Influence of ontogenetic changes in prey selection on the survival and growth of rohu, Labeo rohita and singhi, Heteropneustes fossilis larvae

When offered a mixed diet of different zooplanktonic items covering a body size range of 75–2200 μm, (a) rohu, Labeo rohita and (b) singhi, Heteropneustes fossilis larvae ingested progressively larger prey as they grew, due to age-related increase in gape. However, a nearly constant prey size/mouth size ratio was maintained for a period of 4wk after hatching. The dominance of rotifers in the diet during the first 2-wk was followed by cladocerans, particularly Moina macrocopa. Significant differences observed in the growth rates of the larvae reared on different diet regimes were related to ontogenetic changes in prey selection. An exclusive copepod diet throughout resulted in the lowest weight gain in the larvae of both species. However, copepods had no apparent adverse effects when present with the preferred rotifers and cladocerans. Although constituting a suboptimal prey size for the older larvae, rotifers alone, when present in sufficient densities, produced growth rates comparable to those obtained on a cladoceran diet. However, a mixed diet regime contributed to the maximum growth. The implications of these findings to rearing larvae of the economically important rohu and singhi are discussed.     

Effects of food deprivation on the development of Coleomegilla maculata (De Geer) (Coleoptera: Coccinellidae)

The lady beetle Coleomegilla maculata (De Geer) is a natural enemy of several insect pests and feeds on pollen and nectar to survive periods when prey is scarce. The effect of the feeding interval on the development, survival, fecundity, and longevity of C. maculata was determined. Newly hatched larvae of C. maculata were reared individually and fed with eggs of the Mediterranean flour moth Anagasta kuehniella (Zeller) at intervals of one, two, and three days under controlled conditions (23 ± 1oC; 60 ± 10% RH; 12 h phtophase). The duration of larval instars and the total larval stage was prolonged as the feeding interval increased. The larval period lasted on average 9.2 ± 0.19 days when the larvae were fed daily with prey, and 14.6 ± 0.48 days when food was offered at three-day intervals. There was an inverse relationship between food intervals, survival, and weight of larvae and adults of the coccinellid. Survival rate of larvae fed daily was 76.8%, while the rate was 50.0% and 23.4% for larvae fed every two and three days, respectively. Coleomegilla maculata showed fecundity of 781.1 ± 149.02, 563.4 ± 80.81 and 109.0 ± 103.0 eggs when fed daily and at intervals of two and three days, respectively.     

Use of prey and non-prey food by the ladybird beetle Eriopis connexa (Coleoptera: Coccinellidae) under laboratory-rearing conditions

Prey for predators can fluctuate in abundance and in quality over time requiring predator strategies to cope with food shortage. Coccinellinae are often associated with sap-sucking pests that exhibit high population unpredictability such as aphids and psyllids. Eriopis connexa (Germar) (Coleoptera: Coccinellidae) is a predator with potential for biological control, especially a well-studied population which is resistant to pyrethroids used to control insect defoliators. Both larvae and adult E. connexa were provided ad libitum prey and non-prey foods (pollen and honey water solution) at increasing intervals from 1 to 10 days. Neonate larvae of E. connexa required eating prey daily to develop into adults. However, non-prey food such as honey water solution did prolong larval and adult survival but neither fulfilled larval development nor adult reproduction. Honey water solution promoted 100% adult survival up to 25 days in the adult stage without prey with oviposition returning after daily feeding on prey. Females subjected to increased feeding intervals over four days reduced oviposition and lived longer, but 10-day feeding intervals correlated with risk to adult survival. These results indicate the importance of non-food sources in E. connexa maintenance and the ability of larvae and adult females to compensate for prey scarcity.     

Nucleic acid contents and growth of first-feeding walleye pollock larvae in response to prey densities typical of sub-Arctic ecosystems

The growth, nucleic acid and protein contents of walleye pollock Theragra chalcogramma larvae reared at prey densities of 10, 30, 50, and 500 prey 1^-1 were measured for the first 9 days after the feeding initiation at 6° C. Incremental growth rates of larvae (mm day^-1) were low and variable for the first 7 days after feeding initiation. Growth rates and rates of RNA, DNA, and protein accumulation by larvae reared at 500 prey 1^-1 were positive while those of larvae reared at the lower prey levels did not differ significantly from zero. The RNA/DNA ratio was variable and exhibited no significant trend among food treatments. Estimates of instantaneous protein growth rates ranged from - 6·7 to 13·2% day^-1 at food densities of 10 and 500 prey 1^-1, respectively, and were moderately correlated with larval RNA/DNA ratios ( r = 0·628). The results suggest that in situ protein growth rates of first-feeding pollock larvae may be influenced by prey fields within the range of ambient food densities reported for sub-Arctic ecosystems.     

