Cork Ring Research Articles

Climatic drivers of cork growth depend on site aridity

Cork is one of the main non-timber forest products in the world. Most of its production is concentrated in the Iberian Peninsula, a climate change hotspot. Climate warming may lead to increased aridification and reduce cork production in that region. However, we still lack assessments of climate-cork relationships across ample geographical and climatic gradients explicitly considering site aridity. We quantified cork growth by measuring cork ring width and related it to climate variables and a drought index using dendrochronology. Four cork oak (Quercus suber) forests located from north eastern Spain to south western Morocco (31.5–41.5° N) and subjected to different aridity levels were sampled. Warm conditions in spring to early summer, when cork is formed, reduced cork width, whereas high precipitation in winter and spring enhanced it. The response of cork to increased water availability in summer peaked (r = 0.89, p = 0.00002) in the most arid and continental site considering 14-month long droughts. A severe drought caused a disproportionate loss of cork production in this site, where for every five-fold decrease in the drought index, the cork-width index declined by a factor of thirteen. Therefore, site aridity determines the responses of cork growth to the soil water availability resulting from accumulated precipitation during winter and spring previous to cork growth and until summer. In general, this cumulative water balance, which is very dependent on temperature and evapotranspiration rate, is critical for cork production, especially in continental, dry sites. The precipitation during the hydrological year can be used as a proxy of cork production in similar sites. Assessments of climate-cork relationships in the western Mediterranean basin could be used as analogues to forecast the impacts of aridification on future cork production.

Beyond width and density: stable carbon and oxygen isotopes in cork-rings provide insights of physiological responses to water stress in Quercus suber L.

As climate change increasingly affects forest ecosystems, detailed understanding of major effects is important to anticipate their consequences under future climate scenarios. The Mediterranean region is a prominent climate change hotspot, and evergreen cork oak (Quercus suber L.) woodlands are particularly climatically sensitive due to cork (bark) harvesting. Cork oak's drought avoidance strategy is well-known and includes structural and physiological adaptations that maximise soil water uptake and transport and limit water use, potentially leading to reduced stem and cork growth. Trees' responses to cope with water-limited conditions have been extensively described based on cork-rings width and, more recently, on cork-rings density, in dendroecological studies. However, so far, tree functional attributes and physiological strategies, namely photosynthetic metabolism adjustments affecting cork formation, have never been addressed and/or integrated on these previous cork-rings-based studies. In this study, we address the relation between carbon and oxygen stable isotopes of cork rings and precipitation and temperature, in two distinct locations of southwestern Portugal-the (wetter) Tagus basin peneplain and the (drier) Grândola mountains. We aimed at assessing whether the two climatic factors affect cork-ring isotopic composition under contrasting conditions of water availability, and, therefore, if carbon and oxygen signatures in cork can reflect tree functional (physiological and structural) responses to stressful conditions, which might be aggravated by climate change. Our results indicate differences between the study areas. At the drier site, the stronger statistically significant negative cork δ 13C correlations were found with mean temperature, whereas strong positive cork δ 18O correlations were fewer and found only with precipitation. Moreover, at the wetter site, cork rings are enriched in 18O and depleted in 13C, indicating, respectively, shallow groundwater as the water source for physiological processes related with biosynthesis of non-photosynthetic secondary tissues, such as suberin, and a weak stomatal regulation under high water availability, consistent with non-existent water availability constrains. In contrast, at the drier site, trees use water from deeper ground layers, depleted in 18O, and strongly regulate stomatal conductance under water stress, thus reducing photosynthetic carbon uptake and probably relying on stored carbon reserves for cork ring formation. These results suggest that although stable isotopes signatures in cork rings are not proxies for net growth, they may be (fairly) robust indicators of trees' physiological and structural adjustments to climate and environmental changes in Mediterranean environments.

Determination of the cumulative and skin-resorptive action of the Zoodizin disinfectant

