Influence of experimental conditions on nitrogenous excretion by Lake MichiganMysis relicta (Lovén): laboratory studies with animals acclimated inFragilaria

In laboratory experiments, rates of excretion of ammonia and urea by Lake MichiganMysis relicta were compared for animals incubated in the presence and the absence of algal food (Fragilaria crotonensis chemostat outflow). Prior to experiments, all animals were acclimated to laboratory conditions and the experimental food for 2–4 weeks. Algae used in experiments were enriched in the dark with nutrients (N and P) prior to experimental incubation.There were no significant differences in the ammonia and urea excretion rates of mysids incubated in filtered water compared to those inFragilaria cultures, or in ammonia excretion rates either as a function of individual size or sex of the mysid or of its having been held at 5 °C, 10 °C, or 15 °C. Ammonia excretion rates measured between 4 and 8 h of incubation were significantly higher than those between 1–3 h and 8–15 h, for mysids held both in filtered water and inFragilaria cultures. The results are compared to those from shipboard incubations in a previous study and are discussed with respect to physiological regulation of nitrogenous waste production.Contribution No. 234, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.Contribution No. 234, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.     

The release of dissolved organic carbon as a result of diatom fragmentation during feeding byMysis relicta

The release of dissolved organic carbon (DOC) byMysis relicta that occurs while feeding may be an important pathway in the recycling of nutrients in Lake Michigan. Laboratory experiments were conducted to determine the levels of DOC released byM. relicta from Lake Michigan while grazing on laboratory-cultured diatoms. Increasing filtering rates were observed at progressively lower chlorophyll concentrations. Higher filtering rates were accompanied by higher fragmentation of diatoms and a higher concomitant release of DOC from broken cells. While the amount of DOC release is small (0.02–3.30 g C · 1^–1· h^–1), it nonetheless is of importance to bacteria and possibly some algae. After assimilation by bacteria and algae, the organic carbon is then available to consumers.Contribution No. 239, Center for Great Lakes Studies, The University of Wisconsin-Milwaukee, Milwaukee, WI 53201, U.S.A.Contribution No. 239, Center for Great Lakes Studies, The University of Wisconsin-Milwaukee, Milwaukee, WI 53201, U.S.A.     

Excretion of total phosphorus,dissolved reactive phosphorus,ammonia, and urea by Lake MichiganMysis relicta

Rates of nutrient release byMysis relicta in Lake Michigan were measured on five nights at a 45-m station near Milwaukee, Wisconsin, U.S.A., in the summer of 1977. Nocturnal vertical migrations of the mysids were monitored with both echosounder tracings and vertical net tows. Estimates of the total areal dry mass of the mysids ranged from 600 to 1 820 mg m–2. Rates of release of dissolved reactive phosphorus, total phosphorus, ammonia, and urea were measured in dark incubations on shipboard. Excretion experiments were initiated immediately after mysids were collected from each of several vertical net hauls. The depths of maximum mysid densities corresponded approximately with a deep phytoplankton peak located in the vicinity of the thermocline. Semiquantitative demands for N and P by phytoplankton within this peak were obtained from¹⁴C estimates of primary production from a previous study, assuming a constant C:N:P ratio for the algae. These algal nutrient demands were compared to potential N and P release by the mysids to obtain a first approximation of the relative rates of nutrient supply and demand for the field phytoplankton populations. Our analysis indicates that mysids may directly supply about 1–10% of the daily N and P demands of the phytoplankton in the deep peak. However, indirect interactions betweenMysis relicta and other organisms, such as small zooplankton and fishes, could be major factors in nutrient recycling within the metalimnion and subthermocline region of Lake Michigan.Contribution No. 238, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Wisconsin.Contribution No. 238, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Wisconsin.     

In situ predatory behavior ofMysis relicta in Lake Michigan

Selectivity coefficients (W) and predation rates on Lake Michigan zooplankton were determined forMysis relicta during spring through fall using anin situ method. W values indicated the following ranked order of prey preference: Cladocera > copepod copepodites and copepod nauplii > adult diaptomids and cyclopoids. With few exceptions, W values for different prey categories remained fairly constant despite greatly changing relative abundances of prey. Predation rates and prey selectivity were similar in most cases to those determined in laboratory studies. Ingestion rates (percent dry body weight · day^–1) were correlated to total prey biomass (r = 0.38) and to effective prey biomass (r = 0.85), where the weighting factors were overall mean selectivity coefficients for the different prey categories. This result suggested that seasonally varying composition of prey caused much of the variation in ingestion rates among experiments. Feeding trials performed at the same depth with daytime and nighttime assemblages of zooplankton indicated that Cladocera may escape heavyMysis predation at night by migrating from the metalimnetic-hypolimnetic interface into the epilimnion.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.     

