 7a). A similar experiment with live Scenedesmus and dead, nonsterile cells revealed weak discrimination against dead cells at low concentrations and stronger discrimination at the high con- centration (Fig. 7b). These results prompted me to test directly for discrimination between dead Scenedesmus with bacteria and dead, sterile Scenedesmus. Here Eudiaptomus spp. showed little or no preference be- tween the two categories of dead algae when offered at low concentrations but exhibited Table 3. The influence of the relative and absolute abundance of food particles on selection between Crucigenia and Chlamydomonas reinhardi by Eudiaptomus spp. Values are means &SE for three replicate beakers, Crucig. : Chlamy. (mm) 1:1 10: 1 5:5 1: 10 Clearance (ml ind.-! rate h-l) Crucigenia Chlamydomonas a to & eO ~ to.004

9 148 DeMott w L- Iy.-&.3 Lu- ok.2 ZC a.- ryd ae.1 W- d Chlamy. rein. Crucigenia 0 Alone Low Paired Low Paired High Alone Pai red Paired Low Low High Fig. 5. Selection between Chlamydomonas reinhardi and Crucigenia tetrapedia by C3 and C4 copepodites (a), and C5 copepodites and adults (b) of Eudiaptomus spp. in the same experimental beakers. Further explanation given in Fig. 3. significant discrimination against dead, sterile cells in the high-concentration treatment (Fig. 8). Taken together, the Scenedesmus experiments demonstrate selection between living algae, dead, sterile algae, and dead, nonsterile algae and provide another example of the shift in selectivity predicted by foraging theory. Discussion Unlike conventional optimal diet models (Stephens and Krebs 1986), Lehman s (1976) food-quality model assumes that handling time is negligible. Experiments with polystyrene spheres support this assumption for suspension-feeding copepods. Even when spheres were 10 times more abundant than algae, there was no hint of reduced feeding on scarce algae (Fig. 1). In experiments of similar design, Paffenhijfer and Van Sant (1986) also found that polystyrene spheres did not interfere with feeding on algae by a marine copepod. Consideration of encounter rates and handling capabilities help explain these results. Maximal encounter rates with spheres, based on clearance rates for algae, were about 1 s-l in both studies. High-speed movies, however, show that handling times for readily ingested particles are very short, only about 50 ms (Paffenhofer et al. 1982). Movies also show that several particles can be handled simultaneously and that additional particles can be captured and ingested while others continue to be manipulated (Strickler 1984; Van derploeg and Paffen hijfer 1985). Since handling time is not a limiting factor, rejection of numerous poor-quality particles need not interfere with feeding on highquality items. The ability of Calanoid copepods to use chemical cues in selecting between individual particles is already well established. Results presented here show that optimal diet theory can be a useful predictor of when selection will be weak or strong and dependent on or independent of food concentration. Predictions vary, depending on whether particles differ in size or nutritional quality and on whether low-quality particles are toxic or nontoxic. Most studies of food selection by Calanoid copepods have focused on particle-size selection. High-speed movies show that larger cells are detected at greater distances and are more likely to be actively captured. Thus, perceptual biases rather than active choice may explain preferences for larger, highquality cells over smaller, high-quality cells (Price and Paffenhofer 198 5; Legier-Visser et al. 1986). If we assume that handling times and handling costs are negligible, energy gain is maximized by the capture and ingestion of all high-quality algae that are detected.

