Ecology and conservation status of endemic freshwater crabs in Lake Tanganyika, Africa

Sedimentation resulting from riparian deforestation has a wide range of detrimental effects on aquatic biodiversity, but predicting the full consequences of such disturbances requires an understanding of the ecosystem’s key functional components. We investigated the ecology and response to sedimentation of the diverse, endemic freshwater crabs of Lake Tanganyika, which may occupy important positions in littoral foodwebs. Our surveys revealed crab distribution patterns to be patchy, and that crabs can be locally abundant (0–28 individuals m⁻²). Crab densities decreased with depth and the dry mass of crab assemblages ranged from 0.0 to 117.7 g m⁻². Comparisons among sites revealed significant effects of sedimentation on crab assemblage evenness, but provided no evidence that sedimentation has altered densities, incidence or species richness. The resilience of crabs to sedimentation might be related to their intraspecific dietary breadth. Stable isotope data (δ¹³C and δ¹⁵N) from crabs and their potential food resources indicated differences in trophic roles among endemic crab species. Overall, crabs occupy higher trophic positions than most other invertebrates, and they draw upon both benthic and planktonic energy pathways. The high biomass and top-predator status of some crab species suggests the potential for cascading effects on organisms lower in the food web.     

Hydrodynamic studies on the streptokinase complexes of human plasminogen, Val442-plasminogen, plasmin, and the plasmin-derived light (B) chain

Sedimentation velocity and sedimentation equilibrium studies have been carried out on the Glu- and Lys-plasminogen-streptokinase complexes as well as on the complexes formed by Val442-plasmin and the light (B) chain of plasmin. Sedimentation equilibrium molecular weights are consistent with a 1 to 1 molar complex in all cases and give values consistent with the differences in size of the plasminogen moieties. Sedimentation velocity determinations in the presence of protease inhibitors give values consistent with the conformational differences already reported for the Glu- and Lys-plasminogen molecules. However, unlike Glu-plasminogen, the addition of epsilon-aminocaproic acid or lysine does not alter the conformation of the Glu-plasminogen complex. The values of the sedimentation coefficient and the molecular weight of the plasmin and the Val442-plasmin-streptokinase complexes increase to those of a dimer when determined in the absence of active-site inhibitors but return to monomer values when these inhibitors are added. Thus, dimer formation requires the presence of an available active site in at least one of the two molecules involved and is reversible.     

The development of sessile epifaunal communities in Kylesalia,Kilkieran bay (west coast of Ireland)

Test panels, which were suspended horizontally at three sites in a shallow inlet on the west coast of Ireland, were sampled monthly over a period of one year to investigate the development of epifaunal communities. In addition, the current regime and sedimentation at these sites were measured. The deposited materials, of high organic value, may enhance the food supply to the sessile organisms through resuspension. Heavy deposition at one site interrupted the study. At the other two sites, however, the experiment was not affected. At one site, initial settlement by colonial bryozoans, followed by solitary ascidians, was noted on the panels over the period studied. In the competition for space, the dense aggregations during settlement, the relative large size, and the rapid growth of these solitary animals may occlude the colonization by colonial bryozoan mats underneath. At the other site, a more diversified community with co-existence of many sedentary polychaete and colonial bryozoan species was recorded on the panels. The continuing availability of free space on the panels suggests that the development of sessile communities at this site may be hindered by the relatively slow current flow and heavy deposition and resuspension. Whilst differences in community development were observed on the panels at these two sites, it would be difficult, through such a short study period, to ascertain whether stable climax states had been reached.     

Short-term sedimentation in Tagus estuary,Portugal: the influence of salt marsh plants

Short-term sediment deposition was studied at four salt marsh areas in the Tagus estuary. In areas covered with Sarcocornia perennis, Sarcocornia fruticosa, Halimione portulacoides and Spartina maritima and also in the non-vegetated areas, sedimentation was measured as the monthly accumulation of sediments on nylon filters anchored on the soil surface, from August 2000 to May 2001. Our experiments were used also to determine the influence of the different plant species in vertical accretion rates. Short-term sedimentation rates (from 2.8 to 272.3 g m⁻² d⁻¹) did show significant differences when the four salt marshes studied in the Tagus estuary were compared to each others. Salt marshes closer to the sediment sources had higher sedimentation rates. Our results suggest that the salt marsh type and surface cover may provide small-scale variations in sedimentation and also that sediment deposition values do change according to the position of the different plant species within the salt marsh. Sedimentation is an essential factor in salt marsh vertical accretion studies and our investigation may provide support to help forecast the adaptative response of the Tagus estuary wetlands to future sea level rise.     

