Distribution,movements and diet of nocturnal fishes on temperate reefs

We counted nocturnal fishes both day and night, and monitored the position of tagged individuals on temperate reefs in New South Wales, Australia. Pempheris affinis and P. multiradiata were the most abundant nocturnal planktivores on Sydneys rocky reefs and showed great differences in diel migration behaviour. Both species were observed in deep shelter sites during the day (5–10m), and most emerged into the water column at night. P. multiradiata was found to undergo extensive vertical and horizontal migrations. In contrast, P. affinis remained within daytime depth strata, with tagged individuals often moving less than 20m at night. Tagged adult P. affinis returned to tagging sites for up to 7weeks, indicating high site fidelity. Dietary analysis demonstrated that small and large pempherids differed in diet and the timing of foraging, suggesting a size-based transition from diurnal to nocturnal foraging. Stratified sampling of planktonic assemblages at different depths during the day and night showed spatial variation in the availability of prey items at different times of the day. Amphipods, the main prey of large fish, were only available during the night, and concentrated in shallow water, whereas decapod larvae, consumed mainly by small fish, were abundant day and night. Large P. affinis also fed on polychaetes, which were never found in the stomachs of P. multiradiata, suggesting that these species may have different prey requirements, or that these polychaetes are only found in deep water where foraging P. affinis were abundant. We found no general model for the Pempheridae. The movements and behaviour of nocturnal fishes varied greatly by species, and this may be due to differences in body size, and/or physiological (e.g. visual ability) and ecological constraints.     

Diel distribution of copepods across a channel of an overwash mangrove island

The distribution of copepod species and their nauplii was studied in a narrow, blind channel on an overwash mangrove island offshore of Belize. Copepodids were sampled with a pump at five stations across the channel during a diel cycle. Diel changes of copepodid stages II – VI were marked by horizontal dispersal of Dioithona oculata, the dominant species, from swarms in the prop roots along the shore during the day to the edge of the prop root habitat at night. Migration of copepodids back to the prop roots appeared to be controlled endogenously because change from a night to a daytime age structure began before first light. Mean copepodid stage at subsurface depths in the channel and prop root edge decreased from 4.2 (with 6.0 = all adults) to 2.9 at predawn to 1.1 during day. The oceanic Oithona nana and O. simplex, and the coastal zone O. fonsecae were evenly distributed with depth and distance from shore during day and night, with avoidance of prop root shoreline during day. These species were much less abundant than Dioithona oculata in the prop roots, but of comparable or greater abundance in the channel. Coastal zone Acartia spinata exhibited evidence of swarming. Nauplii, sampled with a 25m plankton net, were dominated by harpacticoid (50%) and cyclopoid (34%) nauplii, which generally were more abundant at 1m than at the surface and more abundant at night than the day. Lagrangian current measurements indicated velocities at ebb tide twice those of flood tide (1.9 vs. 0.8 cm s^–1) and a minimal residence time of 5 days, which could result in advection of D. oculata nauplii out of the Lair Channel before their recruitment into swarms as copepodid stage II. Previously reported maximum swimming speeds of swarming D. oculata copepodid stages (2.0 cm s^–1) and greater densities in prop roots and near the benthos may help copepodids avoid advection. The swarming behavior and diel horizontal migration (or dispersal) reported for D. oculata appears analogous to that of limnetic zooplankton, which may swarm among macrophytes along shorelines during the day to avoid visual predators and disperse or migrate away from the shoreline at night.     

