Food selectivity and diel vertical distribution of Chaoborus edulis (Diptera, Chaoboridae) in Lake Malawi

1. Diel diet and vertical distribution patterns of the larval instars of Chaoborus edulis were studied in deep water near the central part of Lake Malawi, Africa.
2. First instar larvae contained very little food in their crops and probably depended on reserves from the egg. Second, third and fourth instars fed on zooplankton and were size-selective in their feeding. The mean size of prey eaten by the three instars was significantly different from each other, with larger instars feeding on larger prey. Smallest available prey was selected against and the upper size of prey was probably constrained by larval gape. Nauplii were not found in any of several thousand larvae examined. Phytoplankton did not form a significant part of the diet.
3. There was a progressive and related increase in diel periodicity in feeding and vertical migrations of successive instar stages. Fourth instars migrated particularly large distances. Such migrations removed them from their zooplankton food supply but avoided predators. A refuge from predators is probably found in or near the permanent zero oxygen boundary, at depths greater than 200 m.     

The occurrence and behaviour of adult Chaoborus and Procladius (Diptera: Nematocera) from Lake George, Uganda

A suction trap was used to collect adult Nematocera from the equatorial Lake George, Uganda. The dominant species, Chaoborus (Neochaoborus) anomalus Edwards, Chaoborus (Sayomyia) ceratopogones and Procladius (Procladius) brevipetiolatus Goetghebuer, all showed a lunar periodicity of occurrence in the samples being most abundant in the first quarter of the lunar month and least abundant in the third quarter. Chaoborus also showed a seasonal variation in occurrence, not apparent in Procladius. Observations on sex ratios and on longevity, swarming and oviposition behaviour of Chaoborus made at Lake George and at the neighbouring Lake Edward are also described.     

The Feeding of Chaoborus flavicans Meigen (Diptera,Chaoboridae). and its Predation on Lake Zooplankton

In a study of Chaoborus feeding in a eutrophic lake, selectivity was found to be positive with Crustacea (especially copepodit stages). and negative with Rotatoria. Daily food rations were about 20% for most of the feeding period, but higher (106%). during the month of intensive growth after hatching. Feeding intensity correlated positively with amount of food an temperature, and negatively with Chaoborus concentration. Elimination of Crustacea (in the epilimnion of the central zone of the lake). equalled about 30–40% of Crustacea production in June and September and slightly exeeded the August production (it was almost zero in the remaining months because Chaoborus larvae stayed at the bottom). This applies, however, only in the central zone – about 50% of the lake volume. Chaoborus probably influences both the density of zooplankton and the quantitative relations between zooplankton species.     

The Chaoborus (Diptera: Chaoboridae) component of the invertebrate drift of the Mwenda river,Lake Kariba,Central Africa

Chaoborus larvae were collected from the riverine drift of the Mwenda River during the 1972/3 rainy season. Large numbers of larvae were found in the early stages of flow following the 1972 dry season, fewer were found ten days later.Larvae which reached the lake in this way appeared to show no ill effects. Possible mortality arising prior to reaching the sampling point was doubtful since no dead larvae or parts were found in the material collected.     

Zooplankton populations and the diets of three Chaoborus species (Diptera, Chaoboridae) in a tropical lake

SUMMARY. 1. Zooplankton populations in a small, natural, tropical lake are dominated by a few, small-sized taxa including the copepod Thermo- cyclops consimilis , the cladoceran Moina micrura and several rotifer species.
2. Moina micrura and adults of Thermocyclops consimilis undergo diel vertical migrations within the water column. Population densities of T. consimilis show marked intra-annual variations which may to some extent be related to variations in rainfall and to lunar periodicities In predator abundance.
3. The diets of Chaoborus lavae include other Chaoborus , Cladocera, Copepoda. Rotifera and the dinoflagellate Peridinium. The diets of late instars of the largest species, edulis , were dominated by Crustacea, while those of the two smaller species. C, ceratopogones and C. anomalus , were dominated by Rotifera and the dinoflagellate alga Peridinium , as were the diets of early instars of all species. Algae have not been previously reported to be a large component of the diet of Chaoborus populations in nature.
4. Some of the dietary differences among Chaoborus instars and species are related to the size of each prey species in relation to the mouth gape of each instar. However, there are also important differences in electivity among instars of different species of the same size.     

