The time course of dark adaptation in the bee: a phototactic and electrophysiological investigation

ABSTRACT. The time course of dark adaptation in Apis melifera L. was investigated by analyses of phototactic behaviour and electroretinogram (ERG). The behavioural results give a function for dark adaptation showing that in the dark after strong light adaptation the sensitivity increases exponentially with a time constant of 3 min. The sensitivity changes c. 2.4 log units during the time span of 5–720 s. The electrophysiological results indicate a smaller change in sensitivity at the level of the photoreceptors. Within a time span between 20 and 720 s the sensitivity increases during dark adaptation by a factor of 4.3 on a linear scale.     

Diet,Prey Selection,and Predation Impact of Black-Backed Jackals in South Africa

ABSTRACT To investigate the role of black-backed jackals (Canis mesomelas) as predators, we studied diet, prey selection, and predation impact of jackals on 2 game ranches in South Africa that differed in ungulate diversity and biomass. Results showed that large (>15 kg) ungulate species dominated jackal diets throughout the year on both the less diverse (range of ingested biomass across seasons = 39–78%) and more diverse (26–69%) game ranch. Other important food items included medium-sized mammals (1–3 kg; 1–26%) and fruit (2–69%), whereas small mammals comprised 3–11% of ingested biomass across seasons on both sites. Jackals were not random in consumption of ungulates, and consumption patterns suggested jackals actively hunted certain species rather than consumed them as carrion. During ungulate birthing periods, jackals consumed almost exclusively those ungulate species that were hiders (i.e., fawns were hidden in tall vegetation away from herd) regardless of ungulate densities, suggesting that primarily fawns were preyed upon. Among hiders, there was a negative relationship (P = 0.01) between body size and percent of population consumed by jackals, indicating smaller species were more susceptible than larger species to jackal predation. Consequently, springbok (Antidorcas marsupialis) were always selected over other ungulate species on both sites, and this species was the most impacted by jackal predation. In contrast, ungulate species that were followers (i.e., fawns immediately followed mothers within protection of the herd) were scarcely or not at all consumed by jackals, regardless of body size or density. Medium-sized mammals were selectively consumed over ungulates, and there was a negative relationship (P < 0.01) between consumption of berries and ungulates, indicating alternative food resources influenced consumption of ungulates on our study sites. Our results will help wildlife managers in Africa identify ungulate species susceptible to jackal predation, and can be used to develop management strategies for reducing jackal predation in areas where it is problematic.     

Vertical zonation and production rates of epiphytic algae on Phragmites australis

1. The vertical distribution of chlorophyll in epiphyton on Phragmites australis showed a peak in the middle sections of the submerged parts. Just below the water surface and above the sediment, chlorophyll concentrations were much less. 2. During winter and early spring, loosely attached diatoms were predominant just below the water surface and on the middle sections of the Phragmites plants. Near the bottom, adnate diatoms, parenchymatous thalli of chlorophytes and cyanobacteria were abundant. 3. At high photosynthetic active radiation (PAR) during May, filamentous species of Ulpthricophyceae and Zygophyceae developed dense populations on the middle sections of the stems. 4. Primary production rate was proportional to chlorophyll concentration although production maxima were recorded above the biomass maxima. 5. Where illumination was low, the chlorophyll-specific rate of photosynthesis (P^B rate) decreased proportionally with the vertical decrease of PAR in the littoral zone, independent of the chlorophyll concentration on the stems. 6. When illumination was high, the P^B rate decreased as biomass increased, and was independent of surface radiation. 7. The dependence of primary production rate on chlorophyll concentration produced a saturation curve with a maximum production at 4.6 μgC cm^?2h^?1.     

Cell Surface Interactions between Trypanosoma congolense and Macrophages during Phagocytosis In Vitro

ABSTRACT. Trypanosoma congolense bloodstream forms preincubated with a high titer of anti-variant surface antigen (VSG)-specific antibody, a low amount of anti-VSG plus complement-active mouse serum (MS), MS alone, and trypsin were cocultivated with mouse peritoneal macrophages in vitro. Immunofluorescence as well as transmission and scanning electron microscopy revealed that upon attachment to the macrophages' surface, trypanosomes opsonized with anti-VSG/MS formed opsonized filopodia, which were rapidly internalized by the phagocytes. Although these cells attached as frequently as anti-VSG or trypsin-pretreated parasites, the rate of phagocytosis of anti-VSG/MS pretreated trypanosomes was reduced significantly. Trypanosomes pretreated with high antibody titers alone were lysed on the surface of the macrophages before phagocytosis was completed. Parasites opsonized with complement alone adhered only occasionally and were rarely phagocytosed. Trypsin-treated trypanosomes, which served as positive control cells, rapidly attached and remained intact until ingulfment by the macrophages was completed. Untreated control parasites did not attach to the macrophages and were not phagocytosed. Cocultivation of macrophages with anti-VSG/MS-opsonized trypanosomes caused internalization of the flagellum by membrane fusion. Filopodia formation by T. congolense is thus correlated with a marked reduction in phagocytosis even in the presence of only a sublytic antibody titer.