Trichocerca Similis Research Articles

Fine structure of the retrocerebral organ in the rotifer Trichocerca similis (Monogononta)

AbstractThe retrocerebral organ (RCO) is a complex glandular system that is widely distributed across species of phylum Rotifera (sensu stricto). This system is hypothesized to secrete mucus that aids in benthic locomotion, adhesion, and/or reproduction. Unfortunately, the ultrastructure of the RCO is mostly unknown, having only been partially examined in one species. We used transmission electron microscopy and confocal laser scanning microscopy to describe the RCO in the planktonic freshwater rotifer Trichocerca similis. Results reveal the RCO to be a singular syncytial organ composed of a posterior glandular region, an expansive reservoir, and an anterior duct. The glandular portion has an active synthetic cytoplasm with paired nuclei, abundant rER, ribosomes, Golgi, and mitochondria. Electron‐dense secretion granules accumulate at the anterior end of the gland and undergo homotypic fusion to create larger, more electron‐lucent granules with numerous mesh‐like contents that gradually fuse into tubular secretions that accumulate in the reservoir. Ultrastructure of these secretions suggests they may be hydrated glycoproteins. Cross‐striated longitudinal muscles form a partial sleeve around the reservoir and may function to squeeze the secretions through the single cytoplasmic duct that penetrates the cerebral ganglion. A review of the RCOs from other rotifers suggests that further ultrastructural analyses are required before attempting to discern their functions and homologies.

Effects of macrophyte cultivation on community structure of rotifer in small lakes.

To understand the effects of macrophyte growth on water environment and rotifer community structure, monthly survey lasted for one year from October 2015 to September 2016 was conducted in Lake Minghu and Lake Nanhu at Jinan University in Guangzhou. There was no macrophyte cultivation in Lake Minghu. A kind of macrophyte Hydrilla verticillata grows from March-October in Lake Nanhu and grows vigorously in June-September. In this study, 50 species belonging to 23 gene-ra of rotifer, were recorded in the two lakes. 32 species were found in Lake Minghu and 39 species in Lake Nanhu, respectively. The dominant species in Lake Minghu were Polyarthra vulgaris, Trichocerca pusilla and Liliferotrocha subtilis. The dominant species in Lake Nanhu were Keratella cochlearis, Colurella adriatica and Lecane (Monostyla) bulla. The highest abundance of rotifers in Lake Minghu was at 3790 ind·L-1 in October 2015, and the lowest at 854 ind·L-1 in March 2016. The highest abundance of rotifers in Lake Nanhu was at 3555 ind·L-1 in November 2015, and the lowest at 977 ind·L-1 in January 2016. Results from the ANOSIM (one-way) test revealed that the community structure of rotifers was significantly different between Lake Minghu and Lake Nanhu. The largest contribution rate of the difference between the two lakes was made by Keratella cochlearis and Polyarthra vulgaris. RDA analysis showed that total phosphorus, chlorophyll a, and water temperature had significant impacts on the community structure of rotifer in Lake Minghu. During the period with luxuriant macrophyte in Lake Nanhu, the rotifer community structure was most influenced by total phosphorus, water temperature, chlorophyll a and water depth, dominated by benthic species Squatinella mutica, Colurella adriatica and Lecane bulla. During the period with sparse macrophyte, the community was mainly influenced by total nitrogen and transparency, dominanted by planktonic species Polyarthra vulgaris and Trichocerca similis. During the period with no macrophyte, communities were mainly influenced by pH and dissolved oxygen, and also dominated by planktonic ones Polyarthra vulgaris and Anuraeopsis fissa. Macrophyte could effectively absorb nutrients in water, inhibit growth of phytoplankton, improve water quality, increase spatial heterogeneity, support more species of rotifer, and maintain a stable aquatic ecosystem.