The rational organization and implementation of effective disinfection activities plays an important role in the complex of measures for the prevention of infections. The development and introduction of new disinfectants into production is an urgent issue of modern poultry farming. When developing a disinfectant, it is important to determine the cumulative effect of the drug. The purpose of the study was to determine the cumulative and skin-resorptive action of the disinfectant “Zoоdizin”. For the toxicological study of the drug used healthy white male rats and white female rats weighing 200 ± 10 g 1.5 years of age. To study the toxicity of the drug “Zoоdizin” when applied to the skin used the method of immersion of the tails of rats in a test tube with the test substance. The tail was injected 2/3 into a regular tube with a 5 % solution of the drug “Zoоdizin”. The tube was closed with a cork ring whose diameter was slightly larger than the tail diameter. For 15 days, the tubes were placed daily in a water bath at 28–30 °C for 2 hours. Control animals tails were immersed in distilled water. To establish the local action of the drug “Zoodizin” on the mucous membranes of the study drug was introduced into the conjunctival sac of the right eye of the rabbit at a dose of 50 mg, and in the left eye was buried saline in a volume of 0,05 cm3. When studying the cumulative effect of Zoоdizin, no significant changes in the biochemical parameters in the serum of rats were observed. In the study of possible irritant or damaging effect on the skin and the development of contact non-allergic dermatitis found that a single application of disinfectant “Zoоdizin” on the unaffected skin of the back of white rats in the maximum significant recommended concentration of working solutions (2 %) did not cause signs. The single effect of the drug on the intact areas of the skin did not cause skin irritation, but it can be stated that prolonged daily epicutaneous exposure of high concentration (5 %) of the solution of the drug “Zoоdizin”, which is 2.5 times higher than the maximum recommended concentration, caused a general resorption. When assessing the cumulative properties, it was taken into account that the total dose administered to rats was Zodizin 42000 mg/kg body weight and did not result in animal death. It did not allow to calculate the cumulative coefficients for the “lethal effect”. A single effect of the product on the intact areas of the skin did not cause skin irritation, but it can be stated that prolonged daily epicutaneous exposure of a high concentration (5 %) of the Zodizin solution, which is 2.5 times the maximum recommended concentration, caused a general resorption. In the future, it is planned to study the virulidal properties of the biocide “Zoоdizin”.

Drivers for Annual Cork Growth under Two Understory Management Alternatives on a Podzolic Cork Oak Stand

Understory management practices and stand density characteristics allow one to distinguish a cork oak traditional silvopastoral system (known as a montado) from a cork oak forest system. Although understanding the manner in which different management practices affect cork growth is imperative, there are still only a few outputs from experimental research that contribute to this knowledge. The effect of potential drivers on annual cork growth was analyzed using a linear mixed model approach. Two dimensions of drivers were considered: intraspecific competition, assessed by tree level distance-dependent indices; and interspecific competition, assessed by variables characterizing understory management. The present dataset was collected from an experimental trial established on a cork oak stand in Podzolic soil on the Tagus river basin, covering two different cork growth cycles over the period from 2003 to 2015. The adjusted models considered two understory management alternatives: spontaneous shrubs maintenance and forage application. In both models, annual precipitation displayed a positive effect on annual cork growth, as expected. However, no significant effect of intraspecific competition was found. Additionally, there was a positive effect on annual cork growth associated with the spontaneous shrubs growth and a negative effect associated with lupine presence; both effects linked to different cork ring ages’ thresholds. The study main contributions are the following: (i) the introduction of the interaction between cork growth cycle stage and understory management practices, only possible with cork sample collections from different cork rotation cycles; (ii) the finding that there was no significant effect of intraspecific competition on cork growth.

Cork rings suggest how to manage Quercus suber to mitigate the effects of climate changes

Climate scenarios in the Mediterranean region predicts raising temperatures and more frequent and extreme drought conditions. Cork oak is a Mediterranean species with a large distribution in Portugal from which cork is extracted in a sustainable way and mainly used as the raw material for cork stoppers and insulating materials. To study the response of cork oak to drought and the effect of phellogen age on that response we examined cork growth from a 30-year chronology of trees from 12 sites in the main Portuguese cork oak production area. For the first time in cork, a components resilience study was performed. The research confirmed that drought reduces cork growth and provided extra knowledge on the responses of cork oak to drought: more severe droughts correspond to higher decrease of cork growth and more trees affected but to greater recovery performance. Moreover, cork oak is very tolerant and resilient to extreme droughts. Nevertheless, there are other factors that affect cork growth during and after drought, namely site, tree and the age of the phellogen. In fact, in the first 2 years and in the last 2 years of the production cycle the effects of drought on growth are more pronounced than in the middle of the cycle. The age of the phellogen is significant in the recovery, resistance and resilience but not in the relative resilience. The most noticeable differences occurred in the recovery for phellogen under 3 years (17% lower than that for phellogen with 3 to 6 years of age). Moreover, under drought conditions, there is a strong evidence that forest managers should enlarge debarking rotations, namely if drought occurs in the first 2 years of the production cycle and/or establish new cork oak stands in more humid areas, namely, in higher latitudes than the actual species distribution area.