Deoxyribonucleic acid poymerase of BHK-21/C13 cells. Heterogeneity, molecular asymmetry and subcellular distribution of the enzymes.

Nuclear and cytoplasmic fractions were prepared from exponentially-growing BHK-21/C13 cells; DNA polymerase was extracted from them and analysed by gel filtration and sucrose-density-gradient centrifugation. DNA polymerase I is heterogeneous comprising species covering a considerable range of molecular weights. These have been tentatively identified as four subspecies of apparent molecular weights 900000-1000000 (IA), 460000-560000 (IB), 270000-320000 (IC) and 140000-200000 (ID), as assessed by gel filtration through Sepharose 6B. DNA polymerase II has a mol.wt. of 46000 +/- 4000 as assessed by gel filtration on Sepharose 6B, and 48000 +/- 2000 as assessed by gel filtration on Sephadex G-100. Sedimentation analyses on sucrose density gradients showed that the DNA polymerase I species had sedimentation coefficients predominantly in the range 6-8 S. DNA polymerase II had predominantly a sedimentation coefficient of 3.2 S although a component with lower sedimentation coefficient was found. The lack of correlation between the molecular weights derived from gel filtration and the sedimentation coefficients is attributed to molecular asymmetry. DNA polymerase I was found to be associated predominantly with the cytoplasm although certain types of nuclear preparation contained large amounts of it. DNA polymerase II was found to be mostly if not exclusively in nuclear preparations.     

Mortality and growth of juvenile coral Pocillopora damicornis (Linnaeus)

Early mortality in cohortsof the coral Pocillopora damicornis (Linnaeus) was monitored under experimental conditions on a reef in order to evaluate effects of sedimentation, grazing, predation and competition. Corals that settled in dishes in the laboratory were placed on the reef flat about 3 days after metamorphosis. Six different conditions were tested in each series of experiments: orientation of dishes (upward, vertical and downward) by with and without protection against potential grazers by covering the dish with a net. Survival of juvenile corals on both protected and unprotected dishes facing upward was lower than in the vertical or downward direction. Under the vertical facing and protected conditions, algal growth was more intensive and algae trapped sediment; mortality of juvenile corals by algae and sedimentation increased gradually. In the uprotected and vertical conditions, algal growth on the surface was removed constantly by grazing invertebrates and fishes and the juvenile corals were removed or killed as well. On the downward facing dishes, survivorship of juveniles was relatively high in both protected and unprotected conditions. This habitat attracted many sessile animals that killed some juveniles by competition. Thus, juvenile corals survived better in experimentally manipulated microhabitats not affected by direct sedimentation, not exposed to direct grazing activity, and not occupied by rapidly growing filamentous algae.     

Late Quaternary pelagic sedimentation on the southern Calabrian Ridge and western Mediterranean Ridge,eastern Mediterranean

Cores from plateaus, lying some 150–200 m above the basin floors in areas of strong vertical relief visited during the Cobblestone Project, consist entirely of pelagic marls with sapropels and tephras as minor lithologies. All the plateau cores were correlated by means of isochronous lithologies and biostratigraphically calibrated. A core from Area 3 (No. 29) and two cores from Area 4 (No. 45 and 6) were selected as reference sections and studied quantitatively in terms of grain sizes, carbonate content and microfossil content.Faunal curves based on selected species of planktonic foraminifers correlate between the three cores, which extend back in time to approximately 185 000 yr for Area 4 and to 430 000 yr for Area 3. Winnowing resulted in a reduced sedimentation rate in Area 3, in unusually high percentages of the coarser sediment fraction and in extended hiatuses at the top of the cores.The highest temperature in surficial waters as indicated by the foraminiferal faunas is recorded in all three references sections at the base of an interval correlative with isotopic stage 5e (～ 125 000 yr B.P.), within sapropel S-5.Clay mineralogy indicates basically uniform conditions in the two areas, with the exception that smectite is more abundant in the Calabrian Ridge, which is closer to its source area (Sicily, Sicily Channel). Carbonates include calcite, magnesian calcite and some dolomite.Sapropels, sedimentary expression of stagnation, represent up to 25% of the sedimentary column in an interval encompassing isotopic stage 5 through 7. The average organic carbon content in analysed sapropels is 3.1%, versus 0.29% in normal pelagic sediments. Sapropels S-6 and S-8 are consistently accompanied by a foraminiferal fauna that is indicative of low-salinity cool surficial waters, whereas sapropels S-1, S-5 and S-7 occur in coincidence with the warmest peaks of the climatic faunal curve.Average sedimentation rate is 3.2 cm/1000 yr for Core 29; 5.5 cm/1000 yr for Core 45. Sedimentation rates calculated for basin cores are five to ten times higher. Sedimentation rates of sapropels and tephras are independent from regional sedimentation rates.Hiatuses in plateau cores correlate with debris flows in base-of-slope cores, both occurring during times of enhanced bottom circulation and submarine erosion related to low stands of sea level in cold periods.     