Zooplankton Diversity of Osmansagar Reservoir, Telangana, India

Zooplankton composition and diversity in Osmansagar reservoir, Telangana, India were investigated during the year 2010–2012. Collected data revealed the occurrence of 66 species of zooplankton, of which 50 rotifers, 14 cladocerans and two copepods. The population density of zooplankton varied from 83 to 1080 Ind./L. The highest density was due to rotifer and copepod population. Diversity (H′) was ranged from 0.679 to 2.631, evenness (J′) was 0.257 to 0.904 and species richness was 8–24. The less diversity and evenness was observed in November 2011, March and August 2012, because of copepoda dominance by Mesocyclops leuckarti. The population abundance of zooplankton was Brachionus forficula, Brachionus diversicornis, Brachionus calyciflorus, Keratella tropica, Trichocerca similis and Diaphanosoma sp. especially during the summer seasons in both the years. There was no seasonal periodicity of any zooplankton communities, but the population turnover was observed from rotifers to copepoda over the study period. The significant inter and intra relationship between and within physicochemical and zooplankton indices were noted. It is found that surface water temperature, magnesium and total hardness could be a limiting factor for the species richness. Dominance may be due to the numerical abundance of rotifer and copepod and it has a positive significant correlation with pH.

ZOOPLANKTON AND PHYTOPLANKTON IN ALL ANOXIC LAYERS OF LAKE FUKAMI-IKE

Lake Fukami-ike is a small monomictic and eutrophic lake, located in southern Nagano Prefecture in Central Japan. An uncommon phenomenon of dissolved oxygen 0-1 mgL -1 from the surface to the bottom layer occurred on 16 November, 2013. On November 2, Trichocerca similis (Rotatoria) etc. dominantly distributed from 0-5 m depth depending on distribution DO concentration. Tintinnopsis lacustris (Protozoa) was found in the bottom layer (anoxic condition). The number of cells (Fragilaria rumpens (Bacillariophyceae) etc. abounded from the 0-5 m layer. On November 16, T. lacustris distributed from the surface to the bottom layer. Then, T. lacustris continued to distribute from the surface to the bottom layer, and the genus Synedra was found to have distributed uniformly from the surface to 6 m depth in the next investigation on 21 December, when the DO concentration was about 10 mgL -1 from the surface to the bottom layer. The changes in vertical distribution of T. lacustris, seemed to indicate that lake water was lifted from the bottom layer when shift stagnation periods to circulation periods.

Biodiversity of rotifers in urban water reservoirs of Southern Poland

Abstract Rotifer diversity was studied in three different types of artificial water bodies situated in one of the largest cities in Poland. The bodies of water were as follows: a fountain in a city park, the Nowa Huta dam reservoir, and the Bagry gravelpit. The lowest number of rotifer species (23) was noted in the urban fountain, and the highest (36) in the Bagry gravel-pit lake. Rotifers’ communities in the investigated water reservoirs showed different levels of diversity and density. The most numerous species in the dam reservoir was Trichocerca similis (2511 ind. l-1) and in the Bagry gravel-pit lake was Polyarthra dolichoptera (2634 ind. l-1). However, the most abundant species in the fountain was Brachionus urceolaris (863 ind. l-1). The highest values of Shannon and evenness diversity indices were found in the Bagry gravel-pit lake (H’ = 2.521, J’ = 0.783), while the lowest in the urban fountain (H’ = 0.568, J’ = 0.258). The DCA analysis (Detrended Correspondence Analysis) showed that the diversity of rotifers communities in the studied reservoirs was related to the concentration of magnesium and calcium, and to water conductivity.

Small dams profoundly alter the spatial and temporal composition of zooplankton communities in running waters

We analyzed the influence of a small dam on the temporal and spatial properties of zooplankton communities by comparing the zooplankton composition established prior to the dam's construction to that during 5 years after its construction. The dammed water of a small dam, even just by 1 m, induced spatial changes in the zooplankton communities similar to those induced by a large dam. Changes in the number of taxa and zooplankton densities were observed a few months after the construction of the dam. In particular, in the number of planktonic rotifers such as Keratella cochlearis, Filinia longiseta, Polyarthra longiremis, Trichocerca similis and the larval stages of copepods considerably increased, along with the density of the littoral taxa of rotifers and crustaceans. Cladocerans appeared in the impoundment after the construction of the dam and their abundance increased in the second year after the dam was built. The abundance of all systematic groups of zooplankton increased in subsequent years after the construction of the dam. The small dam had a great effect on the spatial and temporal composition of the zooplankton communities in the stream studied, related to an abrupt change in the physical conditions from those typical of flowing water to those typical of limnetic basins.