Studies on the biology of Chaoborus flavicans (Meigen) (Diptera: Chaoboridae) in a fish-free eutrophic pond,Japan

Population dynamics of Chaoborus flavicans larvae of various instars was studied from November 1986 to December 1987 in a eutrophic, fish-free pond, Japan. First and 2nd instar larvae were observed from late April to late October, indicating a reproductive period of about half a year. C. flavicans overwintered in the 4th instar larvae. In water column samples, total density of all instars was 680–23680 m^-2, and pupal density 0–2600 m^-2; larvae of the 1st, 2nd, and 3rd instars showed 5–6 density peaks in 1987, suggesting that 5–6 generations occur during a year (peaks of the 4th instar larvae were not clear, probably due to their longer development than those of younger instars). In sediment samples, no 1st and 2nd instar larvae were found, 3rd instar larvae were found occasionally but density of the 4th instar larvae was 280–18600 m^-2, and pupal density varied between 0–502 m^-2. Fouth instar larvae accumulated in sediment in the cold season and in the water column in the warm season; high temperature and low oxygen concentration were the most important factors limiting the distribution of larvae in the sediment in summer in the NIES pond. The dry weight of total C. flavicans larvae was 0.08–4.2 g m^-2 in sediment samples and 24–599 μg l^-1 (0.10–2.40 g m^-2) in water column samples. Comparisons of maximum densities in the NIES pond in different years and in waters of different trophic status show that density is generally higher in eutrophic than in oligotrophic habitats. This revised version was published online in July 2006 with corrections to the Cover Date.     

The estimation of crop evacuation rates in Chaoborus larvae (Diptera: Chaoboridae) using natural prey

SUMMARY. 1. Crop evacuation rates were estimated for the first time in Chaoborus larvae, using natural prey.
2. Fourth instar C. americanus Matheson digested copepods (Diaptomus leptopus S.A. Forbes) 48.7% faster than daphnids (Daphnia rosea Leydig) of similar size; meal size did not significantly affect the instantaneous rate of digestion (IRD) within each prey type. Prey specific IRD has not been reported before for zooplankton.
3. Prey specific differences in IRD require the use of natural prey when digestion experiments are to be used to estimate natural rates of food consumption of animals.     

The life history and production of Chaoborus punctipennis (Diptera: Chaoboridae) in Lake Norman,North Carolina,U.S.A.

A study of the life history and production of Chaoborus punctipennis (Say) in Lake Norman, North Carolina, U.S.A. was conducted from February 1978 through January 1979. Four sublittoral (～8 m) and two profundal (～30 m) locations were sampled. Larvae and pupae were collected with a modified Petersen grab and a plankton net, and adults were collected with emergence and light traps. Based on larval, pupal, and adult collections, there appear to be two generations per year — an overwintering spring generation and a summer generation. Annual dry weight standing stock biomass, dry weight production, and P/ B ratio were estimated from each sampling location and depth zone. Production was estimated by the size-frequency method. Standing stock biomass (30.9 mg · m^-2) and production (170.8 mg · m^-2) were highest in the profundal zone. In the sublittoral zone, standing stock biomass and production were 4.7 mg · m^-2, and 29.6 mg · m^-2, respectively. Annual P/ B ratios in the profundal and sublittoral zones were 5.5 and 6.3, respectively.     