Understory effect on tree and cork growth in cork oak woodlands

Aim of study: Cork oak is one of the main forest tree species in Portugal that typically occurs in montado, where operational practices oriented to the tree, crop or animal management may influence several of the ecosystem components. This study aimed at contributing to fulfil the a lack of knowledge on the effect of these practices on the cork and wood growth, by comparing the wood diameter growth and the annual cork increment under two different understory management options.Material and methods: An experimental trial implemented on an uneven-aged cork oak pure stand during a cork rotation period of 9 years, was established with the specific goal of comparing understory management options: a yellow lupine pasture versus spontaneous vegetation. Cork samples were taken at the beginning and end of the period and were used to measure cork thickness and annual cork rings. The differences between treatments were assessed performing a non-parametric test and a more robust approach using linear mixed model. Precipitation and treatment levels were jointly considered on the analysis.Main results: A slight effect was found on the cork thickness regarding the treatment with lupine application. However, no distinct effect was found, regarding wood and the annual cork increment pattern. Additionally, annual cork ring width showed a positive correlation with precipitation and a negative correlation with ring age.Research highlights: The results of this study indicate no distinct pattern regarding the annual cork and wood increment when comparing the understory effect of yellow lupine pasture versus spontaneous vegetation.

Climate Signal in Cork-Ring Chronologies: Case Studies in Southwestern Portugal and Northwestern Algeria

In temperate regions, the analysis of climate signals encoded in the wood rings allows assessing tree sensitivity to climate and its potential effects on growth and yield. In Mediterranean regions, the cork oak (Quercus suber L.) has received limited attention for dendrochronological studies because tree rings are faint and cork rings with a clear annual banding have been rather neglected. We analyzed the climatic signal of cork-ring chronologies for AD 1996–2010 from distinct regions in western Algeria [a mountain oak forest called Hafir-Zarieffet] and in southern Portugal [a peneplain wood-pasture called Benavente]. The goal was to evaluate the strength and consistency of climate signal and to assess cork growth sensitivity to climate variables (precipitation and temperature). Our results suggest that cork growth encodes a climatic signal. Trends of cork growth, via correlation analysis, including climate variables (on a monthly and seasonal basis) and Lang's index (on an annual basis) were successfully assessed. Drought-driven cork growth reduction is a threshold function of a P-T ratio, and with expected increase in the drought occurrence under changing climate, cork growth is likely to be similarly affected in both regions.

A comparative transcriptomic approach to understanding the formation of cork

The transcriptome comparison of two oak species reveals possible candidates accounting for the exceptionally thick and pure cork oak phellem, such as those involved in secondary metabolism and phellogen activity. Cork oak, Quercus suber, differs from other Mediterranean oaks such as holm oak (Quercus ilex) by the thickness and organization of the external bark. While holm oak outer bark contains sequential periderms interspersed with dead secondary phloem (rhytidome), the cork oak outer bark only contains thick layers of phellem (cork rings) that accumulate until reaching a thickness that allows industrial uses. Here we compare the cork oak outer bark transcriptome with that of holm oak. Both transcriptomes present similitudes in their complexity, but whereas cork oak external bark is enriched with upregulated genes related to suberin, which is the main polymer responsible for the protective function of periderm, the upregulated categories of holm oak are enriched in abiotic stress and chromatin assembly. Concomitantly with the upregulation of suberin-related genes, there is also induction of regulatory and meristematic genes, whose predicted activities agree with the increased number of phellem layers found in the cork oak sample. Further transcript profiling among different cork oak tissues and conditions suggests that cork and wood share many regulatory mechanisms, probably reflecting similar ontogeny. Moreover, the analysis of transcripts accumulation during the cork growth season showed that most regulatory genes are upregulated early in the season when the cork cambium becomes active. Altogether our work provides the first transcriptome comparison between cork oak and holm oak outer bark, which unveils new regulatory candidate genes of phellem development.

The influence of season on carbon allocation to suberin and other stem components of cork oak saplings.