On body size and habitat selection in rotifers in a macrophye-dominated lake Budzyńskie, Poland

Lake Budzynskie is shallow, freshwater lake with a well-developed and differentiated macrophytic vegetation. Zooplankton samples were collected from five stations: two of them in submerged macrophytes (Chara and Myriophyllum), one in the zone of floating leaves (Potamogeton), a rush station (Typha) and one in the open water surrounding the vegetation beds. The mean Rotifera densities differed significantly between the lake parts. Furthermore, different habitats were characterised by differences in body size with the exception of the middle body size group (Keratella cochlearis, Polyarthra vulgaris and Trichocerca similis), which was dominated by limnetic representatives. However, in all the other size-dependent groups both stands of submerged macrophytes were characterised by much higher densities than other zones. Additionally, body size within the examined habitats significantly differed. Thus, the size structure of Rotifera communities was directly related to morphological and spatial structures of the substrata. Two groups of habitats were distinguished: the first one consisting of open water and two vegetated zones of less complicated structure (Potamogeton and Typha), and the second of more complex submerged macrophyte species (Chara and Myriophyllum). The differentiation of the architecture of macrophytes affected the nutritional conditions and refuge effectiveness of these habitats.

Community Structure of Rotifera and Ecological Assessment of Water Quality in Qiandao Lake

The community structure of Rotifera and ecological assessment of water quality, including species composition, population dynamics, standing crops and indicator organisms, E/O and Q-B/T index, was studied in a large, deep lake, Qianda Lake, Zhejiang Province. After one-year monitoring in situ, 70 species of Rotifera were found, 41 species were pollution indicator species, of which oligo- βmesosaprobic, βmesosaprobic andβ-αmesosaprobic species accounted for 41. 5%, 36. 5% and 22. 0% of the total indicator species, separately. In terms of annual average density, the dominant species were Keratella cochlearis, Trichocerca similis and Polyarthra trigla, which accounted for 59.2% total density. Asplanchna spp. accounted for 56.5% of total biomass. The annual average density of Rotifera was 103 ind. /L, and biomass was 0.069mg/L. Except for Keratella cochlearis, which form stable population, the density distribution of other dominant species were varied temporarily from station to station, such as Schizocerca diversicornis density reached SOOind. /L at station II in May, Trichocerca similis density reached lOOOind. /L at station III in July, and Keratella cochlearis density reached 560ind. /L at station VI in September. Biomass changes in space and times were determined by Asplanchna spp, the higher biomass at station I and II in May, reached 0.169 mg/L and 0.275 mg/L, separately. The ranges ofE/O values in Qiandao Lake were 0.29-0. 62(0.46 on average), and the ranges of Q-B/T values were 0.17-1.50(0.68 on average). The regression analysis showed that the transparency of lake water had negative linear relationships with E/O value (r~2= 0.7182) and QB/T value(r~2= 0.7747). The characteristics of Rotifera community in Qiandao Lake showed the rich species number and low density, biomass and QB/T value compared with other large shallow lakes in China. Furthermore, biological assessment of water pollution and trophic condition were evaluated by species indicators, biotic index of E/O and Q-B/T. The trophic type of Qiandao Lake could be regarded as oligo-βmesosaprobic, of which station I , II and V were considered as P mesosaprobic, while others were considered as oligo-βmesosaprobic.

Enhancement of Planktonic Rotifers by Microcystis aeruginosa Blooms: An Enclosure Experiment in a Shallow Eutrophic Lake

ABSTRACT Enclosure experiments with three treatments (sediment addition, sediment + nitrogen enrichment, sediment + phosphorus enrichment) and unfertilized controls were performed in shallow hypereutrophic Lake Donghu during the summer of 2000. Dense Microcystis aeruginosa blooms occurred in all the enclosures during the experimental period but not in the surrounding lake water. Generally, the dominant rotifers were Polyarthra vulgalis, Filinia longiseta, Proales sp. and Asplanchna sp. at the beginning of the experiment, followed by a shift to Brachionus calyciflorus, Trichocerca similis, Cephalodella catellina and Anuraeopsis fissa, and finally to F. longiseta, Proales sp. and Keratella cochleris. M. aeruginosa blooms strongly suppressed the larger Diaphanosoma brachyurum but enhanced the development of the smaller cladocerans and rotifers that probably efficiently utilized organic matter from M. aeruginosa through the detritus food chain. The smaller cladoceran and rotifers coexisted successfully throughout the experimental period.