Ecological notes on the Haplochromis (Pisces: Cichlidae) species-flock of Lake George, Uganda (East Africa)

Lake George, a shallow equatorial lake, supports a Haplochromis species-flock of 17 species. Of the five species that dominate the species-flock, two feed mainly on benthic and mid-water dipteran larvae, one is a piscivore, one exploits microcrustacea and another the abundant blue-green phytoplankton population. There is little stenophagy in this group. There is a seasonal variation in the total gill-net catch of the three dominant Haplochromis of the sandy shoal habitat, with high catches in ths wet season and low catches in the dry. Similar fluctuations are known to occur in the commercial catches of Sarotherodon niloticus. No seasonal changes were observed in the proportions of the individual species making up this community and the individuals breed all the year round.
A combination of ecologically unspeeialized species, resulting in an efficient exploitation and stability of the ecosystem may account for the observed high fish biomass and the high numbers of individuals. Taking into account the effect of the monotony of the environmental conditions, the physical continuty of the available habitat types and the relatively short time that the lake has been in existence, coupled with the apparent lack of rigid ecological requirements by the individual members of the Haplochromis group, it is suggested that this species-flock may still be at an early stage in its evolutionary develoment.     

Factors contributing to coexistence of Chaoborus flavicans and C. punctipennis (Diptera,Chaoboridae) in a small meromictic lake

In Crawford Lake, a small meromictic water body in southern Ontario, Canada, the life cycles of planktonic Chaoborus flavicans and C. punctipennis were out of phase by about two months, the former pupating from mid May through mid June and the latter from July through September. C. flavicans possibly produced a second annual generation. Fourth instars of both species were strong diel migrators and occupied similar strata at most times. C. punctipennis fourth instar diet consisted almost entirely of rotifers. C. flavicans ate rotifers but fed heavily also on daphnids in May and August. C. punctipennis fourth instars showed little growth until early spring, possibly owing to a sparseness of rotifers, then grew rapidly until pupation. C. flavicans had a slow, but relatively constant growth rate at all times during the open water season, presumably because its greater mouth gape allowed it a wider range of food items.     

Isolation of polymorphic microsatellite loci in three phantom midge species of the genus Chaoborus (Diptera: Chaoboridae)

Because of its widespread distribution in lakes and ponds Chaoborus is of great interest to many freshwater ecologists. Interestingly some species are restricted to small fish‐less water bodies, whereas other species live mostly in large lakes. To eventually test the genetic and evolutionary implications of these different lifestyles we identified microsatellite loci in three species in this preliminary study: C. obscuripes, C. crystallinus and C. flavicans. Using a biotin/streptavidin capture technique of repetitive sequences in a 96 well format, we obtained microsatellite‐enriched genomic libraries for all three species and identified six polymorphic microsatellite markers for each species.     

Persisting stemma neuropils in Chaoborus crystallinus (Diptera: Chaoboridae): Development and evolution of a bipartite visual system

Stemmata or “larval” eyes are of crucial importance for the understanding of the evolution and ontogeny of the hexapod's main visual organs, the compound eyes. Using classical neuroanatomical techniques, I showed that the persisting stemmata of Chaoborus imagos are connected to persisting stemma neuropils neighboring the first and second order neuropils of the compound eyes, and therefore also the imago possesses a stemma lamina and medulla closely associated with the architecture and the developmental pattern of those of the compound eyes. The findings are compared with other arthropods, e.g. accessory lateral eyes in Amandibulata and Myriapoda, suggesting some ancestral rather than derived character states. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Diurnal stratification, photosynthesis and nitrogenfixation in a shallow, equatorial Lake (Lake George, Uganda)