10 Optimal foraging by copepods 149 I Staurastrum Dead Ib) I fm Alone Paired Paired Low Low High Alone Paired Paired Low LOW High Fig. 6. Discrimination between living Staurastrum punctulatum and dead cells of the same species by Eudiaptomus spp. Dead cells were (panel a) kept sterile (s) or (panel b) allowed to become contaminated by bacteria (b). Further explanation given in Fig. 3. Unless detection capabilities are influenced by food concentration or learning (Price and Paffenhijfer 1984), optimal foraging theory predicts particle-size preferences independent of both the absolute concentration and relative abundance of food species. The weak but consistent preference for larger C. sphaeroides over smaller S. acutus observed here (Fig. 3a) is consistent with the notion of fixed size preferences. Studies that have carefully controlled particle quality and particle production artifacts have documented preferences for larger particles independent of both absolute and relative concentrations (Frost 1977; Vanderploeg et al. 1984). In one clear exception, Vanderploeg et al. (in press) have shown that Diaptomus sicilis discriminates against large, difficult-to-handle, net diatoms when Chlamydomonas is abundant. In this situation, handling times and handling costs could be important. Cladocerans which ingest toxic bluegreens often suffer higher mortality rates than experienced during starvation (Lam- a pco.001 W &-.6 (r -& W-7 z -ci.4 C a +- %z.2 w- u Alone Paired Paired Low Low High Fig. 7. As Fig. 6, but with living.3.2.l Alone Paired Paired Low Low High Scenedesmus acutus.

11 150 DeMott i - u- p -z I--- - Alone Paired Pai red Low Low High Fig. 8. Discrimination between dead, sterile (s) and dead, nonsterile (b) cells of Scenedesmus acutus by Eudiaptomus spp. Further explanation given in Fig. 3. pert 1981; Fulton and Paerl 1987b). Calanus pac$cus exhibits acute symptoms, including loss of muscular control and regurgitation, following the ingestion of toxic dinoflagellates (Sykes and Huntley 1987). Strong, invariant selection against toxic algae is consistent with the optimal foraging paradigm; even when high-quality algae are scarce or absent, the ingestion of highly toxic cells would not be beneficial. The behavior of Eudiaptomus spp. feeding on Monoraphidium and toxic Microcystis (Fig. 3 b) was similar to that observed for Diaptomus reighardi feeding on C. reinhardi and toxic Microcystis (Fulton and Paerl 1987b) and for two marine calanoids feeding in mixtures of toxic and nontoxic dinoflagellates (Huntley et al. 1986). Consistent with expectations, toxic algae were ingested at very low rates whether offered alone or with low or high densities of high-quality algae. Con- trary to my experiments with polystyrene spheres, both Huntley et al. (1986) and Fulton and Paerl(l9876) found that high densities of toxic algae caused reduced feeding on nontoxic algae. Selection between algae that differ in di- u gestibility - the specific situation for which Lehman s (1976) optimal diet model was formulated-had not previously been tested. Results presented here show consistent, qualitative agreement with predictions of the model. In each experiment, discrimination against digestion-resistant algae was weak at low concentrations of food and stronger at high concentrations. Some experiments illustrate interactions between particle size and particle quality. For example, higher feeding rates on Planktosphaeria than C. reinhardi at low concentrations of food probably reflect a perceptual bias for the larger, low-quality alga (Fig 3~). Preference for C. reinhardi over Planktosphaeria at the high concentration of food presumably results from the active rejection of the larger low-quality alga. As predicted by Lehman s (1976) model, experiments with Crucigenia showed a continuous shift in selectivity that was closely linked to the functional response curve (Fig. 4). Moreover, selection between Crucigenia and Chlamydomonas was sensitive to the absolute abundance of particles and rather insensitive to relative abundance. Discrimination against Crucigenia was weaker when the relative abundance of Chlamydomonas was high (Table 3), however-a result opposite to predictions of foraging theory. In a similar situation, Ayukai (1987) suggested that an increased incidence of simultaneous captures could account for a positive relationship between ingestion rates on algae and polystyrene spheres by Acartia clausi. The tendency of copepods to ingest small cells in groups (Vanderploeg and Paffenhiifer 198 5) could lead to reduced selection against low-quality particles when their relative abundance is low. My data are insufficient for testing detailed quantitative aspects of the Lehman (1976) model. Doing so would require quantitative data on assimilation efficiencies, food value, gut packing, and rejection costs. One assumption of optimal diet models is that organisms are able to assess the availability 0.f high-ranking foods. Lehman (1976), however, assumed that gut fullness an d hunger, rather than food concentration per se, control foraging behavior. Thus, ZOOplankton could follow a simple rule: feed