Life history characteristics and distribution of Bythotrephes longimanus Leydig (Crustacea,Onychopoda) in the Biesbosch reservoirs

The diel and horizontal distributions of Bythotrephes longimanus Leydig, Leptodora kindti (Focke) and Anomopoda (cladocerans), as well as several life history parameters of each developmental stage of B. longimanus were studied in the non-stratified Biesbosch reservoirs. Anomopoda avoided the surface layer during the day only in the pelagic zone. In the surface layer near the shore, however, they were very abundant. Because densities in the pelagic zone during the day were very low, and high during the night, the population undoubtedly moved from the littoral zone or from the sediments to the pelagic. Accumulation of zooplankton at the littoral site is either the result of random dispersal or wind-induced movements. Bythotrephes' diel distribution pattern is possibly related to that of their prey, the anomopods. Leptodora mostly exhibited a normal diel vertical migration with a small amplitude. Wind most likely influenced the horizontal distribution of the zooplankton. Bythotrephes body length increased with higher temperatures, whereas spine length was constant throughout the year. Bythotrephes can already be fertile in the first developmental stage, indicating that a rapid adaptation to a favourable environment is possible. The number of parthenogenetically produced eggs per ovigerous female was higher at the start of the growing season and constant throughout the rest of the year. Relatively few resting eggs and males were found in autumn.     

Comparative Ontogenetic Behavior and Migration of Kaluga, Huso Dauricus, and Amur Sturgeon, Acipenser Schrenckii, from the Amur River

We conducted laboratory experiments with kaluga, Huso dauricus, and Amur sturgeon, Acipenser schrenckii, to develop a conceptual model of early behavior. We daily observed embryos (first life phase after hatching) and larvae (period initiating exogenous feeding) to day-30 (late larvae) for preference of bright habitat and cover, swimming distance above the bottom, up- and downstream movement, and diel activity. Day-0 embryos of both species strongly preferred bright, open habitat and initiated a strong, downstream migration that lasted 4 days (3 day peak) for kaluga and 3 days (2 day peak) for Amur sturgeon. Kaluga migrants swam far above the bottom (150cm) on only 1 day and moved day and night; Amur sturgeon migrants swam far above the bottom (median 130cm) during 3 days and were more nocturnal than kaluga. Post-migrant embryos of both species moved day and night, but Amur sturgeon used dark, cover habitat and swam closer to the bottom than kaluga. The larva period of both species began on day 7 (cumulative temperature degree-days, 192.0 for kaluga and 171.5 for Amur sturgeon). Larvae of both species preferred open habitat. Kaluga larvae strongly preferred bright habitat, initially swam far above the bottom (median 50–105cm), and migrated downstream at night during days 10–16 (7-day migration). Amur sturgeon larvae strongly avoided illumination, had a mixed response to white substrate, swam 20–30cm above the bottom during most days, and during days 12–34 (most of the larva period) moved downstream mostly at night (23-day migration). The embryo–larva migration style of the two species likely shows convergence of non-related species for a common style in response to environmental selection in the Amur River. The embryo–larva migration style of Amur sturgeon is unique among Acipenser yet studied.     

Diel vertical migrations of invertebrate predators (Leptodora kindtii ,Thermocyclops taihokuensis, andMesocyclops sp .) in a shallow, eutrophic lake

We studied vertical distribution patterns of three invertebrate predators – Leptodora kindtii, Mesocyclopssp., and Thermocyclops taihokuensis– in a shallow eutrophic lake, Lake Suwa , Japan. From June to October in 2000 and 2001, we collected samples in the lake center in order to examine the vertical distribution patterns and the densities of the predators in the water column during the day (0900) and at night (2330). We also examined phototactic behavior of Leptodora in the laboratory. The three invertebrate predators showed clear migration patterns. Leptodora and Thermocyclops displayed a typical migration, avoiding the surface and maintaining a high abundance in deeper water during the day, and being distributed uniformly during the night. Mesocyclops, on the other hand, showed no clear vertical distribution pattern in the water column. However, Mesocyclops showed higher densities in the water column during the night than during the day. It suggests that they stayed just above the bottom during the day and migrated upward during the night. Leptodora also showed such a density difference between day and night. In the laboratory, Leptodora showed strong negative phototactic behavior. The observed density changes between day and night in Leptodora and Mesocyclops suggests the possible underestimation of their population density by usual sampling methods, and thus the impact of predation on populations of prey zooplankton species may also be underestimated in shallow water bodies.     