The relationship between fecundity and adult body weight in Homeotherms

Summary Bythotrephes cederstroemii Schoedler, a predatory freshwater zooplankter (Crustacea: Cladocera), was first found in the Laurentian Great Lakes in December 1984. The first individuals were from Lake Huron, followed in 1985 with records from Lakes Erie and Ontario. By late August, 1986 the species had spread to southern Lake Michigan (43°N). Bythotrephes has not previously been reported from North America, but has been restricted to a northern and central Palearctic distribution. Its dramatic and widespread rise in abundance in Lake Michigan was greatest in offshore regions. Bythotrephes appears to be invading aggressively, but avoiding habitats presently occupied by glacio-marine relict species that became established in deep oligotrophic North American lakes after the Wisconsin glaciation. Because it is a voracious predator its invasion may lead to alterations in the native zooplankton fauna of the Great Lakes. It offers the chance to study how invading plankton species join an existing community. Judging from its persistence and success in deep European lakes, Bythotrephes may now become a permanent member of zooplankton communities in the Nearctic.     

Suspension feeding of anuran larvae at low concentrations of Chlorella algae (Amphibia,Anura)

Summary Ingestion and filtering rates in larval Xenopus laevis, Bufo calamita, Rana temporia and Bufo bufo fed suspensions of Chlorella fusca were investigated. Concentrations were measured with a Coulter Counter. (1) For all species, filtration occurred at concentrations far below those reported by other authors for Rana sylvatica feeding on Chlorella pyrenoidosa. For Bufo bufo, only larvae near metamorphosis showed ingestion at low particle concentrations. Since buccopharyngeal ventilation continues even in the absence of food particles, this threshold feeding behaviour in the younger larvae must be due to different mechanisms to those found in Daphnia and Calanus studied by other authors: probably reduction of the buccal pumping rate and the mucus production of the filter apparatus. (2) For B. calamita, R. temporaria and X. laevis the highest suspension feeding efficiency was at early tadpole stages, corresponding with the high growth rate of these stages. (3) The life histories of the species provide the basis for understanding their different retention efficiencies and functional responses.Supported by the Deutsche Forschungsgemeinschaft-DFG     

Functional response of suspension feeding anuran larvae to different particle sizes at low concentrations (Amphibia)

The influence of particle size, initial particle concentration and larval stage on the ingestion rate, ‘retention efficiency’, and filtering rate of anuran larvae with varying filter apparatus anatomy and different life histories was investigated for four species. Larvae of premetamorphic Stages 28 and 32 and prometamorphic Stage 40 were selected for filtering experiments on the basis of their different growth rates. Three different sizes of silica gel particles were offered as mock food. Particle concentration was measured photometrically. The Michaelis-Menten model was used to describe the dependency of ingestion rate, filtering rate, and ‘retention efficiency’ upon initial particle concentration, and to calculate maximum ingestion rate, threshold concentration, and the half-saturation constant. (1) The highest ingestion rates, filtering rates and ‘retention efficiencies’ were achieved by Xenopus laevis larvae, followed by Bufo calamita larvae. Bufo bufo larvae lay at the opposite end of the scale. Rana temporaria larvae were placed between B. calamita and B. bufo larvae. This order is attributed to differences in life histories, especially the different breeding environments in which these larvae occur. (2) The larger the particle size and the older the stage, the greater the tendency toward saturation of the ingestion rate, filtering rate and ‘retention efficiency’. These filtration parameters are graded according to particle size. The ingestion rate (number of particles), filtration rate and ‘retention efficiency’ are greatest for PS3. Ingestion volume is greatest for PS 1. The difference between PS3 and PS2 on the one hand, and PS1 on the other, is often great; for Stage 28 X. laevis it is very great. This shows that larvae ingest large particles more effectively, and that the most effective ingestion takes place at Stages 28 and 32, owing to the growth function of these stages. The ability of larvae to ingest large particles effectively is possibly a very basic phylogenetic characteristic. (3) The threshold concentration is lowest when the particles are at their largest. In accordance with conclusions drawn by other authors, threshold feeding is attributed to regulation by buccal pumping and mucus production. Considerable importance is attributed to threshold feeding with respect to larval adaptation to oligotrophic environments.     