The growth pattern of cork oak (Quercus suber L.), an important component of South Mediterranean woodlands, is seasonal. Seasonality has been found for shoot, radial and cork ring growth as well as for carbon (C) photoassimilation, nutrients remobilization and water relations, among other physiological aspects. However, little is known about the seasonality of C allocation to cork oak chemical compounds, including suberin, a major component of cork. In order to achieve this goal, an isotopic tracer experiment was conducted using 18-month-old cork oaks so that the fate of C photoassimilated in different seasons could be traced into biochemical (main organic) stem components. Two distinct patterns of C allocation, associated with the stages of active plant growth and dormancy, were identified and described. Evidence was provided that translocation of photoassimilated C to stems does not cease during the dormancy period and that suberin is the major C sink for the C assimilated throughout the whole active growth period, as compared with other stem components.

Climate response of cork growth in the Mediterranean oak (Quercus suber L.) woodlands of southwestern Portugal

In the Mediterranean climate regions, drought events are expected to affect the growth of forests ecosystems by changing trees growth rates and eventually inducing shifts in their growth patterns. Cork oak (Quercus suber L.) is a strictly western Mediterranean tree species periodically harvested for its bark, the cork. So far, cork oak has received limited attention for dendroclimatological studies due to its typical faint and erratic tree wood rings. Moreover, its distinct cork rings chronologies have been completely neglected. In this study we introduce an approach using cork ring chronologies dated back 9–10 years for climate response. Despite enhancing interannual variability and increasing statistical response to short-term climatic variability, still poorly understood, this study will possibly allow infer long-term climate response. We analyzed the cork ring chronologies of 55 cork samples collected in mature (under exploitation) trees in three distinct locations in southwestern Portugal. Cork growth recorded a high climate signal, with highly significant and coherent responses to the yearly climate-related sources of variation. We successfully assessed trends of cork growth via correlation analysis including selected climate variables among mean monthly temperature, monthly precipitation and, on an annual basis, eight precipitation indices. The high mean sensitivities and inter-series correlations found for cork ring chronologies combined with the significant variance explained by climate variables suggest that climate is likely one dominant signal that affects cork growth, but local environmental stresses can decisively affect this (climate) signal. Assuming cork growth as a proxy for cork oak growth, it seems conceivable that despite the trees being highly resistant to drought stress, cork oak woodlands in southwestern Portugal would have to face lesser growth in a global warming scenario.

Predicting mature cork biomass with t years of growth from one measurement taken at any other age

Cork is a natural product that is extracted from the outer bark of the cork oak tree. According to Portuguese legislation, the interval between two consecutive cork extractions on the same tree must be equal or greater than 9 years. Although the majority of the cork oak stands are debarked at the end of this period, this rotation may not be the optimum in many cases. The existing models for cork weight prediction can only be used for trees debarked at a 9 years or, in one model, at a 10-years rotation period, since the data used for its development was limited to these growth periods. The development of a method that allows for the prediction of the mature cork biomass with t years of growth, based in one measurement taken at any other age, was the main objective of this work. The method is based on the knowledge that the density of the cork tissue is constant between the inner and outer cork rings, being significantly different from density of the cork back. It can be implemented using two different equations that were developed during this work: a model to estimate cork biomass with 9 years of age and a model to estimate the cork back weight proportion at 9 years of age. For the first model, four different alternative models were developed, considering different levels of information collected during forest inventory. The model to estimate the cork back weight proportion leads to the biomass of cork tissue. Cork biomass at t years is obtained by decreasing or increasing the biomass of cork tissue according to the difference in cork thickness between t and 9 years of growth. The proposed method was evaluated by comparing the observed and estimated values of cork biomass from an independent data set with corks with 9, 10 and 11 years of age. The results showed similar precision for corks with 9, 10 or 11 years of age. As expected the precision of the predictions increases when the model to estimate cork biomass with 9 years of age uses more information. The presented method should be an important tool for cork oak stand management, for the prediction of the evolution of carbon stocks in cork oak stands, and will allow analyzing the impact in cork biomass production of decreasing or extending the interval between two consecutive cork extractions.

A new method for measurement of annual growth rings in cork by means of autofluorescence

A new method for measuring annual growth rings in cork has been developed. It is based on the fact that excitation of Quercus suber L. cork cross-sectional planes with UV light and also with blue light results in enhanced autofluorescence at the growth ring borders. This distinct autofluorescence band is located in the cork produced at the end of the vegetation period, with its smaller cells and thicker cell walls. The enhanced autofluorescence of polyphenolics mainly results from a very thick suberin layer in the walls of the small late cork cells. The gradient in autofluorescence from late cork to first early cork is steep. The best visibility of cork annual rings was found under the epifluorescence microscope in 60 μm thick microtome cork cross sections. For fast screening of high sample numbers scanning the blue-excited (473 nm) fluorescence of water wetted polished cross-sectional surfaces of cork pieces with a laser-equipped fluorescence image analyzer was most suitable. Evaluation of visibility of cork rings showed a clear improvement in comparison with reflected light image analysis; thus data obtained with this new autofluorescence scanning method are excellent for modeling the yearly growth increment of cork.