From a series of experiments and measurements covering one 24-h period in Lake George the diurnal patterns of photosynthesis, nitrogen-fixation and stratification are described. The thermal cycle shows three distinct phases of isothermy, intense stratification and mixing. During stratification the surface water temperature reached 36°C, whereas the bottom temperature remained at 25°C. The phytoplankton are evenly distributed at dawn, sink out during the day and tend towards an even redistribution as thermal stratification breaks down. Depth-time profiles of oxygen concentration and pH values showed that intense photosynthetic and respiratory activity occurred. The shortness of the predominant limnological cycle is thought to be instrumental in restricting species fluctuations. Changes of total CO2 concentration in situ within the euphotic zone indicated that 2.25 g C/m².12 h were photosynthetically fixed during daytime. From changes of in situ oxygen concentration, gross photosynthesis was estimated to be 12 g O₂/m².12 h and net particulate production over 24 h as zero. Photosynthetic activity was also measured with the i^C and oxygen techniques by enclosing algal samples in light and dark bottles. The patterns of activity recorded by the two methods were similar, although quantitatively the oxygen method gave rather higher values than did the method. Daily gross photosynthesis was estimated as 15.56 g O₂/m².12 h and 4.5 gC/m².12 h. These figures are compatible with one another and with the production estimates calculated from Tailing's model of integral daily photosynthesis. The balance between respiration and photosynthesis is described as a function of the underwater light climate and the relative rate of respiration. Both the experimental and theoretical approaches suggest that the column 24-h net particulate production is an extremely small percentage of gross photosynthesis. Nitrogen-fixation was measured using both the ¹⁵N₂ and the acetylene reduction techniques. The day-long incubation of the ¹⁵N₂ experiment showed that 11 mg N/m².day were fixed. The series of shorter acetylene reduction experiments gave a value of 58 mg N/m².day. The acetylene technique showed relatively high nitro-genase activity at very low light intensities. The ¹⁵N₂ results indicated little N2-fixation at low light intensities. The differences between the two techniques are discussed. Gross photosynthesis estimates calculated from seven different methods run simultaneously are compared. The figures show reasonable agreement, but since the gaseous environment in the euphotic zone is particularly suitable for photo respiration the values are not considered reliable estimates of phytoplankton photosynthesis.     

Observations on the Cladocera of Lake George, Uganda

Seven species of Cladocera were recorded from the shallow, equatorial Lake George in Uganda, during the period 1967–72. Of these only three, Moina micrura Kurz, Ceriodaphnia cornuta Sars and Daphnia barbata Weltner, are found in the open water and they form a small proportion of the total zooplankton biomass, which is dominated by a cyclopoid copepod. All three species are found throughout the year and carry parthenogenetic eggs at all times. They are smaller species than those predominating in some other East African lakes and some comparisons are also made between C. cornuta and four other species of that genus studied in Great Britain. The mean body lengths and clutch sizes of ovigerous females of C. cornuta vary little through the year. This is, presumably, a reflection of the relatively constant environment in Lake George. Alona poppei is only found in the open water intermittently and is thought to be swept from the peripheral swamps during heavy rains.
The general scarcity of Cladocera in Lake George is thought to be due, either to a shortage of food particles of suitably small size in the phytoplankton, which is dominated by large blue-green algae, or to predation. Although Cladocera are certainly eaten by fish, quantitative data, with which to substantiate either of these hypotheses, is lacking.     

Phytoplankton biomass and distribution in a shallow eutrophic lake (Lake George,Uganda)

Summary Lake George, a shallow lake in western Uganda, supports a permanent and dense crop of phytoplankton, and may be regarded as eutrophic although the ambient concentrations of inorganic nitrogen and phosphorus are low. The figures for the annual nutrient loadings (Viner and Smith, 1973) would suggest, however, a eutrophic lake when analysed on the scheme of eutrophication proposed by Vollenweider (1968).The horizontal distribution of chlorophyll a shows a concentric pattern, with the maximum values occurring towards the center. This horizontal variation is thought to reflect water movements, and a general pattern of water flow is proposed.The majority of the species show no seasonal variation but populations of both Anabaena and Melosira show annual variations. These two species also have a horizontal distribution pattern which is the reverse of other species. This pattern is used to support the proposed movements of water.Phytoplankton generation times are discussed in relation to the diurnal cycle occurring in Lake George.     