Vulnerability of larval white perch,Morone americana,to fish predation

Synopsis The vulnerability of white perch, Morone americana, larvae to yearling bluegill, Lepomis macrochira, predators was examined in relation to larval size, nutritional condition and relative abundance of alternative prey. Short-term (15 min) predation trials were conducted in 381 tanks in the laboratory. Larval vulnerability was measured as the proportion of larvae killed and the proportion of successful attacks per predator in each 15 min trial. No significant differences in vulnerability were apparent among larvae regardless of feeding history at sizes < 6 mm SL. At larval lengths > 6 mm SL, size of larvae was the crucial determinant of their vulnerability. Percentage of larvae killed in 15 min was nearly 100% at sizes < 6 mm SL, decreased to 30% at a length of 12.0 mm SL and dropped to 18% at 14.0 mm SL. Larvae initially feeding at low food levels for 2–4 d exhibited decreased growth of 13–25% over the first 3 wks of life, and simulations based on laboratory results indicated that these growth deficits could result in 5- to 68-fold decreases in survivorship at 38 days after hatching (DAH). The relative abundance of alternative prey also had a pronounced effect on mortality of larvae. A 10-fold increase in alternative prey (Daphnia magna) abundance decreased bluegill predation rates on white perch larvae by 10–20%, while a 100-fold increase in Daphnia density decreased larval mortality by 75–90%.     

Feeding behaviour of Japanese flounder larvae under laboratory conditions

Tank–reared Japanese flounder larvae, Paralichthys olivaceus , had a major feeding peak in the morning and a secondary peak in the afternoon throughout the larval development, with light being the primary factor regulating their feeding activity. The larvae consumed rotifers in preference to Artemia for up to 10 days, after which the food preference shifted to Artemia . Feeding rates of the larvae prior to 10 days post–hatch depended on prey density, but in the old larvae, feeding rates were independent of prey density. Maximum feeding rate occurred at 19° C. The occurrence of the attack posture, after its onset at first feeding (2 days post–hatch), increased up to 25 days, began to decrease when the larvae prepared to settle down, then disappeared after settlement. The occurrence frequency of the attack posture was positively related to fish density, but inversely related to starvation duration, and occurred most frequently at 19° C. This posture depended on prey density in larvae prior to 10 days post–hatch, but became independent of prey density as the larvae developed. It was obvious that, for flounder larvae, attack posture was a behavioural character closely related to feeding and subject to larval development and environmental factors.     

Feeding Patterns in Different Size Larvae of Walleye Pollack, Theragra chalcogramma (Pallas, 1814) on the Shelf of Western Kamchatka

The stomach contents were examined in 373 walleye pollack larvae of different sizes. The diet of pollack larvae included more than 20 plankters of different sizes. Phytoplankton was the major food source of 4- to 6-mm-long larvae. The proportion of phytoplankton in the larval diet decreased as the larvae grew, and the late larval stages shifted entirely to a diet of zooplankton. As the larvae increased in size, the spectrum of food organisms changed from smaller to larger sizes. In the larvae 4 to 31 mm long, the length of prey varied from 0.1 (Coscinodiscus) to 6.0 mm (Neocalanus plumchrus). The average daily repleteness of pollack larvae varied from 32 to 210. The minimum repleteness was registered in 4- to 6-mm-long larvae with mixed feeding (endogenous and exogenous); in 30- to 35-mm-long fish the repleteness was the greatest. The daily food rations calculated for the most abundant size groups of larvae ranged from 4.3 to 6.6% of their body weight.     