12 Optimal foraging by copepods 151 selectively on high-quality particles when satiated and feed less selectively when hungry. Runge (1980) found that hunger had little if any effect on food selection by C. pacz&kus, but his experiments tested for selection based on particle size, not particle quality. The importance of hunger in diet choice is supported by experiments with predatory zooplankton and noxious or difficult-to-handle prey. Starved Mesocylops (Williamson 1980) and starved Chaoborus (Pastorok 1980) fed less selectively than animals acclimated to ambient food concentrations. If hunger per se is important for suspension feeders, factors which inhibit ingestion rates, such as crowding or the presence of predators (Folt 1987), would also lead to weaker discrimination against lowquality particles. This study demonstrates discrimination against digestion-resistant, toxic, and heatkilled algae. Low-quality cells, including blue-greens and digestion-resistant greens, often comprise > 80% of the phytoplankton biomass in mesotrophic and eutrophic lakes (Porter 1977; DeMott and Kerfoot 1982; Sommer et al. 1986). Heat-killed algae are considered models of detritus. Detritus is often more abundant than living phytoplankton in both freshwater lakes (Melack 1985) and marine nearshore environments (Poulet 1983). Experiments with live and dead Scenedesmus revealed stronger discrimination against dead cells at higher food concentrations. Similarly, Eudiaptomus spp. feeding in mixtures of Chlamydomonas and Chlamydomonas-flavored spheres showed stronger discrimination against flavored spheres at higher concentrations of algae (DeMott 1988). These results suggest that optimal diet theory is applicable to copepods feeding on mixtures of algae and detritus. Contrary to expectations, however, Eudiaptomus exhibited strong, invariant selection against dead S taurastrum (Fig. 6). This result may illustrate a limitation on the use of chemical cues to assess food value. Just as a chemical coating can trick copepods into eating polystyrene spheres (DeMott 1986, 1988), the suface properties of dead cells could lead to underestimation of their food value. The almost complete lack of feeding on dead, sterile Staurastrum, even when offered alone, suggests that these cells lacked the necessary chemical cues to be recognized as food. The robust cell wall of Staurastrum may have contributed. Detritus varies in nutritional value according to its source and age, and cannot, therefore, be considered a single food category (reviewed by Poulet 1983; Melack 1985). Results presented here show that Eudiaptomus spp. is very sensitive to the surface properties of detritus. The presence of bacteria increased the palatability of dead Staurastrum and dead Scenedesmus. Since bacteria are indicators of fresh, readily degraded and digested matter (Paerll980), the presence of attached bacteria and bacterial films could be used by copepods as an indicator of the food value of detrital particles. My experiments tested for discrimination between particles that differed markedly in food quality. Future studies should also test whether copepods can detect more subtle chemical differences (among and within species of algae) which make some cells more nutritious and hence preferable to others (e.g. Scott 1980; Houde and Roman 1987). Optimal diet theory predicts that selection based on small differences in food quality would only be significant at high concentrations of food. All of the selection experiments reported here involved choices between only two kinds of particles. It is not yet clear whether results from simple laboratory mixtures can be extrapolated to natural environments. Selection behavior in complex mixtures of natural seston could be different, particularly if learning and experience influence feeding on particular foods (e.g. Price and Paffenhijfer 1984). Studies of food selection in natural seston have focused on particle size, not particle quality (e.g. Poulet 1978; Cowles 1979; Lampert and Taylor 1985). Despite difficulties in obtaining unbiased, quantitative results, analyses of gut con tents and fecal pellets provide the most direct information on diets in nature. An analysis of the gut contents of E. gracilis from Lake Balaton provides evidence for strong discrimination against suspended sediments and certain blue-green bacteria (G.-Toth et al. 1987). Other more qualitative analyses