Influenza del clima sulla crescita del sughero in soprassuoli produttivi siciliani

Cork oak (Quercus suber L.) is usually dominant in silvopastoral systems in many areas of Sicily, where the trees are debarked periodically for cork production. In spite of the importance of cork and cork oak stands in Sicilian forests and the potential economic scenarios, few research works have been carried out on these systems. Given the importance of cork thickness in cork quality evaluation, the main objective of this work is to study cork growth in cork oak productive stands spread on the north (Nebrodi Mountains) and south-east (Iblei Mountains) of Sicily. Image analysis techniques were used on cork surfaces of transverse sections of planks to measure cork rings. Dendrochronological analysis was applied to study annual fluctuation on rings growth in relation to various climate parameters in a cork cycle production. Results showed that rainfall, summer drought and temperature are determining factors in controlling cork growth. In siliceous areas of Nebrodi Mountains correlation between cork growth index and rainfall indicates that the rain period from May to September strongly influences phellogen activity. Temperature and water stress indices, on the other hand, show a negative correlation with cork growth. In clay and evolved soils of volcanic plateau of Iblei Mountains January precipitation shows a positive correlation with cork growth index. Also absolute minimum temperature in June and absolute maximum temperature in September show a positive correlation when temperature possibly has influence on phellogen activity during growing season.

The effect of cork-stripping damage on diameter growth of Quercus suber L.

Summary The Mediterranean cork oak (Quercus suber L.) agro-forestry system is oriented towards cork production, with cork being extracted from tree stem and branches as planks by cutting with an axe and stripping off. The effect of damage to the tree during cork stripping was studied in cork oaks, weakened by wounding, by following the diameter growth and its seasonality during a 9-year production cycle, and comparing them with healthy cork oaks. Tree wounding decreased diameter growth during the following cycle, e.g. 8.5 mm a ‐1 and 9.8 mm a ‐1 for weakened and healthy trees, respectively, mostly in the 2 years immediately following the cork stripping. The beginning of annual growth in spring and the occurrence of the highest increments in June‐August were delayed by about 1 month in the weakened trees. The cork produced by weakened trees was reduced by 13 per cent in thickness, with average cork ring widths of 3.3 mm a‐1 vs. 3.8 mm a‐1 for healthy trees.

Influence of climate on the seasonality of radial growth of cork oak during a cork production cycle

The radial growth of mature cork oaks (Quercus suber L.) was studied during a 9-year production cycle using monthly dendrometer measurements and cork ring widths. The mean tree radial increase was 5.1 mm yr -1 and the cork increment accounting for 3.8 mm yr -1 . The radial growth curves for cork, wood and stem were very similar, showing a decreasing trend along the production cycle. Cork increment was always the largest fraction of tree growth (on average 74%) and showed less inter-annual variations as compared to wood. Tree radial growth presented a clear within-the-year seasonal pattern, extending from March to October, with a maximum in June- July. The overall pattern of monthly growth distribution was similar among the years of the production cycle, but some inter-annual va- riations occurred with 1-2 month shifts or monthly growth rate decreases, related to climatic factors. Early spring growth was enhanced by winter rain, autumn growth by high summer rain and June increments by high temperatures during this month.

Laboratory Improvement with Fun

Due to a series of advantages such as chemical and heat resistance, the use of cork rings as supports for round-bottomed glass apparatus has survived many possible substitutes available with the technical advances made in composition materials. One drawback to cork rings is their price. An alternative to purchasing the rings is discussed here.

EVOLUTION OF SOCIAL BEHAVIOR IN A RESOURCE-RICH, STRUCTURED ENVIRONMENT: SELECTION EXPERIMENTS WITH MEDAKA (ORYZIAS LATIPES).

