Significant Effect Of Density Research Articles

Population size regulation is density-dependent in Rhodnius prolixus (Hemiptera: Reduviidae) through an irritability mechanism.

Physical factors can determine the level of triatomine abundance, but do not regulate their population densities, and neither do natural enemies. To identify the processes associated with density-dependent triatomine population regulation. We set-up a laboratory experiment with four interconnected boxes; the central box harbored Rhodnius prolixus bugs and one hamster. Stage 5 and adult densities of 10, 20, 30, 40, and 60 bugs per hamster, were replicated four times (except the density of 60 bugs). Hamster's irritability and several triatomine responses were measured: feeding, development time and longevity, mortality, fecundity, dispersal, and the net reproductive value (R o ). Density had a statistically significant effect on irritability, but not on the percent of bugs feeding. Density was significant on blood meal size ingested in bugs that did not move between boxes, but not significant when the bugs moved. Density and irritability affected the proportion of stage 5 nymphs molting, and the proportion of adult bugs dying per day and over a three-week period. There was a highly significant effect of density and irritability on R o . We showed that a density-dependent mechanism, acting through the irritability of the host, seems the most plausible process regulating populations in triatomines.

Intra- and Interspecific Competition Altered the Competitive Strategies of Alternanthera philoxeroides and Trifolium regens under Cadmium Contamination

Heavy metal accumulation in soils has been one of the environmental and ecological issues, as it caused life and biodiversity problems. However, many invasive plants can survive in heavy metal polluted areas, but little is known about the invasiveness while under different densities either with native species or themselves. In this study, a greenhouse experiment was performed to examine how cadmium contamination with different concentrations (0, 100, and 200 mg/kg) may influence the interspecific competition between invasive plant Alternanthera philoxeroides and the landscape grass T. regens, as well as the intraspecific competition of A. philoxeroides with different densities. The results showed that stronger interspecific competition would alleviate cadmium damage to both A. philoxeroides and T. regens, but the two species adopted different allocation strategies. A. philoxeroides allocated more biomass to belowground and less to aboveground, while T. regens showed exactly the opposite allocation strategy. There was a significant density effect of intraspecific competition on A. philoxeroides. That is to say, with the increase of A. philoxeroides density, the cadmium stress on the growth of A. philoxeroides decreased. Our findings provide a theoretical basis and technical support for the effective control of A. philoxeroides invasion, as well as the restoration and reconstruction of green vegetation.

The impact of gadolinium-based MR contrast on radiotherapy planning for oropharyngeal treatment on the MR Linac.

Gadolinium-based contrast agents (GBCAs) may add value to magnetic resonance (MR)-only radiotherapy (RT) workflows including on hybrid machines such as the MR Linac. The impact of GBCAs on RT dose distributions however have not been well studied. This work used retrospective GBCA-enhanced datasets to assess the dosimetric effect of GBCAs on head and neck plans. Ten patients with oropharyngeal squamous cell carcinoma receiving RT from November 2018 to April 2020 were included in this study. RT planning included contrast-enhanced computed tomography (CT) and MR scans. A contrast agent "contour" was defined by delineating GBCA-enhanced regions using an agreed window/level threshold, transferred to the planning CT and given a standardized electron density (ED) of 1.149 in the Monaco treatment planning system (Elekta AB). Four plans were per patient calculated and compared using two methods: (1) optimized without contrast (Plan A) then recalculated with ED (Plan B), and (2) optimized with contrast ED (Plan C) then without (Plan D). For target parameters minimum and maximum doses to 1cc of PTVs, D95 values, and percent dose differences were calculated. Dose differences for organs-at-risk (OARs) were calculated as a percentage of the clinical tolerance value. For the purposes of this study,±2% over the whole treatment course was considered to be a clinically acceptable dose deviation. Wilcoxon-signed rank tests were used to determine any dose differences within and between the two methods of optimization and recalculation (p<0.05). Pearson's correlations were used to establish the relationship between gadolinium uptake volume in a structure (i.e., proportion of structure covered by a density override) and the resulting dose difference. The median percent dose differences for key reportable dosimetric parameters between non-contrast and simulated contrast plans were<1.2% over all fractions over all patients for reportable target parameters (mean 0.34%, range 0.22%-1.02%). The percent dose differences for maximum dose to 1cc of both PTV1 and PTV2 were significantly different after application of density override (p<0.05) but only in method 2 (Plan C vs. Plan D). For D95 PTV1, there was a statistically significant effect of density override (p<0.01), however only in method 1 (Plan A vs. Plan B). There were no significant differences between calculation methods of the impact of contrast in most target parameters with the exception of D95 PTV1 (p<0.01) and for D95 PTV2 (p<0.05). The median percent dose differences for reportable OAR parameters as a percentage of clinical planning tolerances were<2.0% over a full treatment course (mean 0.65%, range 0.27%-1.62%). There were no significant differences in dose to OARs within or between methods for contrast impact assessment. Dose differences to targets and OARs in oropharyngeal cancer treatment due to the presence of GBCA were minimal, and this work suggests that prospective in vivo evaluations of impact may not be necessary in this clinical site. Accounting for GBCAs may not be needed in daily adaptive workflows on the MR Linac.