Distribution and habitat of Nitellopsis obtusa (Characeae) in the Laurentian Great Lakes

Nitellopsis obtusa, a macroalga (Characeae) native to Europe and Asia, was found in U.S. waters of the St. Clair-Detroit River system in 1983, thus extending the range of this taxon into the Laurentian Great Lakes about 850 km from the St. Lawrence River where it was first discovered in North America in 1978. Its occurrence only in water frequented by commercial shipping vessels suggests that it is distributed via this mechanism. In the St. Clair-Detroit River system, N. obtusa was collected with a Ponar grab at four locations, and with a grapnel at one additional location. It was the ninth most frequently found macrophyte and it was most abundant at Belle Isle in the Detroit River, where the mean dry-weight biomass in Ponar samples was 0 g m^-2 in June, 37 g m^-2 in August, and 32 g m⁻² in September. Maximum biomass of this taxon in one Ponar grab at this location was 289 g m^-2 in September. The alga occurred primarily in water of relatively low current velocity (11.3 cm s⁻¹) and in association with Vallisneria americana, Myriophyllum spicatum, Potamogeton richardsonii, Najas flexilis, and Elodea canadensis. Contribution 654, Great Lakes Fishery Laboratory, Ann Arbor, MI 48105, USA Contribution 654, Great Lakes Fishery Laboratory, Ann Arbor, MI 48105, USA     

Feeding at different plankton densities alters invasive bighead carp (<Emphasis Type="Italic">Hypophthalmichthys nobilis</Emphasis>) growth and zooplankton species composition

Invasive Asian carps Hypophthalmichthys spp. are an ecological threat to non-native aquatic ecosystems throughout the world, and are poised to enter the Laurentian Great Lakes. Little is known about how these filter-feeding planktivores grow and impact zooplankton communities in mesotrophic to oligotrophic systems like the Great Lakes. Our purpose was to determine how different plankton densities affect bighead carp H. nobilis biomass and how bighead carp affect zooplankton species composition. We conducted a 37-day indoor mesocosm experiment (volume = 678 l) with high and low plankton treatments (zooplankton dry mass ≈ 1,900 and 700 μg l⁻¹; chlorophyll a = 25 and 14 μg l⁻¹, respectively) in the presence and absence of juvenile bighead carp (mean = 5.0 g, 8.5 cm). Carp lost weight in the low plankton treatment and gained weight in the high plankton treatment, suggesting that food availability may be a limiting factor to bighead carp growth in regions of low plankton densities. In the presence of carp, zooplankton shifted from Daphnia to copepod dominance, while in the absence of carp, Daphnia remained dominant. Chydorids and ostracods increased in the presence of carp, but only in the low plankton treatment, suggesting that the impact of bighead carp on zooplankton species composition may vary with zooplankton density. Chlorophyll was higher in the absence of carp than in the presence. Chlorophyll and zooplankton densities in many Great Lakes ecosystems are substantially lower than our low treatment conditions, and thus our results suggest that Asian carp establishment in these regions may be unlikely. Handling editor: S. Declerck     

Ingestion of live filamentous diatoms by the Great Lakes amphipod,Diporeia sp.: a case study of the limited value of gut contents analysis

Individuals of the Great Lakes amphipod, Diporeia sp. (formerly named Pontoporeia hoyi) were collected from a 45-m deep station in southeastern Lake Michigan and isolated in small laboratory feeding vessels at 4 °C, after the animals had voided their guts over a 24-hour period. Over a 20-day period, following introduction of a single ration of live cells of the filamentous diatom, Melosira varians, 9 of 10 animals had ingested this material, and 7 of these 9 individuals had deposited fecal pellets. Subsequent examination of gut contents and fecal pellets showed that although animals had ingested whole algal cells/filaments, little of the material in gut contents or fecal pellets bore any identifiable structural similarity to cells/filaments prior to ingestion. The results suggest that earlier studies of pontoporeiid gut contents may seriously underestimate the importance of algal components in the amphipod's diet and imply that Diporeia sp. growth and production may be more closely linked to primary production than previously thought.     