To migrate, or not to migrate: partial diel horizontal migration of fish in a temperate freshwater reservoir

The diel horizontal migration (DHM) of fish between the inshore and offshore zones of the ?ímov Reservoir (Czech Republic, deep, stratified, meso-eutrophic) was investigated by a combination of horizontal and vertical hydroacoustic surveys at 3-h intervals over 48 h and day/night purse seining in August 2007. An overwhelming majority of fish were aggregated within the epilimnetic layer. Considering only the horizontal surveys, cyclic diel fish movements between inshore and offshore habitats were apparent, while the total fish biomass remained constant between day and night. A higher fish biomass was detected in the offshore zone during daytime by both hydroacoustics and purse seining. In contrast, a higher fish biomass was recorded at night in the inshore zone. Bream Abramis brama, roach Rutilus rutilus, and perch Perca fluviatilis dominated the daytime offshore fish assemblage whereas bleak Alburnus alburnus prevailed at night. Bream and roach decreased in abundance at night while perch completely disappeared from the offshore habitat. The diel differences in size distributions and direct catches suggested the population-wide horizontal offshore migration of bleak and inshore migration of all perch during dusk. On the other hand, partial inshore migration of bream and roach adults was observed during dusk (52 and 80%, respectively). The different proportions of offshore residents among species and size classes suggested that differences in size, and, therefore, predation vulnerability, contributed to the observed migration patterns.     

Light responses ofBeggiatoa

Studies on nativeBeggiatoa demonstrated diel vertical migration into, and out of, sediments at the bottom of warm spring pools. Laboratory experiments withBeggiatoa in natural sediments suggested that high light was the cause of the downward movement. The nature of this presumed photomotion was clarified by microscopic observation of individual filaments of nativeBeggiatoa at light/dark boundaries where the light was varied in intensity and quality. Using white light, a negative photo-response was demonstrated, and a dose-response curve was constructed which indicates an increasing response to light over three orders of magnitude of intensity. A coarse action spectrum implicated a pigment with a peak in the blue region as the receptor. Pure culture studies showed the negative response to be a step-up phobic one. The light intensity increase necessary to invoke reversals was a smaller percentage of the initial intensity for higher initial intensities. The light intensity levels and gradient strengths necessary to evoke reversals in single filaments were consistent with the hypothesis that the step-up response accounts for the disappearance in the field. This response has adaptive significance since full sunlight was completely inhibitory toBeggiatoa growth, even when filaments were aggregated in tufts. Dilute suspensions were also inhibited by as little as 5000 lux (fluorescent lamps).     

The pineal complex and melatonin affect the expression of the daily rhythm of behavioral thermoregulation in the green iguana

Daily variation in the body temperature of the green iguana (Iguana iguana) was studied by telemetry in laboratory photo-thermal enclosures under a 12Light12Dark (LD) photoperiod. The lizards showed robust daily rhythms of thermoregulation maintaining their body temperatures (T_b) at higher levels during the day than during the night. Some animals maintained rhythmicity when kept in constant darkness. On lightdark cycles parietalectomy produced only a transient increase of median T_b in the first or second night following the operation. Pinealectomized lizards on the other hand maintained their body temperatures at significantly lower levels during the day and at significantly higher levels during the night than did sham-operated or intact lizards. This effect was apparently permanent, since one month after pinealectomy lizards still displayed the altered pattern. Plasma melatonin levels in intact animals were high during the night and low during the day and were unaffected by parietalectomy. Pinealectomized lizards showed low levels of plasma melatonin during both the day and the night. A daily intraperitoneal injection of melatonin in pinealectomized animals given a few minutes after the light to dark transition decreased the body temperatures selected by the lizards during the night and increased the body temperatures selected during the following day. Control injections of saline solution had no effect. The significance of these results is discussed in relation to the role of the pineal complex and melatonin in the mediation of thermoregulatory behavior.Abbreviations LD LightDark - T _b body temperature - PAR-X parietalectomy - PIN-X pinealectomy     