Density-dependence and seasonal variation in reproductive output and sporophyte production in the kelp, Ecklonia radiata.

The kelp, Ecklonia radiata, is an abundant subtidal ecosystem engineer in southern Australia. Density-dependent changes in the abiotic environment engineered by Ecklonia may feedback to affect reproduction and subsequent recruitment. Here, we examined: 1) how the reproductive capacity of Ecklonia individuals in the field (zoospores released · mm-2 reproductive tissue) varied with adult density and time, and 2) how the recruitment of microscopic gametophytes and sporophytes was influenced by zoospore density at two times. Zoospore production did not vary with adult density, with only one month out of ten sampled over a 2-y period showing a significant effect of density. However, zoospore production varied hugely over time, being generally highest in mid-autumn and lowest in mid-late summer. There were strong effects of initial zoospore density on gametophyte and sporophyte recruitment with both a minimum and an optimum zoospore density for sporophyte recruitment, but these varied in time. Almost no sporophytes developed when initial zoospore density was <6.5 · mm-2 in spring or <0.5 · mm-2 in winter with optimum densities of 90-355 · mm-2 in spring and 21-261 · mm-2 in winter, which resulted in relatively high recruitment of 4-7sporophytes · mm-2 . Sporophyte recruitment declined at initial zoospore densities >335 · mm-2 in spring and >261 · mm-2 in winter and was zero at very high zoospore densities. These findings suggest that although adult Ecklonia density does not affect per-capita zoospore production, because there is a minimum zoospore density for sporophyte production, a decline in population-level output could feedback to impact recruitment.

Neighbourhood effects on psychotic and depressive symptoms in the context of religious sectarianism in Northern Ireland: A data linkage study.

The incidence of psychotic disorders is higher in ethnic minorities groups. The 'ethnic density effect', in which living in a neighbourhood with a low own-group proportion increases the risk of psychosis, is one explanatory factor. The density effect in the ethno-religious and sectarian context of Northern Ireland has been found to be reversed, particularly for Catholics, in which there is harmful effect of high own-group density areas. This is partly explained by high urbanicity, deprivation and unemployment, but is otherwise not well understood. This study aimed to examine the density effect at the level of symptomology (positive and negative psychosis symptoms and depressive symptoms) in a representative sample of people with a first episode of psychosis in Northern Ireland. Data linkage methodology was used drawing on data from the Northern Ireland First Episode Psychosis Study (NIFEPS) and the 2001 Census of Northern Ireland. In total, 223 people between the ages of 18 to 64 were included in the study. A significant density effect was found for Catholics for total psychosis scores, but not for positive, negative and depressive symptoms, nor for general psychopathology, after adjusting for individual and area characteristics. The model accounted for just over 12% of the variance. No effect was found for Protestants. The findings suggest that the density effect for Catholics is unrelated to the core features of psychosis (hallucinations, delusions and anhedonia) but rather to broader cognitive and emotional disturbances and area deprivation. Explanations of exposure to social adversity and inequality are proposed, with implications for public mental health and social policy.