Relationships between larval nutritional experience, larval growth rates, juvenile growth rates, and juvenile feeding rates in the prosobranch gastropod Crepidula fornicata

Planktonic larvae experiencing short periods of starvation or reduced food supply often grow and develop more slowly, have poor survival, fail to metamorphose, metamorphose at smaller sizes, or grow slowly as juveniles. In this study, we examined the impact of short periods of food limitation at various stages of larval development on larval and juvenile growth in Crepidula fornicata. In addition, we considered whether juveniles that were stressed as larvae grew poorly because of reduced rates of food collection due to impaired gill function. For 5 experiments, larvae were either starved for several days beginning within 12 h of hatching or were starved for the same number of days following 1 or more days of feeding at full ration (cells of the naked flagellate Isochrysis galbana, clone T-ISO, at 18×10⁴ cells ml⁻¹). In one experiment, larvae were transferred for 2 or 4 days to seawater with extremely low phytoplankton concentration (1×10⁴ cells ml⁻¹). In all experiments, larvae were returned to full ration following treatment. Control larvae were fed full ration from hatching to metamorphosis. When larvae reached shell lengths of about 900 μm they were induced to metamorphose and then reared individually at full ration in glass bowls, with phytoplankton suspension replenished daily. Larval and juvenile growth rates were determined by measuring changes in shell length (longest dimension) over time. Juvenile feeding rates were determined by monitoring changes in phytoplankton concentration over 2–3 h at the end of the growth rate determinations. In general, larval growth rates for the first 2 days after the resumption of feeding were inversely proportional to the length of time that larvae were starved. However, larval growth rates ultimately recovered to control levels in most treatments. Starving the larvae caused a significant reduction in initial juvenile growth rates (first 3–4 days post-metamorphosis) in most experiments even when larval growth rates had recovered to control levels prior to metamorphosis. Juvenile growth rates were not significantly reduced when larvae were subjected to reduced food availability (1×10⁴ cells ml⁻¹), even for treatments in which larval growth rates were compromised. Mean weight-specific filtration rates for juveniles were significantly reduced (p<0.05) following larval feeding experience in only one or possibly 2 of the 4 experiments conducted. Our data suggest that although larvae of C. fornicata may fully recover from early nutritional stress, the resulting juveniles may exhibit poor initial growth due to impaired gill function, reduced digestive capability, or reduced assimilation efficiency.     

Trypsin activity and physiological aspects in larval rearing of European sea bass (Dicentrarchus labrax) using live prey and compound diets

The aim of this study was to evaluate the potential of a compound diet as a live prey substitute for feeding European sea bass larvae ( Dicentrarchus labrax L.). The effect of a commercial diet (Nippai ML feed) and live prey ( Artemia nauplii) on tryptic enzyme activity, protein content, growth (standard length) and survival rates of sea bass larvae were tested during a 27-day rearing experiment. Sea bass larvae were divided into two groups. The live food group (control group) was fed exclusively on newly hatched Artemia nauplii (Inve AF grade), the test group was fed exclusively with the compound diet from day 15 onwards. As trypsin has been demonstrated to be a useful indicator for evaluating digestibility of food and the nutritional condition of fish larvae, individual tryptic enzyme activity was determined in both feeding groups. Larvae older than 14 days after hatching and fed on live food showed a significantly higher tryptic enzyme activity than larvae fed the compound diet. A similar relationship between tryptic activity and standard length in both test groups was detected only in small larvae (standard length < 7 mm). The usefulness of proteolytic enzyme activity measurements in larval fish research, as well as its use in aquaculture nutrition research, was confirmed. Protein content, increase in length and survival rates of the sea bass larvae were additionally determined in order to evaluate an influence on the diet. The protein content of larvae fed the Artemia nauplii was higher and the growth of larvae fed the compound diet was reduced. Larval mortality was not affected by the diet given.     

Egg production, hatching rates, and abbreviated larval development of Campylonotus vagans Bate, 1888 (Crustacea: Decapoda: Caridea), in subantarctic waters

Early life history patterns were studied in the caridean shrimp, Campylonotus vagans Bate, 1888, from the subantarctic Beagle Channel (Tierra del Fuego). As a consequence of very large egg size (minimum 1.4 mm), fecundity was low, ranging from 83 to 608 eggs per female (carapace length [CL] 11-22.5 mm). Egg size increased continuously throughout embryonic development, reaching prior to hatching about 175% of the initial diameter. Due to low daily numbers of larval release, hatching of an egg batch lasted for about 2-3 weeks. The complete larval and early juvenile development was studied in laboratory cultures fed with Artemia sp. nauplii. At 7.0±0.5 °C, development from hatching to metamorphosis lasted for about 6 weeks. It comprised invariably two large zoeal stages and one decapodid, with mean stage durations of 12, 17, and 15 days, respectively. Larvae maintained without food survived on average for 18 days (maximum: 29 days), but did not reach the moult to the zoea II stage. Size increments at ecdysis were low in all larval stages (2.1-3.9%), indicating partial utilisation of internal energy reserves. A clearly higher increment (14%) was observed in the moult from the first to the second juvenile stage. Low fecundity, large size of eggs and larvae, an abbreviated mode of larval development, high larval survival rates during absence of food, demersal behaviour of the early life history stages, and an extended hatching period with low daily release rates are interpreted as adaptations to conditions typically prevailing in subantarctic regions, namely low temperatures (causing long durations of development) in combination with a pronounced seasonality in plankton production (i.e., short periods of food availability).     

