Constancy of synaptic ribbon numbers in the retina of the arctic chary,Salvelinus alpinus

Synopsis At high latitudes, such as in Iceland, the daily photoperiod varies from almost continuous darkness in winter to virtually constant light in summer. Previous studies of detailed retinal structure in vertebrates have shown significant daily and annual effects of photoperiod. We sampled arctic charr in Iceland during the summer, including fish that were both light- and dark-adapted, during both day and night. We observed retinomotor responses characteristic of light- and dark-adaptation, but found no difference in the number of synaptic ribbons in the retina. The morpho-physiological changes, appearing as retinomotor responses, are thus not expressed at the synaptic level.     

蓝藻型富营养湖泊藻量的昼夜变化节律

杭州西湖为蓝藻型富营养湖泊,根据1980年9月及11月二次昼夜分层采样调查,该湖浮游藻在一昼夜中有二次上下垂直移位,使近表层藻量形成规则或不规则的双峰型昼夜变化曲线,双峰分别出现在日照开始与日落后2h左右,认为昼夜光暗交替与其昼夜变化有一定相关关系,同时,双峰的形成与蓝藻门优势藻种对光照强度的适应性能相关。文中对其优势藻种的浮沉与日照关系作了讨论。     

大豆-根瘤菌共生体系光合与固氮关系研究

水培大豆和田间生长的大豆,接种根瘤菌 Rhizobium B16-11C 后植株全氮含量、叶片叶绿素含量和净光合速率及种子产量都明显增加。比较 Clark 大豆的结瘤品系和不结瘤品系获类似结果。摘除根瘤后3天内叶片净光合速率无明显变化。大豆植株遮阴、去叶或切掉地上部导致根瘤活性明显下降。但去豆荚不能提高根瘤固氮的比活性。根瘤活性的日变化不能用根瘤蔗糖、淀粉含量或周围温度的变化来解释,其控制因子尚待深入研究。     

Effect of soil temperature on stem sap flow,shoot gas exchange and water potential of Picea engelmannii (Parry) during snowmelt

Summary The effect of cold soils on stem sap flow, shoot gas exchange and water potential of Picea engelmannii (Parry) was investigated during the snowmelt period in the Medicine Bow Mountains, Wyoming, USA. Shoot net photosynthetic rates were higher in young trees (1.5–1.8 m in height) growing in cold soils (<3.5° C) associated with snowpack, than trees in warm soils until about 1500 h. Higher shoot photosynthetic rates of trees in cold soils continued after snow was removed and could not be completely explained by higher visible irradiance over highly reflective snow. Following soil warming higher photosynthetic rates were evident in these trees for five days. High nutrient availability associated with snowmelt may improve shoot nutrient status leading to higher gas-exchange rates during snowmelt. Shoot conductance to water vapor was higher in trees in cold soil until midday, when declining shoot conductance led to lower intercellular CO₂ concentrations. Midday through afternoon shoot water potentials of trees in cold soils were similar or higher than those of trees in warm soils and the lower afternoon shoot conductances in cold soils were not the result of lower bulk shoot water potentials. Decline in net photosynthesis of trees in cold soils at 1500 h paralleled increases in intercellular CO₂ concentrations, implying a nonstomatal limitation of photosynthesis. This scenario occurred consistently in mid-afternoon following higher morning and midday photosynthesis in cold soils, suggesting a carbohydrate feedback inhibition of photosynthesis. Diurnal patterns in stem sap flow of all trees (cold and warm soils) reflected patterns of shoot conductance, although changes in stem sap flow lagged 1–3 h behind shoot conductance apparently due to stem water storage. Total daily stem sap flow was similar in trees in cold and warm soils, although diel patterns differed. The morning surge and night-time drop in sap flow commenced 1–2 h earlier in trees in cold soils. Overnight stem sap flow was lower in trees in cold soils, possibly due to higher resistance to root water uptake in cold soils, which may explain lower predawn shoot water potentials. However, midday shoot water potentials of trees in cold soils equalled or exceeded those of trees in warm soils. Higher resistance to root water uptake in P. engelmannii in cold soils was apparently overshadowed by transpirational forces and significant shoot water deficits did not develop.     

Life histories and habitat selection in Daphnia: divergent life histories of D. magna clones differing in phototactic behaviour

To test the hypothesis of co-adaptation of life histories and daytime vertical distribution (vertical migration behaviour) in Daphnia, life history characteristics were analysed for two positively, three negatively, and four intermediately phototactic Daphnia magna clones. Clones with different phototactic behaviour were found to have divergent life history strategies, with positively phototactic clones being good exploiters under the non-limiting conditions provided in the laboratory, i.e. low density (1 ind./1), high food concentration (6,5–7 10⁵ Scenedesmus cells/ml, restored daily) and high temperature (20° C). They realized a high intrinsic rate of increase at a small adult body size through rapid development, at a cost of producing small neonates. Negatively and intermediately phototactic clones had larger adult body sizes, and produced larger neonates that were more starvation-resistant than those of positively phototactic clones. Selection for high intrinsic rate of increase in intermediately phototactic clones was mediated through the production of large clutches.     