Seasonal variations of zooplankton abundance in the freshwater reservoir Valle de Bravo (Mexico)

Information on the density and diversity of zooplankton from drinking water reservoirs in Mexico is meagre. This is important not only from the point of view of lake management but also for providing clean drinking water for human populations. In the present work, we provide quantitative information on the seasonal variations of zooplankton and selected physico-chemical variables from Valle de Bravo, a large man-made reservoir in the State of Mexico. Based on the nutrient data, this reservoir can be regarded as mesotrophic. However, we found a high density of phytoplankton. Among Cyanophyceae, Anabaena, Microcystis, Nostoc and Oscillatoria were encountered, particularly during the warmer months. Microcystis blooms were observed from June to September. Diatoms dominated the phytoplankton during the remaining months of the year. Among zooplankton, Rotifera comprised the highest number of species. The most common species occurring throughout the year were Keratella chochlearis, Polyarthra vulgaris, Trichocerca capucina, Trichocerca similis, Asplanchna priodonta and Synchaeta pectinata. The highest abundance of K. cochlearis was observed in May at site 5 (340 ind 1−1 ). P. vulgaris occurred in high numbers throughout the year. Both Trichocerca capucina and T. similis were encountered in low numbers (mean annual average: 10 ind 1−1). Among the crustacean zooplankton, Bosmina longirostris occurred in higher numbers (up to 105 ind 1−1) regardless of the time of the year. Copepods were predominantly from the genera Mesocyclops and Megacylops; calanoids were rare. Naupliar stages were dominant during winter months (up to 670 ind 1−1). These results have been discussed from the point of view of Microcystis control in water bodies.

Zooplankton Grazing in a Potomac River Cyanobacteria Bloom

During summer, bloom-forming cyanobacteria, including Anacystis, Aphanizomenon, and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 108 cells l−1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing experiments were conducted in July and August 1987. Using size-fractionated river phytoplankton assemblages, zooplankton grazing rates were determined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on 14C-labeled river assemblages, 14C-labeled river assemblages enriched with unlabeled cyanobacteria, and unlabeled river assemblages enriched with 14C-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mira, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trichocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither rotifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl· [individual · h]−1 while copepod rates ranged from undetectable to 135 μl · [copepod · h]−1. Grazing rates on labeled river phytoplankton assemblages were 4–100 times higher than noted for zooplankton feeding on cyanobacteria. The addition of the colonial alga to labeled river phytoplankton assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no added cyanobacteria. Total zooplankton demand for cyanobacteria and river phytoplankton assemblages was estimated for the study period July–August 1987. Rotifer plus C. vernalis herbivory would have removed 1–5% and 49%, respectively, of the standing stock of the two autotroph pools each day. Literature-derived clearance rates for Bosmina indicate, however, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungrazed and may be transported to the lower estuary for salinity-induced aggregation and sedimentation.

Toxicity of a soluble peptide from Microcystis sp. to zooplankton and fish

SUMMARY. A soluble cell fraction lethal to Daphnia magna was isolated by the disruption of net phytoplankton from eutrophic Aculeo Lake. The phytoplankton of this lake is dominated by Microcystis sp. The soluble fraction was also lethal to common native zooplankton, including the rotifers Keratella sp., Trichocerca similis, the copepod Boeckella sp., and the cladoceran Chydorus sphaericus. In addition, a partially purified soluble component of the lethal fraction caused the death of the fish Gambusia affinis. The specific toxicity of the soluble fraction from phytoptankton varied four‐ to six‐fold seasonally with peaks during periods of warm temperature. Our results suggest that releases of a toxic peptide from Microcystis sp. may be the cause of massive fish kills at the study site, Aculeo Lake.