Influence of density on growth and survival of freshwater prawn Macrobrachium americanum (Bate, 1868) (Caridea: Palaemonidae) cultured in a cage-pond system

The domestication and culture of freshwater shrimp native of tropical and subtropical American Pacific zone requires the study and development of technologies in open systems where their response to types of farming, feeding and management strategies generate practical information for the production of this species. The aim of this study was to evaluate the effect of stocking density on growth and survival in M. americanum juvenile-adult, cage-cultured and to determine the optimal density for grow-out production. The caridean shrimp Macrobrachium americanum was cultured for 152 days in a 16 cage-culture (3 m3 each) at densities of 1, 3, 6 and 9 org / m3, respectively, with stocking sizes from 12.1 ± 1.7 to 13.5 ± 2.3 g. The prawns were fed twice daily with Camaronina 35%. Water quality parameters were within standard range for caridean shrimp culture. There was a significant effect of density on final weight, growth rate, specific growth rate, K condition and survival, and an inversely proportional relationship with the feed conversion ratio. The growth was affected by density, resulting to a maximum increase of 6 org / m3 in the asymmetry of the prawn and also increasing the number of small organisms by 9 org / m3. However, it is necessary to study such aspects as the rate and frequency of feeding, the initial size-grade and the implementation of shelter, among others.

Interpretation of small and intermediate strain characteristics of Hostun sand for various stress states

Small and intermediate strain properties of soils are key parameters to assess ground motion characteristics during an earthquake and other dynamic events. These properties are affected by various parameters. Among them, the effect of anisotropic stress state on the soil specimen is interest of investigation in this paper. Experiments were carried out using the modified resonant column device at Ruhr Universität Bochum on dry Hostun sand with relative density of 35%-95%. The results show the significant effect of density and anisotropic stress states on the small strain properties, Gmax, of Hostun sand. At intermediate strain level, the results show the significant effect of anisotropic stress state on the shear stiffness and damping ratio. In addition, it is concluded that the effect of compression stress component on the small and intermediate strain properties is more significant than the effect of the deviatoric stress component. At the end, the paper shows the successful application of stress-based approach to describe G(γ) and η(γ) in Hostun sand subjected to the isotropic and anisotropic stress states.

Suitable Planting Density for Chaiya Rice using Parachute Planting Method

The effect of planting density on growth and grain yield of Chaiya rice using parachute planting was examined at Pathum Thani Rice Research Center, from September 2014 to January 2015. The particular objective was to examine the suitable planting density when parachute planting was applied. Five densities of Chaiya rice seedlings were thrown similar to parachuting. Those densities were 22,400, 30,720, 38,400, 48,000 and 56,320 clumps per rai (1 rai = 0.16 ha). Pre-heading growth varied greatly among densities; this variation was partly related to the number of tillers, lower densities having more tillers per clump than higher densities, respectively. The relationship between tiller number and plant height was not similar. The heights of rice at densities of 22,400, 30,720, 38,400, 48,000 clumps per rai were not significantly different within the group but these were significantly different with the planting density of 56,320 clumps per rai.&#x0D; Low densities promote head development as the number of heads per clump was higher than those of high densities in most densities. No significant effect of density on filled-grain and unfilled-grain per head. Seed weight per clump showed significant difference between densities except between those of 38,400 and 48,000 clumps per rai. Grain yield varied among densities. Chaiya rice with 22,400 clumps per rai had the highest yield at 786.38 kg per rai. However, it was not significantly different from yields of rice at densities of 30,720 and 38,400, clumps per rai. However, the grain yield of rice planted at 48,000 and 56,320 clumps per rai was significantly lower than that of 22,400 clumps per rai. It is concluded that there was a consistent effect of plant density on growth and yield of Chaiya rice using parachute planting.

Ecological and evolutionary consequences of tri‐trophic interactions: Spatial variation and effects of plant density

The factors driving variation in species interactions are often unknown, and few studies have made a link between changes in interactions and the strength of selection. We report on spatial variation in functional responses by a seed predator (SP) and its parasitic wasps associated with the herb Ruellia nudiflora. We assessed the influence of plant density on consumer responses and determined whether density effects and spatial variation in functional responses altered natural selection by these consumers on the plant. We established common gardens at two sites in Yucatan, Mexico, and planted R. nudiflora at two densities in each garden. We recorded fruit output and SP and parasitoid attack; calculated relative fitness (seed number) under scenarios of three trophic levels (accounting for SP and parasitoid effects), two trophic levels (accounting for SP but not parasitoid effects), and one trophic level (no consumer effects); and compared selection strength on fruit number under these scenarios across sites and densities. There was spatial variation in SP recruitment, whereby the SP functional response was negatively density-dependent at one site but density-independent at the other; parasitoid responses were density-independent and invariant across sites. Site variation in SP attack led, in turn, to differences in SP selection on fruit output, and parasitoids did not alter SP selection. There were no significant effects of density at either site. Our results provide a link between consumer functional responses and consumer selection on plants, which deepens our understanding of geographic variation in the evolutionary outcomes of multitrophic interactions.