Daily Rhythms of Body Temperature in Acornys russatus: The Response to Chemical Signals Released by Acomys cahirinus

Two species of spiny mice of the genus Acomys—the golden spiny A. russaturs and the common spiny A. cuhirinus—are syrnpatnc in the and and hot parts of the Rift Valley in Israel. The coexistence of these two species is due to exclusion of A. russatus mice by A. cuhirinus mice from nocturnal activity. The aim of this research was to study if odor signals released by A. cahirinus mice can play a role in the exclusion of A. russatus mice. A. russatus mice with an implanted transmitter recording body temperature (T_b) were kept alone in a metabolic chamber under constant conditions of ambient temperature (27°C) and photoperiod (12 h light: 12 h dark). After 5 days of recording, chemical signals from an A. cuhirinus mouse were added through the air tube going into the metabolic chamber of the A. russatus mice. This treatment caused a shift of ∼ 2 h inT_b daily rhythm of the naive tested A. russutus mice, whereas no shift was observed in A. russatus mice that had been kept in the same room with the A. cahirinus mouse before measurements. These results strongly support the idea that chemical signals released by A. cahirinus mice can entrain the T_b rhythms of A. russatus mice. Therefore, it may be assumed that the exclusion of A. russatus mice from nocturnal activity by A. cuhirinus mice could be achieved through the odor released by the latter.     

Aspects of the biology of the euryhaline Asian cichlid,Etroplus suratensis

Synopsis Aspects of the biology of the euryhaline Asian cichlid Etroplus suratensis in a coastal lagoon of Sri Lanka were studied for a period of 15 months. Comparisons are made from the catches from the fishery of manmade, inland freshwater reservoirs. Changes in the stock structure in the sublittoral region of the lagoon indicate that recruitment takes place twice a year, during the high rainfall/low salinity periods. The feeding habits of E. suratensis were different in the two habitats. In the lagoon it fed mainly on molluscs and in the freshwater reservoirs on macrophytes. Feeding chronology based on diurnal surveys indicate that it feeds mainly during the daylight hours. The dentition of E. suratensis is adapted for both tearing and crushing (pharyngeal teeth). Distinct differences in the mean relative intestinal length between populations from the coastal lagoon and inland reservoirs were evident and these differences are correlated to their respective feeding habits. Diurnal changes in feeding activity are associated with changes in the stomach pH. The egg-diameter distribution of mature fish indicate that E. suratensis is a single spawner, that it sheds its mature eggs all at once. It is inferred that E. suratensis breeds twice a year but that an individual female is capable of spawning only once a year.     

Diurnal variation of phytoplankton in Loch Lomond

A study of diurnal variation over a 48 hour period was undertaken in July 1973 to ascertain the extent and timing of some major chemical, physical and biological variables in Loch Lomond. The phytoplankton population was dominated by the diatom Tabellaria fenestrata, with a maximum abundance between 04.00 and 06.00 h in surface waters on both days. A distinct diurnal variation in cell numbers was also recorded. Chlorophyll a values also showed a regular pattern of variation with a single peak between 10.00 and 14.00 h each day. Some chemical changes appeared to be a direct consequence of phytoplankton multiplication. Nitrate-nitrogen showed a decrease in concentration coinciding with the period of cell multiplication, whereas dissolved silica concentrations only fell on the completion of this process. Other common diatoms displayed less distinct patterns of variation although Fragilaria crotonensis attained its maximum abundance in surface waters. Considerable variation was recorded in the number of organisms and chlorophyll a levels at 25 cm intervals in the upper metre of the water column, with large variations in standing-crop and chemical parameters in the space of one hour. Diurnal oscillations in the position of the thermocline were recorded, with the thermal discontinuity being at its greatest depth in the early hours of the morning. The hypolimnion and thermocline regions clearly acted as a source of nutrient supply to the epilimnion. From this investigation it is apparent that for the proper understanding of diurnal variation a 24 hour study alone is insufficient and may give rise to misleading results.Department of Botany, University of GlasgowDepartment of Botany, University of Glasgow     

A dead giveaway: Foraging vultures and other avian scavengers respond to auditory cues

Carrion represents an unpredictable and widely distributed primary food source for vultures and other avian scavengers. Avian scavengers in African savanna ecosystems are reported to rely exclusively on visual stimuli to locate carcasses. However, carnivores’ predation of large mammalian herbivores and subsequent competition for access to the carcass can result in considerable noise, often audible over long distances and for prolonged periods. Vultures and other avian scavengers may therefore detect and respond to these auditory cues, as do the mammalian carnivores alongside which vultures have coevolved, but this has not been investigated to date. Working in the Serengeti ecosystem, Tanzania, we used diurnal auditory broadcasts to simulate predation and competitive carnivore feeding interactions. Based on the current understanding of avian scavenger ecology, we hypothesized that avian responses to call‐in stations would be evoked exclusively by visual, rather than auditory, cues. We therefore predicted that (a) the arrival of avian scavengers at call‐in stations should be preceded and facilitated by mammalian carnivores and that (b) the arrival of avian scavengers would be positively correlated with the number of mammalian scavengers present, which would increase detectability. We recorded 482 birds during 122 separate playback events. In 22% of these instances, avian scavengers arrived first, ruling out responses based exclusively on visual observations of mammalian carnivores, thereby contradicting our first prediction. Furthermore, the first avian arrivals at survey sessions were inversely related to the number of hyenas and jackals present, contradicting our second prediction. Since no bait or carcasses were used during the experiments, these responses are indicative of the birds’ ability to detect and respond to audio stimuli. Our findings challenge the current consensus of sensory perception and foraging in these species and provide evidence that avian scavengers have the ability to use sound to locate food resources.