Morphological and ecological characteristics of Spirochaeta plicatilis

Summary Two types of large spirochetes, differing in size, and in coiling and motility patterns, were observed in sulfide-containing mud collected from a brackish marsh. The spirochetes measured 50–250 m in length and 0.5–0.75 m in diameter. One type was identified as Spirochaeta plicatilis Ehrenberg, inasmuch as its general characteristics corresponded to those described for this species by other authors. The second type of large spirochete was designated large spirochete type B. Both types possessed ultrastructural features typical of spirochetes, e.g. outer sheath, axial fibrils and protoplasmic cylinder. The axial fibrils were arranged in a bundle and, in type B spirochetes, were inserted in a row near each end of the cell. The outermost layer of type B cells, interpreted to be the outer sheath or a main component of it, appeared to consist of an array of fibril-like elements which transversely circumscribed the cell. Transverse septa were observed in S. plicatilis, but not in type B spirochetes. In the mud samples both types of spirochetes were present near the mud-water interface within mats of sulfur-containing Beggiatoa trichomes, together with a variety of other microorganisms. A striking increase in the number of large spirochetes consistently occurred during lysis of the Beggiatoa trichomes, at low levels of H₂S. The large spirochetes were attracted to the lysing trichomes and grouped about them. Under strictly anaerobic conditions S. plicatilis remained motile for 8 days, and type B for 25 days. Experiments involving the use of chemical and biological indicators showed that S. plicatilis is either an anaerobe which can tolerate low O₂ tensions, or a microaerophile able to metabolize and possibly grow in the absence of O₂. Type B spirochetes behaved as strict anaerobes.     

Identification of a Circadian Clock-Controlled Neural Pathway in the Rabbit Retina

Background

Although the circadian clock in the mammalian retina regulates many physiological processes in the retina, it is not known whether and how the clock controls the neuronal pathways involved in visual processing.

Methodology/Principal Findings

By recording the light responses of rabbit axonless (A-type) horizontal cells under dark-adapted conditions in both the day and night, we found that rod input to these cells was substantially increased at night under control conditions and following selective blockade of dopamine D₂, but not D₁, receptors during the day, so that the horizontal cells responded to very dim light at night but not in the day. Using neurobiotin tracer labeling, we also found that the extent of tracer coupling between rabbit rods and cones was more extensive during the night, compared to the day, and more extensive in the day following D₂ receptor blockade. Because A-type horizontal cells make synaptic contact exclusively with cones, these observations indicate that the circadian clock in the mammalian retina substantially increases rod input to A-type horizontal cells at night by enhancing rod-cone coupling. Moreover, the clock-induced increase in D₂ receptor activation during the day decreases rod-cone coupling so that rod input to A-type horizontal cells is minimal.

Conclusions/Significance

Considered together, these results identify the rod-cone gap junction as a key site in mammals through which the retinal clock, using dopamine activation of D₂ receptors, controls signal flow in the day and night from rods into the cone system.     

Removal and predation of Quercus liaotungensis acorns by animals

Two types of acorns (fresh, current-year acorns and old, previous-year acorns) of Quercus liaotungensis were used under two different situations (night and day) to examine the removal and predation by animals in the field. The acorns disappeared very fast. During the day, all of the 280 this-year acorns (TA) and all of the 140 last-year acorns (LA) were removed by animals within 6 h and 5 h, respectively, after acorns were put in place. At night, all of the 280 TA acorns were removed by animals within 10 h after acorns were distributed, and 71 out of 140 LA acorns were removed within 10 h. Animals consumed a small proportion of acorns in situ (11.8%). The number of TA acorns consumed in situ at night was higher than that during the day. During the day, the number of LA acorns consumed in situ was significantly higher than TA acorns. This result suggests that the most important harvesters of Q. liaotungensis during the day were visually orientated diurnal animals, and the most important harvesters at night were olfactorily orientated nocturnal animals. Furthermore, the latter are more efficient than the former at finding and harvesting acorns, because they spend less energy on harvesting the same number of sound acorns.     