Production of Hexagenia limbata nymphs in contaminated sediments in the Upper Great Lakes Connecting Channels

In April through October 1986, we sampled sediments and populations of nymphs of the burrowing mayfly, Hexagenia limbata (Serville), at 11 locations throughout the connecting channels of the upper Great Lakes, to determine if sediment contaminants adversely affected nymph production. Production over this period was high (980 to 9231 mg dry wt m^-2) at the five locations where measured sediment levels of oil, cyanide, and six metals were below the threshold criteria of the U.S. Environmental Protection Agency and the Ontario Ministry of Environment for contaminated or polluted sediments, and also where the criterion for visible oil given in the Water Quality Agreement between the U.S.A. and Canada for connecting waters of the Great Lakes was not exceeded. At the other six locations where sediments were polluted, production was markedly lower (359 to 872 mg dry wt m^-2). This finding is significant because it indicates that existing sediment quality criteria can be applied to protect H. limbata from oil, cyanide, and metals in the Great Lakes and connecting channels where the species fulfills a major role in secondary production and trophic transfer of energy.Contribution 733, of the National Fisheries Research Center-Great Lakes, U.S. Fish and Wildlife Service, 1451 Green Road, Ann Arbor, MI 48105.     

Feeding habits of larval Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret (Israel) II. Experimental study

Interactions between the larvae of Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret, Israel, and their zooplankton prey were studied experimentally. Prey species preference and size selectivities were measured. Larvae were hatched in the lab from eggs collected in the field, and fed different food items in various concentrations. The food items included lake zooplankton, algae, and commercial pellets. It was shown that small, first feeding larvae (7–8.5 mm SL) prefer small bodied zooplankters (< 180 µ). The effect of these food sources on larval growth was measured. It was found that larval Mirogrex grew at a higher rate when fed zooplankton prey sized from 63 µ–250 µ. Food items smaller than 63 µ, larger than 250 µ and Scenedesmus sp., produced less than optimal growth rates. The importance of Mirogrex feeding habits and their potential influence on the Kinneret ecosystem is considered.     

Morphological variation in a larval salamander: dietary induction of plasticity in head shape

We examined diet-dependent plasticity in head shape in larvae of the eastern long-toed salamander, Ambystoma macrodactylum columbianum. Larvae in some populations of this species exhibit trophic polymorphism, with some individuals possessing exaggerated trophic features characteristic of a cannibalistic morphology in larval Ambystoma; e.g. a disproportionately broad head and hypertrophied vomerine teeth. We hypothesized that 1) head shape variation results from feeding upon different types of prey and that 2) cannibal morphs are induced by consumption of conspecifics. To induce variation, we fed three groups of larvae different diets: 1) brine shrimp nauplii only; 2) nauplii plus anuran tadpoles; 3) nauplii, tadpoles and conspecific larval salamanders. Comparisons of size (mass)-adjusted means revealed that this manipulation of diet induced significant variation in six measures of head shape, but not in the area of the vomerine tooth patch. For five of the six head traits, larvae that ate tadpoles and brine shrimp nauplii developed significantly broader, longer and deeper heads than did larvae that only ate brine shrimp nauplii. The ingestion of conspecifics, in addition to nauplii and tadpoles, significantly altered two head traits (interocular-width and head depth), compared to larvae only fed nauplii and tadpoles. Canonical discriminant function analysis detected two statistically reliable canonical variables: head depth was most highly associated with the first canonical variable, whereas three measures of head width (at the jaws, gills and eyes) and interocular width were most highly associated with the second canonical variable. Despite this diet-enhanced morphological variation, there was no indication that any of the three types of diet (including conspecific prey) induced the exaggerated trophic features of the cannibal morph in this species. These results illustrate that ingestion of different types of prey contributes to plasticity in head shape, but that some other proximate cue(s), either alone or in combination with diet variation, is essential to induce the extremes of trophic polymorphism in this species.     