Molecular weight of the acetylcholine receptors of electric organs and the effect of Triton X-100.

Studies are reported on the influence of Triton X-100 on the molecular weight and functional properties of the acetylcholine receptor. Results are presented principally for receptors purified from Torpedo californica and Torpedo marmorata with a limited number of observations on the receptor from Electrophorus electricus. In equilibrium dialysis measurements Trito, X-100 greatly reduced acetylcholine binding to the high affinity sites of the receptor from T. californica, but had only a small effect on the sites of lower affinity. Sedimentation equilibrium experiments on receptor in the absence of added Triton X-100 revealed average apparent molecular weight values of 510,000 for receptor from T. californica and 665,000 for T. marmorata. Under those conditions 0.113 mg of residual Triton X-100 were found per mg of protein as determined by using 3H-labeled Triton X-100. The sedimentation data indicated the presence of more than one molecular species, involving a unit with an apparent molecular weight of 330,000 and higher aggregates. Upon addition of Triton X-100, the higher aggregates were reduced, and above 0.1 percent Triton X-100 the 330,000 unit was the principal component present for receptor from all three species examined. Various structural models are considered in the light of this value, the polypeptide size from Na dodecyl sulfate-gel electrophoresis, and the protomer size determined by the molecular weight of an acetylcholine binding site.     

Structure and dimerization of translation initiation factor aIF5B in solution

Translation initiation factor 5B (IF5B) is required for initiation of protein synthesis. The solution structure of archaeal IF5B (aIF5B) was analysed by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) and was indicated to be in both monomeric and dimeric form. Sedimentation equilibrium (SE) analytical ultracentrifugation (AUC) of aIF5B indicated that aIF5B forms irreversible dimers in solution but only to a maximum of 5.0-6.8% dimer. Sedimentation velocity (SV) AUC at higher speed also indicated the presence of two species, and the sedimentation coefficients s(20,w)(0) were determined to be 3.64 and 5.51±0.29 S for monomer and dimer, respectively. The atomic resolution (crystallographic) structure of aIF5B (Roll-Mecak et al. [6]) was used to model monomer and dimer, and theoretical sedimentation coefficients for these models were computed (3.89 and 5.63 S, respectively) in good agreement with the sedimentation coefficients obtained from SV analysis. Thus, the structure of aIF5B in solution must be very similar to the atomic resolution structure of aIF5B. SAXS data were acquired in the same buffer with the addition of 2% glycerol to inhibit dimerization, and the resultant monomeric aIF5B in solution did indeed adopt a structure very similar to the one reported earlier for the protein in crystalline form. The p(r) function indicated an elongated conformation supported by a radius of gyration of 37.5±0.2 ? and a maximum dimension of ~130 ?. The effects of glycerol on the formation of dimers are discussed. This new model of aIF5B in solution shows that there are universal structural differences between aIF5B and the homologous protein IF2 from Escherichia coli.     