Schooling and aggression in fish are known to be partially inherited traits. Their genetic relationship to growth rate and to each other is, however, not fully understood. In this study we present evidence that schooling, social tolerance, and agonistic behavior in medaka (Oryzias latipes) are altered as an indirect result of selection on growth in two environments that differed in the intensity of social interactions required to obtain access to food. In the high interaction environment food was provided to excess inside a floating cork ring, which limited access to the food and allowed fish to attempt to monopolize it. In the low interaction environment the same amount of food was spread over the container's surface. After two generations of selection on growth the correlation of agonistic behavior and mean growth of broods was negative in the line selected for fast growth, when selection took place in a high interaction environment, but not when it took place in a low interaction environment. School cohesion was higher in the lines selected for fast growth than in those selected for slow growth when selection and rearing environments were both the same, either high or low interaction, but not when they were different. The correlation of social tolerance with growth was significantly more positive in the line selected for fast growth than in that selected for slow growth, but only when selection took place under high social interaction. It appears from these experiments that when resources are aggregated, but unlimited in quantity, competition favors individuals that avoid wasting time and energy on unnecessary and ultimately futile attempts to monopolize food and that also exhibit higher tolerance of nearby conspecifics. The results are interpreted in terms of a hypothesized stimulus-response threshold level for agonistic responses to conspecifics. We suggest that this threshold, which is altered by selection on growth, could provide a common causal (genetic) link between growth and the observed aspects of social behavior. By combining the probabilities from the separate behavioral experiments to obtain an overall significance of our hypothesis we conclude that the probability of no change in threshold is in fact low (P < 0.01).

RAPID BEHAVIORAL CHANGES IN MEDAKA (ORYZIAS LATIPES) CAUSED BY SELECTION FOR COMPETITIVE AND NONCOMPETITIVE GROWTH.

Behavior is a major component of growth rate variation in fish, but the genetic relationship between behavior and growth is largely unknown. In particular, it is not known how behavior responds to natural or artificial selection on growth. It has been argued that artificial selection for fast growth in competitive environments might lead to higher levels of overall aggression, and therefore there would be no net gain in assimilation efficiency or growth. The contrary has also been argued. It is not immediately obvious, therefore, whether fish that avoid interacting with conspecifics should grow faster, or whether more aggressive, dominant individuals should grow faster. In the present study, we artificially selected fish on the basis of growth in two environments that differed in the intensity of social interactions. In the high interaction environment (HI), food was provided to excess inside a floating cork ring, which limited access to the food and allowed fish to attempt to monopolize the food supply. In the low interaction environment (LI), an equivalent amount of food was spread over the container's surface. In each social environment, fish were selected within family for fast and for slow growth rate during two generations (G1 and G2 ). The response was measured in the third generation (G3 ) on the directly selected trait (growth) and on a correlated trait (agonistic behavior). The magnitude of the direct response to selection on growth in medaka was directly related to population density, i.e., selection was most effective at high densities. Agonistic behavior was inversely related to growth when individuals were raised and selected in an environment where enforced social interaction took place, so long as food was not limited in quantity.

Behavioural and growth responses to the intensity of intraspecific social interaction among medaka, Oryzias latipes (Temminck and Schlegel) (Pisces, Cyprinodontidae)

When access to food is restricted, faster growing fish may be those whose behaviour is relatively unaffected by the presence of nearby conspecifics. Behavioural experiments were carried out to determine the relation between growth and motor activity levels in crowded and uncrowded conditions, and measures of aversion/attraction to groups of conspecifics. Two experimental groups of Or)‐ius laripes (Temminck and Schlegel) were grown for several weeks in two environments manipulated so as to maximize differences in social interactions. In the high interaction environment (HI), food was provided inside a floating cork ring. In the low interaction environment (LI), food was spread over the container's surface. Fish were measured at the end of the growth period and tested for their activity levels in the presence of conspecifics and for their preference for, or tolerance of crowded conditions. The correlation between motor activity and growth was significantly more positive in the HI environment than in the LI environment. The relationship between preference for crowded conditions and growth was negative for both groups of fish, although less so for HI than for LI. We conclude that artificial selection for faster growth may produce more aggressive fish only under very high levels of forced social interaction (competition), if at all. Under conditions of reduced social interaction, the social‐aversive or socially indifferent fish grow faster.

Mode of infection and biology of infective larvae of Molineus barbatus chandler, 1942

The infective larvae of Molineus barbatus penetrated the skin of a young mouse stretched on a cork ring when applied in saline solution at blood temperature. This was confirmed by percutaneous exposure of ferrets. Infections were also initiated by mouth. The development of larvae was slow after skin penetration and rapid after oral exposure. The larvae are positively geotropic, thermotropic, phototactic, thigmotropic and unable to withstand even brief desiccation. Longevity in water of stage 3 larvae was studied at temperatures ranging from 5 °C to 40 °C. The higher temperatures were lethal to larvae, which exsheathed before death in many cases.