13 152 DeMott ofgut contents and fecal pellets have shown that the natural diets of Calanoid copepods include most available phytoplankton taxa and also detritus and occasional inorganic particles (e.g. Horn 198 1; Turner 1984; Infante and Edmondson 198 5). If one assumes that food is often limited, relatively weak selection on the basis of food quality is consistent with optimal diet theory and my results. In nature, changes in selectivity in response to food concentration could be important over a range of spatial and temporal scales. One can predict that the selectivity of copepods will vary with the changes in food availability that often occur seasonally and during vertical migration within a given lake, as well as between oligotrophic and eutrophic habitats. For example, Lewis ( 1977) found that a tropical Chaoborus was less selective while feeding on scarce prey in deep waters during day and more selective while feeding on abundant prey in surface waters at night. Although most experiments presented here involved late copepodite stages (C5 and adults), an experiment with earlier stages (C3 and C4) revealed essentially the same foraging strategy (Fig. 5). The grouping of two copepod species (E. gracilis and E. graciloides) is a weakness, since possible interspecific differences are obscured. The low variability in my estimates of feeding rates and selectivity coefficients, however, suggests that the two species of Eudiaptomus exhibit very similar feeding behaviors. Previous experiments with individual copepods showed that the two species of Eudiaptomus discriminated more strongly against polystyrene spheres than did two marine calanoids of similar body size (DeMott 1988). Clearly, further experiments are needed to compare patterns of chemically mediated selection between copepod species. Contrasts between Eudiaptomus spp. and D. pulicaria feeding within the same containers raise questions about the ecological significance of differences in feeding mode among distantly related taxa. For example, the tendency of calanoids to feed selectively on the basis of both particle quality and size should reduce the intensity of competition with nondiscriminating Daphnia (Richman and Dodson 1983). Moreover, the tendency of calanoids to feed selectively may help explain why copepods seem to be less effective than daphnids in strongly depressing total phytoplankton biomass in lakes (e.g. HrbaCek et al ; Lynch and Shapiro 1981; Edmondson and Litt 1982; Vanni 1986). Reft?rences AYUKAI, T Discriminate feeding of the calanoid copepod Acartia cluusi in mixtures of phytoplankton and inert particles. Mar. Biol. 94: 579-,587. BOYII, C. M Selection of particle sizes by tilterfeeding copepods: A plea for reason. Limnol. IOceanogr. 21: 175-l 80. CHE~SON, J The estimation and analysis of preference and its relationship to foraging models. Ecology 65: I COWLES, T. J The feeding response of copepods from the Peru upwelling system: Food size selection. J. Mar. Res. 37: DEMOTT, W. R Feeding selectivities and relative ingestion rates of Daphnia and Bosmina. Limnol. Oceanogr. 27: Relations between filter mesh-size, feeding mode, and capture efficiency for cladoc- erans feeding on ultrafine particles. Ergeb. Limnol. 21: The role of taste in food selection by freshwater zooplankton. Oecologia 69: Discrimination between algae and artificial particles by freshwater and marine copepods. Limnol. Oceanogr. 33: AND W. C. KERFOOT Competition among cladocerans: Nature of the interaction between Bosmina and Daphnia. Ecology 63: DONAGHAY, P. L., AND L. F. SMALI, Food se- lection capabilities of the estuarine copepod Acartia cluusi. Mar. Biol. 52: EDMONDSON, W. T., AND A. H. LITT Daphnia in Lake Washington. Limnol. Oceanogr. 27: ELNER, R. W., AND R. N. HUGHES Energy maximization in the diet of the shore crab, Carcinus maenus. J. Anim. Ecol. 47: 103-l 16. FEKNANDEZ, R Particle selection in the nauplius of Calanuspacificus. J. Plankton Rcs. 1: FOILT, C. L An experimental analysis of costs and benefits of zooplankton aggregation, p In W. C. Kerfoot and A. Sih [eds.], Predation: Direct and indirect impacts on aquatic communities. New England. FROST, B. W Feeding behavior of Calanus paczjicus in mixtures of food particles. Limnol. Oceanogr. 22: FULTON, R. S., AND H. W. PAERL. 1987a. Effects of colonial morphology on zooplankton utilization

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