Impacts of sedimentation and nitrogen enrichment on wetland plant community development

Many factors influence which plant species are found in a particular wetland. The species pool is composed of the species present in the seed bank and species able to disperse into the wetland, and many abiotic and biotic factors interact to influence a species performance and abundance in the plant community. Anthropogenic activities produce specific stressors on wetland systems that alter these abiotic and biotic interactions, potentially altering species composition. We simulated three common wetland hydrogeomorphic (HGM) subclasses in a greenhouse to examine the effects of two stressors-sedimentation and nitrogen (N) enrichment-on the performance of 8 species grown in artificial communities. Species establishment, height, biomass, and foliar N and P concentrations were measured to explore species responses to stressors and competition, as well as the potential impacts of changes in species composition on ecosystem processes. Species were affected differently by sedimentation and N enrichment, and there were differences in overall community sensitivity to stressors between wetland subclasses. Sedimentation generally reduced seedling establishment, while N enrichment produced variable effects on height and biomass. Interspecific competition had little effect on establishment but significantly reduced most species biomass. Sedimentation generally lowered community biomass, diversity, and richness, while enrichment increased community biomass. Establishment, biomass, and foliar nutrient concentrations significantly differed between many species, suggesting that shifts in species composition may impact ecosystem processes such as nutrient cycling and carbon storage. Phalaris arundinacea, an aggressive clonal graminoid, universally dominated all wetland subclasses. This dominance across a range of environmental conditions (sedimentation, fertility, and hydrology) has important implications for both restoration and predicting the impacts of human activities on species composition. Our results suggest that, in regions where P. arundinacea is common, restoration projects that establish communities from seeds and human activities that cause vegetation removal are likely to become dominated by P. arundinacea.     

Detection and characterization of actin monomers, oligomers, and filaments in solution by measurement of fluorescence photobleaching recovery

Fluorescence photobleaching recovery (FPR) was measured to determine the diffusion coefficient of fluorescein-labeled G-actin in low-salt buffer. The result obtained, 7.15 +/- 0.35 X 10(-7) cm2/s, is in good agreement with that computed from the molecular weight, partial specific volume, and sedimentation coefficient, but is higher than previously obtained values. It is demonstrated from theory that at low ionic strength, the electrostatic contribution to the intrinsic viscosity leads to an overestimate of the hydrodynamic eccentricity of G-actin. Data from FPR, sedimentation, and fluorescence polarization experiments all indicate that the true low-salt form of the actin monomer has an axial ratio less than or equal to 3.0. The G-F transformation of actin was also observed by measurement of FPR during the assembly phase, in the steady state, and in the presence of ligands such as cytochalasin and aldolase. Each FPR record in general yields three data: relative proportion of rapidly and slowly diffusing actin, diffusion coefficient for the high-mobility fraction, and a mean diffusion coefficient for the low-mobility fraction. A relation between the mean low-mobility diffusion coefficient and the number-average filament length is derived and applied to the analysis of FPR data. Under typical conditions, the average filament length was much greater than 10 micron in the steady state. Cytochalasin D was found to decrease filament length and total amount of filament proportionally; total filament number was not greatly affected. In all polymerizations of G-actin, the high-mobility material observed in situ was found to be essentially monomeric actin. Relatively stable oligomers of actin were separated by fractionating G-AF-actin by gel filtration in 50 microM MgCl2 at 4 degrees C. On the basis of the diffusion coefficient, we conclude that monomer and dimer constitute the major particle types present under these conditions. Sedimentation of labeled actin polymerized in 1.0 mM MgCl2 yielded a graded supernatant that contained actin oligomers significantly larger than the monomer.     

Effect of Viscosity on Formation and Sedimentation of Polytomella agilis Vertical Aggregates*

The response of vertical aggregates of Polytomella agilis to increased viscosity of the medium indicates that viscous resistance of the medium is a critical factor controlling the formation and sedimentation of protozoan aggregates. Aggregate formation time increased from 20 sec to 6.5 min as viscosity was increased 1.72 times. Sedimentation rate decreased about 140 μm/sec for each 10% increment in viscosity. Although there was a 60% decrease in speed of falling aggregates when relative viscosity was increased from 1.0 to 1.6, individually swimming P. agilis decreased less than 22% in speed even at a relative viscosity of 1.84 where aggregates did not form. Cells continued to accumulate near the surface by negative geotactic swimming after vertical aggregation ceased. Vertical aggregation is a cyclic process composed of 4 phases: a) accumulation-aggregation; b) aggregate sedimentation; c) ablation and dispersal; and d) negative geotaxis. Phase d is unique to motile microorganisms and only in such populations is vertical aggregation continuously selfperpetuating. Phases a-c occur with both motile and nonmotile bodies and are dependent upon viscous forces of the medium.     