Deposition of fusiform crystals without apparent diurnal rhythm at the growing edge of septa of the coral Galaxea fascicularis

Recent studies suggest a diurnal periodicity in the deposition of fusiform crystals by scleractinian corals. In order to check whether this is universally true in the Scleractinia, the surface structure of skeletons of Galaxea fascicularis (L.), collected at 3 h intervals over 1 day was observed with a scanning electron microscope. Fusiform crystals 0.3–3 m wide and 0.5–5 m long were found on the growing edges of septa of polyps collected at different times of day. There was no apparent diurnal change in the mean diameter of fusiform crystals. The size distributions of these crystals were almost the same by day (1200 h) and at night (2400 h). Small fusiform crystals which appeared to have been newly deposited were observed on septa collected both during the day and at night. The present study suggests that fusiform crystals are deposited continuously with no diurnal rhythm in G. fascicularis.     

Effects of ice cover on the diel behaviour and ventilation rate of juvenile brown trout

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Effects of Light on the Tritrophic Interaction Between Kidney Bean Plants, Two-Spotted Spider Mites and Predatory Mites, Amblyseius Womersleyi (Acari: Phytoseiidae)

By analyzing the volatiles from Tetranychus urticae-infested kidney bean plants (Phaseolus vulgaris) at different times for two days, we found that they were mainly produced in the light. Tetranychus urticae showed a higher oviposition rate and spent more time feeding during the day (in the light) than at night (in the dark). Infested leaves placed in the light attracted the predatory mite Amblyseius womersleyi, whereas those that were placed in the dark for at least 2h in daytime did not. This indicates that presence or absence of light affects the production of herbivore-induced plant volatiles. Amblyseius womersleyi dispersed more frequently and consumed more T. urticae eggs during the day (in the light) than at night (in the dark), whereas their oviposition rate did not differ between day and night. Presence or absence of herbivore-induced plant volatiles in the surroundings did not affect dispersal, predation or oviposition rates of A. womersleyi. These results show that A. womersleyis behavior coincides with the production pattern of herbivore-induced plant volatiles.     

Diurnaler Säurerhythmus bei Tillandsia usneoides: Untersuchungen über den Weg des Kohlenstoffs sowie die Abhängigkeit des CO2-Gaswechsels von Lichtintensität,Temperatur und Wassergehalt der Pflanze

Summary Tillandsia usneoides, in the common sense a non-succulent plant, exhibits CO₂ exchange characterized by net CO₂ dark fixation during the night and depression of CO₂ exchange during the day. Malate has been demonstrated to accumulate during CO₂ dark fixation and to be converted to carbohydrates in light. Thus, T. usneoides exhibits CAM like typical succulents.Net CO₂ uptake during the day is increased with net CO₂ output being suppressed in duration of time and extent when light intensity increases. Furthermore, a slight increase in CO₂ fixation during the following night can be observed if the plants were treated with high light intensity during the previous day.Curves of CO₂ exchange typical for CAM are obtained if T. usneoides is kept at 15°C and 20°C. Lower temperature tend to increase CO₂ uptake during the day and to inhibit CO₂ dark fixation. Temperatures higher than 20°C favour loss of CO₂ by respiration, which becomes apparent during the whole day and night at 30°C and higher temperatures. Thus, T. usneoides gains carbon only at temperatures well below 25°C.Net CO₂ uptake during the day occurs only in moist plant material and is inhibited in plants cept under water stress conditions. However, CO₂ uptake during the night is clearly favoured if the plants dry out. Therefore dry plants gain more carbon than moist ones.Curves of CO₂ exchange typical for CAM were also obtained with 13 other species of the genus Tillandsia.The exhibition of CAM by the non-succulent T. usneoides calls for a new definition of the term succulence if it is to remain useful in characterizing this metabolic pathway. Because CO₂-fixing cells of T. usneoides possess relatively large vacuoles and are relatively poor in chloroplasts, they resembles the assimilatory cells of typical CAM-exhibiting succulents. Therefore, if succulence only means the capacity of big vacuoles to store malate, the assimilatory cells in T. usneoides are succulent. It seems to be useful to investigate parameters which would allow a definition of the term succulence on the level of the cell rather than on the level of the whole plant or plant organs.     

