Consideration of translocation into a growth model of a plant organ

Taking account of the Bertalanffy's differential equation on animal growth, plant growth is also considered as the net result of anabolism and catabolism. When we, however, consider the growth of a plant organ, it is necessary to add a term of translocation because it plays an important role in the growth of plant organs, such as leaf and fruit. Considering translocation, therefore, a growth model of a plant organ was proposed on the basis of the compartment model for estimating the carbon balance in the organ by using the experimental data on translocation, photosynthesis and respiration of a tropical fruit of durian (Durio zibethinus Murray). The present growth model of a plant organ belongs to an extended Bertalanffy's growth equation, and was possible to be transformed into the simple Bertalanffy's growth equation on the basis of the proportionality between the growth and translocation rates. The Bertalanffy's growth equation of a plant organ was also possible to apply to that of the whole plant on the assumption of the allomeric relationship between a plant organ and the whole plant.     

Mathematical analysis of change in forest carbon use efficiency with stand development: A case study on Abies veitchii Lindl.

Changes in carbon use efficiency (CUE), which is defined as the ratio of net primary production (NPP) to gross primary production (GPP), were analyzed for Abies veitchii Lindl. forests with respect to stand development by developing a simple mathematical model incorporating data on physiological variables and leaf mass ratio. A decrease in CUE with stand development was successfully expressed as a function of stand biomass (y) based on the following three assumptions: (1) a power-law relationship between mean respiration and mean individual tree mass, (2) a power-functional relationship between mean gross primary production and mean individual tree mass, and (3) self-thinning relationship between stand biomass and density. Based on this model, a parameter of CUE–y relationship was defined, and it was clarified that CUE decrease with stand development is caused not by the ratio of specific respiration rate to specific gross photosynthetic rate, but by leaf mass ratio. Since CUE is high in young forests, helpful information on selecting woody species when planting seedlings was provided from the viewpoints of reducing CO₂ in the atmosphere and global warming.     

Male mating competitiveness and age-dependent relationship between testosterone and social rank in bighorn sheep

In males, the acquisition and development of behavioral and morphological secondary sexual traits typically depends on testosterone and correlates with mating success. Testosterone level could affect competition for mates and thus be a target of sexual selection. We sought to relate testosterone levels to male mating competitiveness, by teasing apart the relationships between testosterone, behavior, and growth before the mating period. We monitored 24 adult bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, from 2008 to 2011. Using linear mixed models, we tested the relationships between testosterone metabolites in feces, social rank, and both growth and size of two sexually selected traits: horns and body mass. The correlation between testosterone and social rank varied with age. Testosterone and rank were weakly and negatively correlated for young rams, positively correlated for prime-aged rams, and negatively correlated for older rams. Although testosterone had an increasingly positive effect on total horn length until 8 years of age, we could not detect any effects on annual growth rate of horns or body mass. Testosterone may be related to male’s ability to compete for mates through its relationship with behaviors determining social rank, rather than by influencing the development of morphological traits. Differences in testosterone levels among competitors may be a proximate cause of variance in fitness.     

The influence of per offspring investment (POI) and starvation on larval developmental plasticity within the palaemonid shrimp,Palaemonetes varians

At the inter-specific level, per offspring investment (POI), degree of abbreviated development, and lecithotrophic potential all increase with increasing latitude and freshwater penetration among crustaceans. These traits are considered adaptations to conditions of decreasing growth potential. We hypothesise that this relationship between POI and abbreviated development also occurs at the intra-specific level. We studied the caridean shrimp, Palaemonetes varians, to investigate the hypothesis that under food-limited conditions, higher POI enables development through fewer larval instars. Under starvation stress, larvae from broods of greater POI (measured as hatchling brood average dry weight, DW) generally developed through fewer larval instars. With increasing starvation period, larval development time increased, whilst larval growth rate, juvenile DW, juvenile carbon mass, and juvenile carbon:nitrogen (C:N) ratio all decreased. Larval development time generally decreased with increasing brood average dry weight. In contrast, larval growth rate, juvenile DW, juvenile carbon mass, and juvenile C:N ratio all increased with increasing larval brood average DW. The relationship between POI and larval instar number (abbreviation of development) reported here is consistent with that at the inter-specific level and supports the concept that macro-ecological trends in development modes at the inter-specific level may be driven by selection occurring on POI at the intra-specific level.     