Tree species mixing can increase maximum stand density

Mixed-species stands are on the advance in Europe. They fulfil many functions better than monocultures. Recent papers show that mixed stands can have higher yields, but it remains open whether mixed stands simply grow faster along the same self-thinning lines as pure stands or have higher maximum stand densities. We analyzed the effect of species mixing on maximum density based on triplets of pure and mixed stands at approximately maximum density. Most considered mixtures include Norway spruce (Picea abies (L.) H. Karst.). We show that (i) in mixed stands, maximum density is, on average, 16.5% higher than in neighbouring pure stands, and (ii) species mixtures with Norway spruce exceed densities of pure stands by 8.8%, on average. For individual species mixtures, we find a significant density effect of +29.1% for Norway spruce mixed with European larch (Larix decidua Mill.) and +35.9% for Scots pine (Pinus sylvestris L.) in association with European beech (Fagus sylvatica L.). No significant links with stand variables such as age and mean tree size and site fertility were found. The results indicate that species mixing substantially increases stand density, indicating a higher carrying capacity caused by a higher supply and use efficiency of resources. The implications for inventory, silviculture, and forest modelling are discussed.

Copulation order, density cues and variance in fertilization success in a cestode

Simultaneous hermaphrodites maximize their fitness by optimizing their investment into male or female functions. Allocation of resources to male function (tissues, traits, and/or behaviours increasing paternity) is predicted to increase as density, and the associated level of sperm competition, increases. We tested whether the simultaneous hermaphroditic cestode Schistocephalus solidus uses cues of potential partner densities in its fish intermediate host to improve its male reproductive success in the final host. We had two worms, one originating from a multiple infection in the fish intermediate host and one from a single infection, sequentially compete to fertilize the eggs of a third worm. The fertilization rates of the two competitors nearly always differed from the 50-50 null expectation, sometimes considerably, implying there was a 'winner' in each experimental competition. However, we did not find a significant effect of density in the fish host (single vs multiple) or mating order on paternity. Additional work will be needed to identify the traits and environmental conditions that explain the high variance in male reproductive success observed in this experiment.

Testing for enemy‐mediated density‐dependence in the mortality of seedlings: field experiments with five Neotropical tree species

The coexistence of plant species in species‐rich tropical forests can be promoted by specialised enemies acting in a negatively density‐dependent manner. While survival of tropical tree seedlings is often negatively density‐dependent, the causes have rarely been identified. We tested whether insects and plant pathogens cause density‐dependent seedling recruitment and survival in five forest tree species in Belize, Central America. We manipulated densities of seeds or newly germinated seedlings in small (1 m2 or 0.25 m2) plots close to fruiting conspecific trees. Using a factorial design, we excluded enemies from subsets of the plots with fungicides and insecticides. Seed germination (for two species) and early seedling survival (for all species) were monitored at approximately weekly intervals for up to eight weeks, during the period when plants are likely to be most susceptible to natural enemies. In Terminalia amazonia, seed germination was negatively density‐dependent and the proportion of seeds germinating increased when insects were excluded. However, the magnitude of the insecticide effect was independent of density. The only significant density effect for survival of young seedlings was in Acacia polyphylla; counter to expectation, seedling survival was higher at high densities. In a few cases pesticide application had a significant effect on seedling survival, but in only one case (Terminalia amazonia) was a significant pesticide × density interaction detected. Our results caution against generalising from studies conducted on a single species at a single time and place and illustrate the challenges of experimentally testing for enemy‐mediated negative density‐dependence. Experimental outcomes are likely to depend on the spatial scale at which the principal enemies disperse and respond to plant density, and the timescales over which they act. Gathering information on these variables will improve our understanding of the natural histories of tropical forest species and help inform the design of future experiments.

Effect of density, cultivar and irrigation on spring sown monocrops and intercrops of wheat (Triticum aestivum L.) and faba beans (Vicia faba L.)