Larval age, growth and mortality in the oceanic squid Sthenoteuthis pteropus (Cephalopoda, Ommastrephidae) from the eastern tropical Atlantic

An investigation was carried out on larvae of the oceanic tropicalsquid Sthenoteuthis pteropus in the equatorial Atlantic (230'N–7S;12W–830'E) The age of the larvae was calculated from thestatolith microstructure of 20 larvae; mortality was estimatedfrom the size structure of 1128 larvae. The larval stage lasts32–38 days. At ages ranging from 14 to 38 days. the dailyrelative growth rates of mantle length decrease from 7.5 to2.8% day^–1 and from 14–16 to 5.8% of body weightday^–1 At age 12–24 days, mortality rates were estimatedusing both raw catch data and corrected data accounting fornet avoidance. The mean value of raw mortality rates was 0.189,the corrected value was 0.158. During the proboscis division(transformation of the larva into juvenile) at age 25–35days, a sharp decrease in larval growth rates and a simultaneousincrease in mortality rates (raw 0.443, corrected 0.379) wereobserved.     

The impact of food type, temperature and starvation on larval development of Balanus amphitrite Darwin (Cirripedia: Thoracica)

The impact of diatom food species (Chaetoceros calcitrans and Skeletonema costatum), temperature and starvation on the larval development of Balanus amphitrite was evaluated. Starvation threshold levels for different ages of larvae (0- to 5-day-old) fed with C. calcitrans and S. costatum and then starved at 5, 15 and 25 °C temperature were estimated as ultimate recovery hour (URH; denoting the starvation point in hours at the end of which larvae can recover and continue development). Effect of temperature on starvation threshold varied significantly with larval age and food species. The URH declined with larval age at 5 °C, but not at 15 and 25 °C. The URH and grazing rates were high for early instars fed on C. calcitrans, and for advanced instars fed on S. costatum. Carbon gain through feeding was maximum for 2-day-old larvae when fed with C. calcitrans and decreased with larval age. However, when fed with S. costatum carbon gain increased with larval age. This confirms that with development the utility of food types changes. The differences in the carbon gain can be attributed to differences in grazing rate due to variations in the size of the diatom cells, larval intersetular distance, diatom sinking rate and the photo-taxic behavior of larvae. Molting was observed at times when larvae were undergoing starvation and this could be viewed as stress-induced molting, and it differed with the larval age and food organisms.     

Combined study of daily growth variability and nitrogen-carbon isotopic signature analysis of schooling Sardina pilchardus larvae

A combined study of Alborán Sea Sardina pilchardus larval daily growth and isotopic signature was carried out to elucidate the cause of morphometric differences observed in a single larval school whereby the standard length (L(S)) v. dry mass (M(D)) relationship differentiated a heavier-by-size larval group from a lighter larval group. The daily growth analysis revealed that this difference originated from two larval growth patterns, where a fast growing population (F) in contrast to a slow-growing larval population (S) was distinguished. The S-growing larval cohort had a significantly higher nitrogen (N) content as a result of greater somatic mass build up with time in the form of structural proteins. Alternatively, the F-growing population showed a significantly greater amount of carbon (C) content with age, indicating faster metabolic rates of C accretion compared to the S-growing group. In consequence, the C:N ratios of the F-growing larvae were significantly higher than the S-group. C:N ratios of both larval populations showed significant linear decrease with age (and size), while K showed an inverse relationship. The stable isotopes of N did not show significant differences between the S and F-growing larvae. In F-growing larvae, however, a significant linear increase in δ(15)N (by age class) was observed, indicating that as larvae undergo ontogenetic development, trophic level tends to increase. This was also made evident by the significant decrease in age of δ(15)N coefficients of variation (by age class). The higher δ(13)C values in the S-growing larvae were related to the lower growth rates observed in this group. These results suggest a broader trophic flexibility in younger larvae, but moving towards a trophic specialization and more selective diets with age. This trophic specialization shows a tendency of a greater prey size with age. These findings suggest that S-growing larvae have a more omnivorous diet than the F-growing ones.     