Daily rhythms of emergence of small invertebrates inhabiting shallow subtidal zones: A comparative investigation at four locations in Japan

Many small invertebrates inhabit the shallow subtidal zone and some of them emerge at times into the water column. The daily timing of their emergence is affected by the day/night and tidal cycles, and shows various patterns of synchrony with these cyclical factors, depending on the species. To detect possible regional differences in their emergence patterns, sampling was carried out at four locations in Japan: a boreal sea (Akkeshi), a temperate sea (Sugashima), an inland sea (Ushimado) and a subtropical sea (Iriomote-jima). The emergence patterns of major taxa were examined by visual inspection and by two statistical methods (periodogram and autocorrelogram). The composition of the taxa collected by the pump system, mostly crustaceans, was similar in each location. The number of taxa that emerged revealed a day/night rhythm in every location. This characteristic was clearest at Iriomote-jima and least clear at Sugashima. The daily fluctuation in the number of individuals in each taxon varied widely, from very clearly nocturnal to weakly diurnal patterns. In Iriomote-jima, the major taxa all showed well-demarcated nocturnal patterns, so these patterns were classified as either level N₂ or N₃ with regard to the degree of synchrony with the day/night cycle. With regard to the synchrony with the tide, the majority of patterns in all locations showed a double-tidal interval. Many patterns were slightly modified by the tidal cycle. These patterns were classified as level T₁ or T₂ with regard to the degree of synchrony with the tidal cycle. The synchrony with the tide was comparatively strong at Ushimado. The synchrony with day/night and tidal cycles varied even within the same species or closely related species. In benthic invertebrates, hiding or resting in the bottom substrates and swimming in the water column would occur alternatively. In planktonic animals, aggregation near the bottom and dispersal in the water column would occur alternatively. The daily timing of such activities may be synchronized with the day/night and tidal cycles to various degrees among species or populations, resulting in a wide variety of emergence patterns in subtidal small invertebrates. This type of behavior is not daily (diel) vertical migration; it should rather be called daily emergence/dispersal. Strong winds, rough waves and unknown seasonal factors would also affect emergence patterns. Furthermore, the transparency of the seawater may also strongly affect these patterns. Nocturnal patterns may be an adaptation to avoid vulnerability to sighted predators. Variation of synchrony with the tide indicates that by definition, the tidal rhythm can only be distinguished from the day/night rhythm. Hence, the daily patterns that are weakly modified by the tides (levels T₁ and T₂) should be called the tidal rhythms. As the period of such rhythms cannot be determined exactly by using statistical methods, lengthy field investigations and visual inspection of each pattern is essential to assess the influence of tides.     

Plankton community composition and vertical migration during polar night in Kongsfjorden

The polar night in the Arctic is characterized by up to six months of darkness, low temperatures and limited food availability. Biological data on species composition and abundance during this period are scarce due to the logistical challenges posed when sampling these regions. Here, we characterize the plankton community composition during the polar night using water samplers and zooplankton net samples (50, 64, 200, 1500 μm), supplemented by acoustics (ADCPs, 300 kHz), to address a previously unresolved question–which species of zooplankton perform diel vertical migration during the polar night? The protist community (smallest plankton fraction) was mainly represented by ciliates (Strombidiida). In the larger zooplankton fractions (50, 64, 200 μm) the species composition was represented primarily by copepod nauplii and small copepods (e.g., Microcalanus spp., Pseudocalanus spp. and Oithona similis). In the largest zooplankton fraction (>1500 μm), the euphausiid, Thysanoessa inermis, was the most abundant species followed by the chaetognath Parasagitta elegans. Classical DVM was not observed throughout the darkest parts of the polar night (November–mid-January), although, subtle vertical migration patterns were detected in the acoustic data. With the occurrence of a more distinct day–night cycle (i.e., end of January), acoustical DVM signals were observed, paralleled by a classical DVM pattern in February in the largest fractions of zooplankton net samples. We suggest that Thysanoessa spp. are main responsible for the acoustical migration patterns throughout the polar night, although, chaetognaths and copepods may be co-responsible.     