Ecological Studies and Trophic State Evaluation of a Tropical Impoundment in West Africa

Weekly base‐line data (September 1983 … March 1984) on plankton community structure, abundance, and primary productivity of a small (A = 3.7 ha, Zm = 16m) water‐supplying reservoir (Kongingiri Dam, alt. 1343 m) in Jos Plateau, Nigeria are presented. Supporting ecological observations include thermal and optical properties, pH, conductivity, dissolved oxygen, alkalinity, chloride and dissolved organic matter.Water characteristics of the exosystem showed varying turbidity (Secchi=1.0−2.8 m), a circumneutral character (pH, x̄=7.1), predominance of bicarbonate (x̄x=0.26 meq/l) system, and moderate ionic composition (x̄x=23 μS/cm).The plankton community was dominated by Bacillariophyceae and Chlorophyceae in population density, and Chlorophyceae in species richness. The important forms were Nitzschia spp. Synedra ulna, Navicula spp. Melosira italica, Pinnularia sp. Staurastrum spp., Cosmarium, Closterium, Eurastrum platycerum, Micrasterias americana, M. crux‐melitensis, Teilingia granulata Desmidium swartzii, Scenedesmus sp. Pediastrum boryanum. The zooplankton community had numerical dominance of rotifers. The principal zooplankton species included Brachionus falcatus, Keratella cochlearis, K. tropica, Trichocerca similis, Pompholyx complanatus, Bosmina longirostris, Ceriodaphnia cornuta, and species of Mesocyclops, Thermocyclops, and copepod nauplii. The ecological characteristics and productivity rates indicate an oligo‐mesotrophic state of the aquatic ecosystem.

Grazing on Bacteria by Zooplankton in Australian Billabongs

Five species of rotifer (Brachionus lyratus, Conochilus dossuaris, Keratella cochlearis, Keratella quadrata and Trichotria tetractis) ingested bacteria at a prey density of 5-7 × 109 cells L.-1, and a sixth (Keratella procurva) did so at a higher bacterial concentration of 4 × 1010 cells L-1. Gastropus minor, Polyarthra vulgaris, Synchaeta pectinata and Trichocerca similis were not bacteriovores. Fluorescent-stained (DATF or acridine orange) bacteria were not suitable for determining which rotifers were potential grazers. However, bacteriovory was detected easily with safranin-red or methylene-blue stained bacteria. Clearance rates for the zooplankton community larger than 51�pm, determined with 3H-thymidine labelled bacteria, varied from <0.1-71 mL L-1. H-1, but were usually <5 mL L-1 h-1. Significant grazing was detected in >200�m, 90-200�m and 51-90�m fractions: it was impossible to ascribe bacteriovory to discrete taxonomic groups, because zooplankton taxa were not completely separated by these mesh sizes.

Differential use of food resources by the instars of Chaoborus punctipennis

SUMMARY. 1. Differential use of food resources by instars I‐1V of Chaoborus punctipennis was examined in a mesotrophic New Hampshire lake during midsummer when all instars coexisted. Diet composition and prey preference were quantified first, at night when all instars were feeding at the same depth, and second, during the day when the early instars (I‐II) and instar III were segregated by depth. Relative abundance, per cent biomass, and per cent frequency of occurrence of soft and hard‐bodied rotifers, flagellated phytoplankton, protozoans, and crustaceans in C. punctipennis diets were quantified using crop content analyses.2. All four instars ingested large, flagellated phytoplankton (mainly Dinobryon and Ceratium). This unexpected result suggests that the effects of phytoplankton on Chaoborus growth and survival should be investigated. All instars also consumed rotifers (mainly Kellicottia, Gastropus, Polyarthra), but only instars III and IV fed upon crustaceans (mainly Daphnia, Bosmina and Diaphanosoma). Small rotifers (Gastropus spp., Keratella cochlearis, Trichocerca similis) occurred more frequently and were more abundant in early instar diets than late instar (III and IV) diets, whereas large rotifers (Asplanchna priodonta and Keratella crassa) were eaten only by instars III and IV. Zooplankton with gelatinous sheaths (e.g. Ascomorpha, Collotheca and Holopedium) were rarely ingested.3. Developmental increase in gape diameter of C. punctipennis seemed to be the major proximate mechanism causing dietary differences among instars. Body widths of hard‐bodied prey in C. punctipennis crops were always less than gape diameter. The relationship between prey body width and Chaoborus gape diameter, coupled with knowledge of prey escape behaviour, should be useful for predicting the presence of hard‐bodied prey taxa in diets of other Chaoborus species.4. All four instars of C. punctipennis selected soft‐bodied, or weakly loricate, rotifer prey over crustaceans and phytoplankton. Early instars preferred the small rotifer T. similis and the protozoan Difflugia sp. to other rotifers and phytoplankton when feeding in the epilimnion or thermocline during the day or night. Late instars positively selected the rotifer, Asplanchna. Prey value (prey weight ingested per unit handling