Laboratory evaluation of Toxorhynchites splendens (Diptera: Culicidae) for predation of Aedes albopictus mosquito larvae

Biology of the mosquito Toxorhynchites splendens (Wiedemann) was studied in the laboratory to provide baseline data for using the predatory larvae of this species against those of Aedes albopictus (Skuse) in a biological control programme. The mean incubation time of Tx.splendens eggs was 43.8 h and the time required for newly-hatched larvae to initiate predation was 2.5 h. Mean numbers of prey larvae consumed and killed by each Tx.splendens larva totalled 389 and 345 respectively. The larval period of Tx.splendens was not significantly different for rearing individually or in groups of nine, with equal prey density, and duration of larval development was proportional to prey density. In mass rearing, larval cannibalism was usually observed during days 1-3 post-eclosion. The incidence of cannibalism decreased sharply on the fourth day after hatching when some larvae became fourth-instar. Adult female Tx.splendens usually commenced oviposition on day 4 after emergence. The number of eggs laid daily increased on day 7 and the peak oviposition of 6.3 eggs/female/day occurred on day 11. When oviposition containers were provided only intermittently, gravid females of Tx.splendens scattered most of their eggs on the dry floor of the cage. Viability of eggs laid by females aged 4-14 days was high (60-90%) but decreased to less than 40% as the females aged.     

The effect of a predatory leech,Nephelopsis obscura,on mortality,growth, and production of chironomid larvae in a small pond

Summary The effect of a predatory leech, Nephelopsis obscura, on survivorship, growth, and production of chronomid larvae was studied by enclosure experiments carried out in a small pond. The prey population was composed almost entirely of the tubiculous, microphagous chironomid larvae, Chironomus riparius and Glyptotendipes paripes. Nephelopsis significantly reduced chironomid survivorship within the enclosures, and accounted for most of the measured mortality of fourth instar larvae. The cropping by Nephelopsis was not significantly biased toward either prey species. In long-term experiments (66 d) chironomid biomass in enclosures without leeches reached much higher levels than in enclosures containing Nephelopsis. This increase in biomass was due to growth of surviving larvae, rather than recruitment, since emergence and oviposition were not going on during the course of the experiments. The enhanced survivorship of larvae within leech-free enclosures was eventually accompanied by reduced growth and specific production (daily production/biomass) for C. riparius, which made up about 90% of the larval population. Growth and specific production of G. paripes (10% of larval population) was not affected. Short-term experiments (25 d) involving manipulation of densities and species ratio (9:1 CR:GP and 1:9 CR:GP) of larvae revealed that growth of the majority species was strongly influenced by larval density, whereas growth of the minority species was not. The same pattern was observed both in the presence and in the absence of Nephelopsis and was a result of differences in resource utilization between the two species. In the shortterm experiments, growth rates estimated for larvae exposed to leeches were significantly less than those for larvae in leech-free enclosures. This could be due either to size-biased consumption of larvae by Nephelopsis, or possibly a disturbance factor leading to reduced larval food intake and/or increased metabolic costs.     

Bioenergetic model of planktivorous fish feeding, growth and metabolism: theoretical optimum swimming speed of fish larvae

The feeding activity of an individual fish larva is described by an equation which includes parameters for the area successfully searched, probability of food capture multiplied by the cross-sectional perceptive visual field, larval swimming speed and the time required to consume a unit of food energy. The proportion of ingested food energy used for metabolism increases exponentially with increasing swimming speed. The model predicts that food consumption rate increases asymptotically whereas metabolic rate increases exponentially. This results in a predicted growth rate curve that reaches a maximum at a certain swimming speed and decreases at both higher and lower speeds. The model can be used to predict the influence of type of prey, prey density, water temperature etc. on larval growth. An expression describing how many hours per day fish larvae must forage in order to grow at a certain daily body weight gain allows the limits of environmental conditions for positive, zero and negative growth rate to be set. Results of simulations demonstrated that the optimum swimming speed for maximum growth of coregonid larvae increased with an increase in food density, decrease in water temperature or decrease of prey vulnerability. At optimum ‘theoretical’ swimming speed an increase in water temperature from 5 to 17° C required the food density to be increased from 20 to 80 copepods l^?1 in order to maintain a daily growth increment of 2%. The minimum Artemia density required for maintenance metabolism increased from 10 to 30 items 1¹ over the same temperature increase from 5 to 17° C, and food densities required for 8% growth rates were 26 and 56 Artemia nauplii l^?1 at 5 and 17° C, respectively. Contrary to previous findings, results of the present study suggest that metabolic rates of actively feeding fish larvae may be from 5 to 50 times the standard metabolic rate: earlier studies suggested that a factor of 2–3 may be generally applicable.     