Spring-sown intercrops of wheat (Triticum aestivum L.) and faba beans (Vicia faba L.) were grown in three experiments at the University of Reading, UK. One wheat cultivar, Axona, and one (experiment 1) or two bean cultivars (experiments 2 and 3) Scirocco and Maris Bead, were grown as sole crops and intercrops at 50%, 75% and 100% recommended density. Experiments were rainfed but irrigation was an additional treatment in experiment 3. Biomass and seed yields of both wheat and faba beans were greater when monocropped than when intercropped. There was no evidence that radiation use efficiency (RUE) of intercrops was significantly different from sole crops. In all intercrops there was no significant effect of density on biomass, RUE or seed yield, though there were compensating changes in yield components. Seed yields of Maris Bead were significantly greater than Scirocco in experiment 3 but not experiment 2. There was no significant effect of irrigation on RUE or on wheat biomass and seed yield, but there was a trend for irrigation to increase faba bean biomass (P=0.07) and seed yield (P=0.06). With later sowing in experiments 2 and 3, time to harvest was shorter and wheat and bean biomass, seed yield and RUE were reduced. All land equivalent ratio (LER) values for both biomass (maximum 1.23) and seed yield (maximum 1.44) were greater than 1, with one exception in experiment 3, indicating that intercrops of wheat and faba beans make more effective use of land than equivalent sole crops. Partial LERs for faba beans were always lower than those of wheat. The tendency was for highest LERs to occur at 75% recommended density.

Size‐symmetric competition in a shade‐tolerant invasive plant

AbstractPlant responses to crowding have been investigated extensively in stands of light‐demanding species, but shade‐tolerant species may react differently. In the present study, we investigated the effect of density on the mortality, size inequality, and biomass allocation of Alternanthera philoxeroides, a shade‐tolerant invasive species. Stem fragments of A. philoxeroides were grown at either low or high densities (6 vs. 24 plants per pot) under three light levels (10%, 34%, and 100% full sun). After 8 weeks, survival was 31% lower in pots with a higher initial density. Both high density and low light levels reduced plant size substantially. Mean plant biomass ranged from 0.23 g in high‐density and low‐light pots to 4.41 g in low‐density and high‐light pots. There were no strong or significant effects of density or light level on size inequality of survivors. Most of the variation in allocation and morphology in response to light level and crowding were due to plant size and allometric growth, with little evidence of true plasticity. There was a small but significant increase in shoot allocation, in the direction predicted by optimal allocation theory, at low light levels. Our results show that intense competition need not be size asymmetric, and suggest that tolerance to low light levels involves a reduction in phenotypic plasticity. Responses of the invasive A. philoxeroides to crowding may be an example of an invasive plant's success in establishing dense stands of closely related individuals that are shade tolerant, cooperative, and follow a relatively fixed allometric trajectory with low plasticity.

Social cues in the expression of sequential alternative reproductive tactics in young males of the peacock blenny, Salaria pavo

Phenotypic change in response to variation in environmental cues has been widely documented in fish. Transitions in social dominance, in particular, have been shown to induce a rapid switch in reproductive phenotypes in many species. However, this effect has been mainly studied in adults and focused on behavioural transitions. The way social cues constraint the phenotypic development of juveniles remains poorly studied in fish. We tested the importance of social dominance and density in the phenotypic development of juveniles of the peacock blenny Salaria pavo. This species shows sequential male alternative reproductive tactics. In the first breeding season males can reproduce as nest-holders or as parasitic males (female-mimicking), or postpone reproduction; from the following season afterwards all males reproduce as nest-holders. Parasitic males have relatively larger testes that lack a testicular gland, present in the testes of nest-holders. The testicular gland is the main source of androgens in the testes and accordingly nest-holders have higher circulating androgen levels. In addition, exogenous androgen administration to parasitic males promotes the development of secondary sexual characters (SSC) only present in nest-holders such as a head crest and an anal gland. We raised juveniles under a high or low-density treatment and monitored social interactions for 1month. No significant effect of density on the development of juvenile males was detected. However, within each replicate, the relative body size of juvenile males at the beginning of the experiment determined their dominance status, with dominant males developing towards the nest-holder morphotype. Dominant males engaged in more nest defence behaviour, showed larger testicular glands, had higher levels of 11-ketotestosterone (11-KT) and testosterone (T) and developed more SSC, as compared to subordinate males. However, these effects of social dominance were moderated by body condition as only dominant males in good body condition developed SSC. The effect of social dominance and of the area of the testicular gland on the development of SSC was mediated by 11-KT and on the expression of nest defence behaviour by T. Interestingly, in spite of the higher androgen levels and more pronounced morphologic development of SSC in dominant individuals, gonadal development was independent of social dominance and most fish still had underdeveloped testis at the end of the experiment. In conclusion, social dominance promoted the development of the testicular gland, an increase in circulating androgen levels and the development of SSC, but did not promote testicular development. This suggests a dissociation of mechanisms underlying sexual maturation and the expression of male reproductive traits. This dissociation seems to be the key for the occurrence of female-mimicking males in this species, which are sexually mature despite lacking the SSC typical of nest-holders.