The rotifers of Warri River, Nigeria

The rotifer assemblage of the dendritic tidal Warri River was investigated at four sampling points from July 1981 to July 1982. Forty-one species of Rotifera were identified. Five of these — Anuraeopsis racenesi Berzins Keratella cochlearis macracantha Lauterbom, Monostyla lunaris Pawlowsky, M. stenroosi Meissner and Trichocerca similis grandis Hauer — are being recorded for the first time in West Africa. The Warri River rotifers are 65% cosmopolitan and 20% pantropical although there was no species limited to African waters only. The dominant rotifers in the Warri River were Keratella tropica and K. cochlearis which were found in the predominantly freshwater sampling station at Udu Bridge to Warri and Orugbo Creek where low brackish water salinities and conductivities are observed from December or January to April each year. The preponderance and longitudinal distribution of these two well-known cyclomorphic rotifers provides an opportunity to study their salinity tolerance and the effect of salinity and conductivity on their cyclomorphosis by sampling along the length of the river from Udu Bridge to Forcados where the Warri River joins the Atlantic Ocean.

Natural changes in the planktonic rotifera of a small acid lake near Sudbury, Ontario following water quality improvements

Swan Lake is a small, acidic, metal-contaminated lake located near Sudbury, Ontario. During the past 8 yr, the lake has experienced a substantial increase in pH, together with significant reductions in the concentrations of heavy metals and base cations. These changes were observed only after acid and metal emissions from Sudbury area smelters were reduced. The composition of the planktonic Rotifera in the lake has changed concurrent with improvements in water chemistry. Dominance of the rotifer community by the acidophile Keratella taurocephala has been sharply reduced in recent years, while significant increases in the densities of Polyarthra spp., Chromogaster ovalis, Conochiloides natans and Trichocerca similis have been observed.

Pelagic rotifers of Lake Glubokoe from 1897 to 1984

After alteration in the pattern of drainage of run-off from the surrounding swamps and changes in the colour and transparency of the Lake Glubokoe water, the previously epilimnial Trichocerca similis, Conochilus unicornis and Keratella cochlearis showed a shift of their maximum numbers to deeper layers. Pompholyx sp. and Trichocerca capucina, which are regarded as indicators of eutrophic waters, have disappeared from the pelagic zone of the lake and Conochilus hippocrepis, Synchaeta pectinata, Gastropus stylifer, Asplanchna herricki, Ascomorpha ecaudis, Ascomorpha saltans, Euchlanis dilatata and Trichocerca porcellus have made their appearance. Most of the new species are considered to be indicators of oligotrophic conditions. However the total density of pelagic rotifers remained at the same level. Apparently the establishment of the new species of rotifers was possible due to some ‘rarefaction’ of the epilimnion, the disappearance of predaceous Mesocyclops leuckarti and the invasion of the pelagic zone by Peridinium cinctum.

Prey utilization by Chaoborus punctipennis Say in a small, eutrophic reservoir

Prey selection by 2nd-, 3rd-, and 4th-instar larvae of Chaoborus punctipennis was evaluated by comparing larval crop contents to the species structure of the zooplankton community in a small, eutrophic lake. The rotifers, Brachionus caudatus and Trichocerca similis, were positively selected by and comprised most of the diet of 2nd- and 3rd-instar larvae. Rotifers comprised 67% of all prey items identified in the crops of even 4th-instar larvae. Whereas 2nd-and 3rd-instar larvae selected the two rotifers with equalfrequency, 4th-instar larvae selected Trichocerca significantly more frequently than they did Brachionus. Although copepods comprised only 8% of all prey identified in 3rd-instar crops, they comprised 33% of the crop contents of 4th-instar larvae. Both 3rd and 4th instars exhibited a positive electivity for Cyclops vernalis significantly more frequently than for Diaptomus pallidus. We conclude that the larvae of Chaoborus punctipennis can, and do, select between Trichocerca and Brachionus and between Cyclops and Diaptomus.