The combined effects of temperature and food consumption on body weight,egg production and developmental time in Chaoborus crystallinus De Geer (Diptera: Chaoboridae)

Summary Chaoborus crystallinus fourth-instar larvae were reared individually at 14°, 17° and 20° C under different food conditions. Daphnia magna of 1.25 mm average length served as prey. The following were measured: amount of prey ingested, larval weight gain, duration of fourth instar, body weight of the adults, and egg number per female. At a given temperature, the body weight, egg-number and developmental rate increased with food consumption. At a given food consumption, higher temperatures caused a decrease in body weight and egg number, and an increase in developmental rate. Gross production efficiencies for fourth-instar larvae were highest at temperatures around 17° C. The results clearly indicate that from an energetic point of view higher temperatures are disadvantageous. In C. crystallinus vertical migration is evidently a way of lowering the temperature to which the animals are exposed and hence optimizing food conversion into biomass and offspring production, especially if prey densities are below the saturation level.     

The benthos of a natural West African lake, with emphasis on the diel migrations and lunar and seasonal periodicities of the Chaoborus populations (Diptera, Chaoboridae)

SUMMARY. 1. Three Chaoborus species dominated in abundance and biomass the mid-lake benthic fauna of a small, natural Nigerian lake. Each population showed distinct seasonal varialitions. C. anomalus being most abundant during the early rainy season, while ( ceratopogones and C edults were most abundant during the dry season. A fourth species. C. fuscinervis (not reported previously from West Africa), was collected only in the adult stage. Also abundant at mid-lake were the Chironomidae. Chironomus seydeli, Prodadius brevipetiolatus and Tanypus brevipalpis. all of which showed a seasonal pattern most resembling that of C. anomalus .
2. Second. third and fourth instar larvae and pupae of the Clutoborus species underwent a diel vertical migration between the lake sediments (diurnal) and the water column (nocturnal) which was strongly related to prevailing light conditions.
3. Lunar periodicities of Chaoborus emergence determined from emergence trap collections are described for the first time. Corresponding lunar periodicities of larval abundance were observed in two of the species. The statistical techniques employed in describing the lunar periodicities permit the estimahon of generation times.
4. C. anomalus is believed to be parthenogenetie in the study lake (no males collected). while the other Chaoborus species are bisexual (1:1 sex ratios). The biology of this species is discussed in relation to the parthenogenetie condition.     

Post-larval development of compound eyes and stemmata of Chaoborus crystallinus (De Geer, 1776) (Diptera : Chaoboridae): Stage-specific reconstructions within individual organs of vision

During metamorphosis, the dioptric apparatus of the larval compound eye of Chaoborus crystallinus (Diptera : Nematocera) is radically reconstructed. The thin larval cornea of the ommatidia is replaced by strongly curved corneal lenses, and the eucone larval cone is replaced by an imaginal cone of the acone type. Curvature of the future lens is already apparent in very young pupae, in which the cornea consists only of a thin epicuticle with corneal nipples. Fibrillary cuticle is secreted by cone and primary pigment cells throughout pupal development. Lens formation is accompanied by movement of the nuclei of the accessory pigment cells. The larval cone disintegrates unexpectedly late in young, images. During late pupal development, 7 cone cell projections emerge. In contrast to the dioptric apparatus, the retinula cells and rhabdom remain almost unchanged during metamorphosis. The main refractive element of the larval ommatidium appears to be the cone, while that of the imaginal ommatidium is the corneal lens. In addition to the compound eyes, the pairs of stemmata are retained during the whole post-larval development. Pupal stemmata show no structural differences from the larval stemmata. The stemmata are still present in 2-day-old images (“retained stemmata”), but the primary stemma loses its dioptric apparatus and is proximally relocated to the basal region of the compound eye. The reconstructions in the visual system of Chaoborus, which occur during ontogeny, are probably connected with the change from aquatic living larvae to aerial adults, and appear to fulfill stage-specific needs of vision.