Evolutionary consequences of habitat fragmentation: population size and density affect selection on inflorescence size in a perennial herb

Habitat fragmentation is considered to be one of the major threats to biological diversity worldwide. To date, however, its consequences have mainly been studied in an ecological context, while little is known about its effects on evolutionary processes. In this study we examined whether habitat fragmentation affects selection on plant phenotypic traits via changes in plant-pollinator interactions, using the self-incompatible perennial herb Phyteuma spicatum. Specifically, we hypothesized that limited pollination service in small or low-density populations leads to increased selection for traits that attract pollinators. We recorded mean seed production per capsule and per plant as a measure of pollination intensity and assessed selection gradients (i.e., trait-fitness relationships) in 16 natural populations of varying size and density over 2 years. Mean seed production was not related to population size or density, except for a marginal significant effect of density on the mean number of seeds per capsule in 1 year. Linear selection for flowering time and synchrony was consistent across populations; relative fitness was higher in earlier flowering plants and in plants flowering synchronously with others. Selection on inflorescence size, however, varied among populations, and linear selection gradients for inflorescence size were negatively related to plant population size and density in 1 year. Selection for increased inflorescence size decreased with increasing population size and density. Contrary to our expectation this appeared not to be related to changes in pollination intensity (mean seed production was not related to population size or density in this year), but was rather likely linked to differences in some other component of the abiotic or biotic environment. In summary, our results show that habitat fragmentation may influence selection on plant phenotypic traits, thereby highlighting potential evolutionary consequences of human-induced environmental change.

The Effect of Organism Density on Bioaccumulation of Contaminants from Sediment in Three Aquatic Test Species: A Case for Standardizing to Sediment Organic Carbon

Laboratory methods for measuring bioaccumulation of organic contaminants from sediment into aquatic organisms continue to improve, but some aspects are still in need of standardization. From a review of published methods, we noted that the loading density of organisms was determined inconsistently and was primarily based on either sediment volume or total organic carbon (TOC). The rationale mainly expressed for standardizing to TOC was to minimize the depletion of sediment contaminants. However, even when density was standardized to TOC, the relative amount of TOC provided (i.e., ratio of TOC to organism dry weight [dw]) was highly variable. In this study, we examined the effect of organism density (standardized to sediment TOC or volume) on bioaccumulation in three freshwater organisms. The oligochaete Lumbriculus variegatus, mayfly nymph Hexagenia spp., and fathead minnow Pimephales promelas were exposed for 28days to two field-contaminated sediments that varied in concentration of PCBs and TOC. Densities tested were 50:1 and 27:1 ratios of TOC to organism dw and 140ml sediment/g wet weight (ww) biomass, yielding low to high organism densities. Bioaccumulation in Hexagenia spp. was significantly higher at the lowest organism density compared with the highest organism density when exposed to site 2 sediment (1.1% TOC) but only with tissue concentrations expressed on a ww basis. Otherwise, there was no significant effect of density on bioaccumulation in organisms exposed to sediments from site 1 (12% TOC) or site 2. Survival of Hexagenia spp. was adversely affected at the highest organism density when the relative amount of TOC was low. The results of this study support the recommendation of standardizing organism density relative to a particular amount of TOC for invertebrate species. A 27:1 ratio of TOC:organism dw was selected as a standard organism density for a new bioaccumulation method because survival, growth, and bioaccumulation were not impacted relative to a 50:1 ratio, and less sediment was required. This density is recommended as an appropriate ratio for sediment bioaccumulation assessments in general.