Diel vertical movements,and effects of infection by the cestode <Emphasis Type="Italic">Schistocephalus solidus</Emphasis> on daytime proximity of three-spined sticklebacks <Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis> to the surface of a large Alaskan lake

We conducted a field study in Iliamna Lake, Alaska, to test the hypothesis that proximity of three-spined sticklebacks Gasterosteus aculeatus to the lake’s surface during the daytime varies with macroparasitic cestode parasite Schistocephalus solidus infection in a manner consistent with enhanced vulnerability to avian predators. Extensive sampling in the lake and likelihood-based modeling revealed that sticklebacks displayed a diel vertical migration, being closer to the surface at night than during the evening and early morning. Additional sampling, also coupled with a likelihood-based modeling approach, showed that fish caught at the surface of the lake during the day were more often parasitized (76 vs. 65%), more heavily parasitized (26.8 vs. 22.7% of their body mass), and had larger individual parasites (0.24 vs. 0.20 g) than those caught at night. Parasite infection was related, non-linearly, to fish size, which also differed between day and night sampling at the surface. We performed statistical competitions among nested hierarchies of models that accounted for this effect of length. The most likely models indicated that fish captured during the day had greater parasite prevalence, higher parasite burdens, and larger parasites than did fish captured at night. Proximity to the surface during the day in this very clear lake would likely increase the vulnerability of sticklebacks to predation from birds, enabling completion of the parasite’s lifecycle.     

Diel vertical migration of major fish-species in Lake Victoria, East Africa

Understanding of migration patterns is essential in the interpretation of hydro-acoustic stock assessment data of partly demersal partly pelagic fish stocks. In this paper we provide this kind of information for some species that were common in the Mwanza Gulf of Lake Victoria in the 1980s, before and after the upsurge of introduced Nile perch (Lates niloticus). Detritivorous haplochromines and Nile tilapia (Oreochromis niloticus), both stay near the bottom during day and night. Feeding seems to occur predominantly during the day. The zooplanktivorous haplochromines and dagaa (Rastrineobola argentea) dwell near the bottom by day and migrate towards the surface during the night. They seem to follow their prey, zooplankton and lake-fly larvae. Piscivorous nembe (Schilbe intermedius) show similar migration patterns to zooplanktivorous fishes, but their behaviour cannot be unambiguously explained by pursuit of prey. Nile perch to some extend migrate into the column at night, though the majority remains near the bottom. Feeding takes place during day and night.     

Effects of soil temperature on nitric oxide emission from a typical Chinese rice–wheat rotation during the non-waterlogged period

Measurements of nitric oxide (NO) emission from a typical Chinese rice–wheat rotation are continuously made during the non‐waterlogged period by using an automatic system based on static chamber techniques. A positive correlation exists between NO emission and soil moisture content when surface soil temperature is > 20 °C. The diurnal variability in NO emission is characterized with day‐peak, night‐peak and irregular patterns, which are in close association with wheat growth. The diurnal NO emission under the day‐peak pattern is correlated with the simultaneously observed surface soil temperature, whereas that under the night‐peak and irregular pattern is dependent on surface soil temperature at 7 ± 2 and 3 ± 2 h before NO observation, respectively. The effect of soil temperature on NO emission is well described by F = α·e^β·T, where F is NO flux, T soil temperature, and α and b empirical coefficients. The parameter Q₁₀; that is, the change in NO emission per 10 °C soil temperature, is correlated with the rates of fertilizer‐N application. An approach orientated from the Arrhenius equation, , is developed in order to predict diurnal NO emission, where T_K is the daily average soil temperature, ΔT_K the deviation of soil temperature from the daily average, E_a the apparent activation energy, R the gas constant, t a given time within the one‐day cycle, t delayed time for appearance of the diurnal NO emission peak, and k an empirical coefficient. Based on the results, the authors recommend that intermittent measurement of NO emission from a similar ecosystem would be best taken around 17:00. The molar ratio of NO/N₂O over the non‐waterlogged period is > 1 when soil moisture content was less than the field capacity, suggesting that NO emission was mainly derived from nitrification under this condition.