Effect of food on immature development, consumption rate, and relative growth rate of Toxorhynchites splendens (Diptera: Culicidae), a predator of container breeding mosquitoes

Food utilization by the larvae of Toxorhynchites splendens (Wiedemann) was studied in the laboratory by offering larvae of Aedes aegypti Linnaeus, Anopheles stephensi (Liston), and Culex quinquefasciatus (Say). Quantitative analyses of data indicated that immature development was significantly faster with increase in food availability. The regression analysis showed that the degrees of the relationship between immature duration (Id) and food availability were higher when offered early instars of prey (first and second instars) than late instars. Consumption rate (Cr) of the predator increased with increase in food availability and this relationship was highly significant when larvae of An. stephensi were offered as food. Consumption rate to food level decreased with increase in the age class of the prey. There was a significant negative correlation between Id and Cr. This aspect helps to increase population turnover of T. splendens in a shorter period when the prey is abundant. Conversely, the predator compensated the loss in daily food intake at low food level by extending Id thereby attains the minimum threshold pupal weight for adult emergence. There was an increase in the relative growth rate (RGR) of the predator when An. stephensi was offered as prey and this was related to the high protein content of the prey per body weight. There was a positive correlation between Cr and RGR. This adaptive life characteristic strategy of this predator is useful for mass-rearing for large scale field release programmes in the control of container breeding mosquitoes is discussed.     

Bile salt-dependent lipase in larval turbot, as influenced by density and lipid content of fed prey

Turbot larvae were fed three different densities of rotifers (1000, 3000 and 7500 rotifers 1⁻¹) with a low lipid level (< 15% of dry weight) or 7500 rotifers 1⁻¹ with a high lipid level (∼30% of dry weight). The larval consumption of rotifers increased with increasing prey densities and the content of bile salt-dependent lipase (BSDL) in larvae was correlated positively with the ingestion rate from days 6 to 8. This suggests that BSDL synthesis was stimulated by the amount of ingested prey in the early larval phase. However, growth was highest in larvae receiving the medium prey densities, which indicates that the larvae were not able to digest properly the ingested prey at the higher density. No significant effect on the BSDL content was seen in turbot larvae fed rotifers with a high or low lipid content.     

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.     

Spatial Distribution,Food and Activity of Gomphus pulchellus SELYS 1840 (Insecta; Odonata; Gomphidae) from a Still Water Habitat

Distribution patterns of Gomphus pulchellus larvae in different sediments with different density of prey organisms were studied in the field in a small gravel pit lake in the south of Germany. Larval burrowing behaviour at different temperatures as well as food preference, consumption rates and activity were studied in laboratory experiments. In the study lake G. pulchellus larvae lived exclusively in places where macrophytes were present and in fine sediments (mean grain size <3 mm) with detritus cover. There was a significant positive correlation between larval density and density of food organisms suggesting that abundance of food is one of the determinants of larval distribution. In late autumn larvae migrated to deeper places probably to survive the winter. Low temperatures simulated in laboratory experiments did not induce larvae to burrow deeper. Larvae were always found in a sediment depth of 0.59–0.74 cm. Experiments with mixed prey showed that G. pulchellus larvae preferred tubificid worms and chironomid larvae over gammarids and ephemerid larvae. However, chironomid larvae which stayed in their tubes had a higher survival rate than those outside of tubes. Single-prey experiments showed that G. pulchellus larvae can prey not only on benthic species but also on Daphnia from the open water. Functional-response experiments showed that one G. pulchellus larva consumes a maximum of 2 to 3 tubificid worms or chironomid larvae per day, which corresponds to a maximum biomass (freshweight) of 5 to 30 mg per day. Video recordings of activity showed that G. pulchellus larvae cover long distances of up to 52 m per night on the substrate surface and that activity on the substrate surface started after midnight and ceased before sunrise. Consumption of Zooplankton prey and high activity above the substrate is interpreted as an adaptation of G. pulchellus larvae to the life in still water habitats.