Stocking density and sex influence individual growth of Senegalese sole ( Solea senegalensis)

Growth is usually inversely correlated with stocking density of fish in culture. Senegalese sole aquaculture is affected by a high size variability and thus, this work tried to investigate the relationship of growth with density of two populations of 96 individually tagged Senegalese sole (318.7 ± 7.9 g; mean ± standard error of the mean). Fish were reared at low (LD) and high (HD) density (60% and 180% of bottom coverage respectively) for 195 days. After 134 days (period 1), density conditions were exchanged between groups. Mean weight, standard length, maximum width and centroid size were calculated for each of the 11 census days of the experiment. White muscle biopsies were taken in 7 of the census days in order to assess the RNA/DNA ratio, as a biochemical indicator of growth. Stocking density had an important effect on growth, as fish reared under HD showed poor or no growth during a ‘lag phase’ on the first 61 days of the experiment, leading to a significantly lower specific growth rate (0.23 ± 0.014) for period 1 compared with LD fish (0.34 ± 0.016). Fitting of linear mixed-effects (LME) models for the first 134 days of experiment showed a significant effect of density and sex on all the assessed biometric parameters. These results could be attributed mainly to the first 61 days of the experiment, as no differences were observed between days 61 and 134 in all the measurements, except for standard length, that showed to be lower for HD fish throughout the whole period. Fish reared under high density tended to grow slower than fish held at low density, while females showed faster growth than males, particularly in HD. Nevertheless, due to high size variability, no significant differences could be found in the mean values of weight or standard length after 134 days (467.2 ± 21.6 g and 28.7 ± 4.2 cm; 502.6 ± 22.5 g and 29.7 ± 4.5 cm for HD and LD fish respectively). Size variability could be an indicator of the onset of hierarchies, being stronger and with more females as dominant individuals than males in HD. After exchanging densities, and up to day 195, a similar lagging effect could be observed in LD fish exposed to high density, suggesting that a sudden change in density, more than density itself, could be the responsible for a detrimental effect on growth. RNA/DNA ratios, were significantly lower for HD fish between days 20 and 61.

Pulse radiolysis of sodium formate aqueous solution up to 400 °C: Absorption spectra, kinetics and yield of carboxyl radical CO 2[rad

Sodium formate neutral aqueous solutions saturated with N 2O were studied by pulse radiolysis techniques from room temperature to 400 °C. The absorption spectra of CO 2 − show a red shift from λ max=235 nm at 25 °C to 280 nm at 400 °C. The second-order decay of CO 2 − is not monotonically temperature-dependent: the rate constant (2 k/ ε) slightly increases with temperature up to 200 °C, then decreases to a valley at 300 °C, and it sharply increases again for temperature >350 °C. Gε(CO 2 − ) increases with temperature from room temperature to 400 °C. Significant density effect on Gε(CO 2 − ) is found at 400 °C. Assumed ε(CO 2 − ) is independent on temperature, at 400 °C, the density dependence of G(CO 2 − ) in the presence of N 2O agrees well with that of { G(e aq −)+ G(H)+ G(OH)}.

Paedomorphosis in Ambystoma talpoideum: effects of initial body size variation and density

Facultative paedomorphosis is the ability of a salamander to either metamorphose into a terrestrial, metamorphic adult or retain a larval morphology to become a sexually mature paedomorphic adult. It has been hypothesized that density and initial body size variation within populations are instrumental in cueing metamorphosis or paedomorphosis in salamanders, yet few studies have adequately tested these hypotheses in long-term experiments. Beginning in the spring of 2004, 36 experimental ponds were used to manipulate three body size variation levels (low, medium, high) and two density levels (low, high) of Ambystoma talpoideum larvae. Larvae were individually marked using visible implant elastomers and collected every 2 weeks in order to measure snout-vent length and mass. Bi-nightly sampling was used to collect new metamorphs as they appeared. Analysis revealed significant effects of density, size variation and morph on body size of individuals during the summer. Individuals that metamorphosed during the fall and following spring were significantly larger as larvae than those becoming paedomorphic across all treatments. These results support the Best-of-a-Bad-Lot hypothesis, which proposes that the largest larvae metamorphose in order to escape unfavorable aquatic habitats. Large larvae may metamorphose to leave aquatic habitats, regardless of treatment, due to the colder climate and lower productivity found in Kentucky, which is in the northern-most part of A. talpoideum's range. By maintaining a long-term experiment, we have provided evidence for the transition of both larvae and paedomorphs into metamorphs during fall and spring metamorphosis events. Furthermore, the production of similar morphs under different environmental conditions observed in this research suggests that the ecological mechanisms maintaining polyphenisms may be more diverse that first suspected.