Expression of the oligopeptide transporter, PepT1, in larval Atlantic cod (Gadus morhua)

The intestinal absorption of di- and tri-peptides generally occurs via the oligopeptide transporter, PepT1. This study evaluates the expression of PepT1 in larval Atlantic cod (Gadus morhua) during the three weeks following the onset of exogenous feeding. Larval Atlantic cod were fed either wild captured zooplankton or enriched rotifers. cDNA was prepared from whole cod larvae preceding first feeding and at 1000 each Tuesday and Thursday for the following three weeks. Spatial and temporal expression patterns of PepT1 mRNA were compared between fish consuming the two prey types using in situ hybridization and quantitative real-time PCR. Results indicated that PepT1 mRNA was expressed prior to the onset of exogenous feeding. In addition, PepT1 was expressed throughout the digestive system except the esophagus and sphincter regions. Expression slightly increased following first-feeding and continued to increase throughout the study for larvae feeding on both prey types. When comparing PepT1 expression in larvae larger than 0.15-mg dry mass with expression levels in larvae prior to feeding, no differences were detected for larvae fed rotifers, but the larvae fed zooplankton had significantly greater PepT1 expression at the larger size. In addition, PepT1 expression in the zooplankton fed larvae larger than 0.15-mg dry mass had significantly greater expression than rotifer fed larvae of a similar weight. Switching prey types did not affect PepT1 expression. These results indicate that Atlantic cod PepT1 expression was slightly different relative to dietary treatment during the three weeks following first-feeding. In addition, PepT1 may play an important role in the larval nutrition since it is widely expressed in the digestive tract.     

A comparison between predation effects on zooplankton communities by Neomysis and Chaoborus

Community level effects of predation by two invertebrate predators, the opossum shrimp (Neomysis intermedia), and the larva of the phantom midge (Chaoborus flavicans) were studied and compared. N. intermedia appeared abundantly in the shallow eutrophic Lake Kasumigaura and had a significant impact on the zooplankton community. The predation pressure by Neomysis was highest on cladocerans, followed by rotifers, and finally copepods. At high densities (maximum nearly 20 000 individuals m^–2), Neomysis excluded almost all cladocerans, rotifers and copepods from the lake.Zooplankton communities were established in experimental ponds, into which C. flavicans was introduced. The predator's density was around 1 individual l^–1, and was probably controled by cannibalism. Although Chaoborus larvae feed on various zooplankton species, their predation impact on zooplankton populations was markedly selective. They eliminated medium- and small-sized cladocerans and calanoid copepods from the ponds, but rotifers increased.Although the feeding selectivities of Neomysis and Chaoborus individuals were similar, the predation effects on zooplankton communities by the two predators were different.     

Feeding behavior of the cichlid,Sarotherodon galilaeum: selective predation on Lake Kinneret zooplankton

In laboratory feeding trials, we analyzed the feeding behavior and selectivity of the cichlid, Sarotherodon galilaeum, for zooplankton prey from Lake Kinneret, Israel. The feeding behavior was dependent on fish size. Fish less than 20 mm SL fed on zooplankton as obligate particulate feeders. Fish from 20 to 42 mm SL fed either as particulate feeders or as filter feeders. Fish larger than 62 mm SL fed as obligate filter feeders. Particulate-feeding fish were size selective and had highest feeding electivities for large-sized zooplankton species. Filter-feeding fish had highest feeding electivities for zooplankton species with poor escape ability. In general, S. galilaeum predation pressure would be greatest on Ceriodaphnia reticulata, a large-bodied and easily captured species which is selected by both particulate-feeding and filter-feeding fish.     

Distribution of Hexagenia nymphs and visible oil in sediments of the Upper Great Lakes Connecting Channels

As part of the study of the Upper Great Lakes Connecting Channels sponsored by the U.S. Environmental Protection Agency, the U.S. Fish and Wildlife Service examined the occurrence of Hexagenia nymphs and visible oil in sediments at 250 stations throughout the St. Marys River and the St. Clair-Detroit River system from May 14 to June 11, 1985. The mean density of Hexagenia nymphs per square meter averaged 194 for the total study area, 224 in the St. Marys River, 117 in the St. Clair River, 279 in Lake St. Clair, and 94 in the Detroit River. The maximum density of nymphs ranged from 1,081 to 1,164 m^-2 in the three rivers and was 3,099 m^-2 in Lake St. Clair. A comparison of nymph density at 46 stations where oil was observed in sediments physically suitable for nymphs showed that densities were lower in oiled sediments (61 m^-2) than in sediments without oil (224 m^-2). Densities of nymphs were relatively high at only four stations where oil was observed in sediments. In general, oiled sediments and low densities of nymphs occurred together downstream from industrial and municipal discharges.Contribution number 736 of the National Fisheries Research Center-Great Lakes, U.S. Fish and Wildlife Service, 1451 Green Road, Ann Arbor, MI 48105.     