Influence of sedimentation in the absence of macrograzers on recruitment of an annual population of Macrocystis pyrifera in Metri Bay,Chile

The effects of sedimentation and substrate orientation on algal and sessile invertebrate assemblages were tested on an annual population of Macrocystis pyrifera in Metri Bay, southern Chile. In the laboratory, M. pyrifera zoospores were seeded on Crepipatella fecunda shells, the primary substrate for M. pyrifera in this system. The seeded shells were deployed at Metri Bay inside cages and were orientated vertically and horizontally under two sedimentation regimes (bottom and suspended). Due to differences in grazer accessibility and the species present between the sedimentation treatments, grazers (>1 cm) were excluded. We followed sporophyte development of M. pyrifera and the natural recruitment of other algal and invertebrate species. Sedimentation rates were significantly higher in the cages attached to the bottom compared to suspended cages (P < 0.001). In total M. pyrifera and three additional algal genera were detected and all algal recruits showed significantly greater recruitment on the horizontally orientated substrate compared to the vertical substrate. Macrocystis pyrifera sporophytes were present only on the horizontal, suspended (less sedimentation) treatment. In contrast, Ulva and Ectocarpus spp. also occurred in the horizontal, high sediment treatment. Invertebrate recruitment (amphipods, barnacles and spirorbids) dominated the vertically oriented shells regardless of sedimentation. Results indicate that high sedimentation negatively affected the development of M. pyrifera sporophytes while other opportunistic species were able to recruit under these conditions.     

Density dependence and temporal plasticity of competitive interactions during utilisation of resources by a community of lepidopteran stemborer species

Interactions within and between species sharing the same resources are characterised by competition or facilitation, and can be influenced by factors such as larval numbers and phenotypic plasticity of the interactions. The effect of larval density on the survival and relative growth rate of the stemborers Busseola fusca (Fuller) and Sesamia calamistis Hampson (both Lepidoptera: Noctuidae), and Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) were studied, as well as the temporal plasticity of their competitive interactions. These stemborers attack maize crops (Zea mays L.) (Poaceae) in sub‐Saharan Africa. Experiments were conducted in the laboratory under controlled conditions at the optimum development temperature (25 °C) for the three species. Surrogate stems filled with artificial diet were intra‐ and interspecifically infested with larvae of each species. The effect of larval density on competition was studied at low (six larvae) and high (12 larvae) levels of infestation, whereas the temporal plasticity of competition was evaluated at 7, 14, 21, or 28 days after infestation. The two experiments involved single‐ and multi‐species infestation treatments. Larval numbers and wet mass in each artificial stem were recorded in each experiment. Survival and relative growth rate of the three species were significantly higher at low‐infestation levels when facing either intra‐ or interspecific competition. The intensity of competition was also temporally plastic among the species and increased as the duration of competition increased. These results are discussed in terms of general infestations of cereal crops by borers.     

Effects of experimental sedimentation on the phenological dynamics and leaf traits of replanted mangroves at Gazi bay,Kenya

相似文献   

The importance of sedimenting organic matter, relative to oxygen and temperature, in structuring lake profundal macroinvertebrate assemblages

We quantified the role of a main food resource, sedimenting organic matter (SOM), relative to oxygen (DO) and temperature (TEMP) in structuring profundal macroinvertebrate assemblages in boreal lakes. SOM from 26 basins of 11 Finnish lakes was analysed for quantity (sedimentation rates), quality (C:N:P stoichiometry) and origin (carbon stable isotopes, δ¹³C). Hypolimnetic oxygen and temperature were measured from each site during summer stratification. Partial canonical correspondence analysis (CCA) and partial regression analyses were used to quantify contributions of SOM, DO and TEMP to community composition and three macroinvertebrate metrics. The results suggested a major contribution of SOM in regulating the community composition and total biomass. Oxygen best explained the Shannon diversity, whereas TEMP had largest contribution to the variation of Benthic Quality Index. Community composition was most strongly related to δ¹³C of SOM. Based on additional δ¹³C and stoichiometric analyses of chironomid taxa, marked differences were apparent in their utilization of SOM and body stoichiometry; taxa characteristic of oligotrophic conditions exhibited higher C:N ratios and lower C:P and N:P ratios compared to the species typical of eutrophic lakes. The results highlight the role of SOM in regulating benthic communities and the distributions of individual species, particularly in oligotrophic systems.     