Hemoglobin transition in relation to metamorphosis in normal and isogenicXenopus

Summary Electrophoretic separation of hemoglobins of normalXenopus laevis and of isogenic animals derived from female hybrids ofXenopus laevis×Xenopus gilli revealed 5–9 components in premetamorphic larvae, and 3–4 components in adult toads. InXenopus laevis the number of larval hemoglobin components showed considerable variation, but this variation was absent in isogenic tadpoles, suggesting a genetic basis for hemoglobin polymorphism in larvae.Electrophoretic separation of larval and adult hemoglobins at different concentrations of acrylamide and treatment of these solutions with mercaptoethanol revealed that larval hemoglobin components are charge isomers, whereas adult hemoglobin was found to contain a minor dimeric component.Estimation of hemoglobin components showed that the main increase in adult hemoglobin, i.e from 30–90% of total hemoglobin, occurs within 4 weeks after completion of metamorphosis. By incroporation of³H amino acids in vivo a switch to preferential synthesis of adult hemoglobin and a corresponding decrease in larval hemoglobin production could be demonstrated during early climax stages. This suggests that thyroid hormones are involved in the hemoglobin transition. Yet chemical inhibition of the larval thyroid by thiourea resulted in a delayed but complete hemoglobin transition without morphological transformation. It is concluded that hemoglobin transition and morphological transformation of theXenopus tadpole require different concentrations of thyroid hormones.Abbreviations Hb hemoglobin - Hb_A adult hemoglobin - Hb_L larval hemoglobin     

Creating conditions for changes in prey community structure by Chaoborus spp. in a lake in Sweden

In Lake G»rdsjön (Southwest Sweden), liming as an experimental improvement of living conditions for pelagic algae, resulted in a significant increase of algal biomass and a reduction of mean cell size. The algal development was beneficial for small sized filter feeding zooplankton, particularly rotifers, which showed a significant increase. The increase in abundance of small sized zooplankton created better food conditions for the smaller instars, and thus a much better overall survival of Chaoborus larvae. The resulting, 6–7 times larger population of Chaoborus larvae significantly changed the structure of the crustacean zooplankton community. Bosmina coregoni, the fastest swimmer of the crustacean species suffered most and was strongly reduced by the increased predation from Chaoborus. The share of cladocerans decreased, while copepods increased in importance.     

Interfacial feeding behavior and particle flow patterns ofAnopheles quadrimaculatus larvae (Diptera: Culicidae)

The interfacial feeding behavior, mouthpart movements, and particle flow patterns of Anopheles quadrimaculatuslarvae were investigated, using videotape recordings, high-speed microcinematography, SEM, and laboratory experiments. While positioned at the water surface, larvae demonstrated 12 behaviors associated with movements of the head. In one of these, a larva rotated its head 180° and directed its mouthparts against the air-water interface. The larva rapidly extended and retracted its lateral palatal brushes (LPBs) at a rate of 5 cycles/s (5 Hz), creating currents and allowing for the collection of particles. Particles moved toward the head at a velocity of 4.31 mm/s, in discrete stops and starts, as the LPBs beat. Our analyses determined that particle movement toward the mouth was governed by very low Reynolds numbers (0.002–0.009). This finding indicated that viscous forces predominated in Anophelesfeeding and no inertial movement of particles occurred. According to this model, the LPBs cannot intercept particles directly, but function as paddles for particle entrainment. We did not observe the pharynx to function in particle filtration but, rather, in food bolus formation. We propose that the maxillary pilose area and midpalatal brush function as interception structures. It appeared that the LPBs do not break the surface film to feed, but collect particles from the surface microlayers. A plume of uningested particles emerged from the sides of the cibarium and descended into the water column. The plume consisted of alternately clear and dark, lenticular laminae formed beneath the larval head during the collecting filtering feeding mode. A comparison of particle sizes from surface microlayers and gut contents of fourth instars showed that larvae ingested mainly small particles in the range of 1.5 to 4.5 pm in diameter. The potential significance of interfacial feeding by anopheline larvae in their aquatic environment is discussed.