Development of a subtidal epibenthic community: factors affecting species composition and the mechanisms of succession

Summary Macroinvertebrate grazers and temporal variability were found to strongly influence species composition of communities that developed subtidally on plexiglas panels. On panels exposed to the naturally high densities of sea urchins and sea stars, only grazer-resistant algal crusts, a diatom/blue-green algal film and short-lived filamentous algae became abundant. On those panels protected from grazers, however, other algae and sessile invertebrates were also common. Both the effects of grazing and the abundance of individual taxa differed on panels immersed at different times of the year.Resident species also affected subsequent recruitment. Some colonists were found more frequently on panels with established communities than on recently immersed plates. Others became more abundant on younger than on older panels. Considerable small-scale spatial variation in the abundance of species was also found among panels within treatments and appeared to persist throughout the 13 months of the study.I suggest that since the interactions that determine which mechanisms are important in succession occur between individuals (generalized here to species), not between successional stages, factors such as those examined that can determine which species will interact, indirectly determine the mechnaisms that are important in the development of a community. Models that deal with interactions between successional stages may lack the detail neccessary to predict or explain changes in species composition in diverse communities.     

Molecular Weight Determination of Sendai and Newcastle Disease Virus RNA

The molecular weights of Sendai and Newcastle disease virus RNA were estimated by sedimentation in sucrose gradients and by length measurements in the electron microscope under both denaturing and nondenaturing conditions. Sedimentation analyses under denaturing conditions yielded molecular weight estimates of 2.3 x 10(6) to 2.6 x 10(6), whereas length measurements yielded estimates of 5.2 x 10(6) to 5.6 x 10(6) for both denatured and nondenatured viral RNA. It would appear that the conditions of denaturation used (99% dimethyl sulfoxide at 26 C, and reaction with 1.1 M formaldehyde for 10 min at 60 C) do not equally denature parainfluenza virus RNA and other RNAs, such as cellular rRNA, 45S rRNA precursor, and R17 RNA.     

Hydrodynamic examination of the dimeric cytoplasmic domain of the human erythrocyte anion transporter, band 3.

Solution studies of the cytoplasmic domain (molecular mass approximately 40kDa) of band 3, the anion exchanger from human erythrocyte membranes, previously suggested a dimeric molecule on the basis of the relative techniques of calibrated gel filtration and calibrated preparative ultracentrifugation. This dimeric behavior is firmly established on an absolute basis by a combination of calibrated gel chromatography and absolute ultracentrifugation techniques. Sedimentation velocity in the analytical ultracentrifuge combined with calibrated gel chromatography give a molecular mass M of (77 +/- 4) kDa, a value confirmed by low-speed sedimentation equilibrium. Velocity sedimentation in the analytical ultracentrifuge gave a single sedimenting species with an s o 20,w of (3.74 +/- 0.07)S. Sedimentation equilibrium analysis was also used to establish the strength of the binding via the dissociation constant Kd, with a value from direct fitting of the concentration distribution curves of (2.8 +/- 0.5) microM, confirmed by a value of approximately 3 microM obtained from fitting a plot of molecular weight Mw,app versus cell loading concentration. Hydrodynamic calculations based on the classical translational frictional ratio showed that the protein was highly asymmetric, with an axial ratio of approximately 10:1, consistent with observations from electron microscopy.     

Sunset,skylight polarization and the migratory orientation of yellow-rumped warblers,Dendroica coronata

Celestial light cues visible at sunset appear to play an important role in the nocturnal orientation of several species of night-migrating birds. The pattern of skylight polarization, an especially prominent geographical reference at sunrise and sunset, influences the orientation behaviour of migratory birds. Yellow-rumped warblers were capable of seasonally appropriate cage orientation at dusk and were sensitive to manipulation of the axis of skylight polarization (E-vector). A series of experimental treatments was designed to examine the relationship between sunset position and skylight polarization. The window panels of hexagonal enclosures were fitted with a depolarizer and a polaroid filter to rotate the E-vector, and mirrors to reflect the position of sunset. The results indicate that this migrant minimizes sunset position as an orientation relative to skylight polarization and may depend upon the latter to orient at dusk. The possibility that yellow-rumped warblers calibrate their sun compass in relation to polarized light remains a question for future research.