Density dependence in social behaviour: home range overlap and density interacts to affect conspecific encounter rates in a gregarious ungulate

Sociality is poorly understood in the context of population processes. We used wild, female elk (Cervus canadensis) equipped with proximity-logging radio collars (n?=?62) from Manitoba, Canada (2007–2009), to test for modifying effects of population density (two areas: 0.42 and 0.22 animals/km²) on the relationship between two measures of sociality. This included the rate at which collared individuals encountered one another per year (encounters logged as animals ranging to within 1.4 m of each other) and the extent to which animals overlapped in annual home range (proportion of shared minimum convex polygon ranges). Overlap was significantly greater in the high density area compared to that of the low, but not if we only considered individuals that directly encountered each other, implying that familiar individuals will maintain a constant degree of range overlap regardless of density. Encounter rate was nonlinearly related to home range overlap. This relationship was also density-dependent, exhibiting negative density dependence at high proportions of overlap, primarily in the high density subpopulation. Sociality, as defined by two interacting measures of behaviour—encounter rate and home range overlap—exhibits a complex nonlinear relationship; we discuss the implications of these results as they pertain to sociobiology, resource competition, and pathogen transmission.     

Dominance rank differences in the energy intake and expenditure of female Bwindi mountain gorillas

Socioecological models predict that contest competition for clumped foods can lead to higher energy intake and lower energy expenditure for higher-ranking individuals. Here, we examine the relationships between dominance rank and energy intake and expenditure of female mountain gorillas in Bwindi Impenetrable National Park, Uganda (Gorilla beringei beringei). Bwindi gorillas have weak dominance relationships, feed on nonreproductive plant parts throughout the year, and consume fruit when it is seasonally available. We used behavioral observations on one group of gorillas and nutritional analysis of their major food items to calculate energy intake rates and estimated energy expenditure. Using linear mixed models, we found a significant positive relationship between dominance rank and energy intake rates, due to higher-ranking females having faster ingestion rates, rather than consuming foods with higher energy concentrations. Lower-ranking females did not spend significantly more time feeding to compensate for their lower energy intake rates. Lower-ranking females spent significantly more time traveling than higher-ranking females, leading to a negative relationship between dominance rank and energy expenditure. The combined results revealed a significant positive relationship between dominance rank and energy balance. Higher-ranking females did not spend longer feeding on fruit than lower-ranking ones, and the relationship between dominance rank and energy intake rates was not stronger when fruit was available. According to socioecological models, these results suggest that contest competition may be occurring with both fruit and nonreproductive plant parts, which would be consistent with growing evidence that nonreproductive plant parts can be contestable.     

氮磷钾对寒害腰果植株恢复生长结果的影响

通过观测不同施肥处理区寒害腰果植株重新开花坐果状况,分析施肥对寒害腰果植株恢复生长的影响,并分析腰果叶片养分含量与结果量之间的关系。结果表明:氮对寒害腰果植株重新开花的花枝数影响最大,钾次之,磷影响最小,随着氮施用量的增加,植株的花枝数逐渐增大;氮是影响寒害腰果植株结果量的主要因素,植株结果量随着氮施用量的增加而增加,磷、钾各处理间的结果量均未达到统计上显著差异;在施氮肥的基础上,配施磷钾肥,可以提高寒害腰果植株结果量,9个施肥处理中以N3P3K2处理的结果量最多。     

Growth dynamics of Saccharina latissima (Laminariales,Phaeophyceae) in Aarhus Bay,Denmark, and along the species’ distribution range

Growth dynamics of Saccharina latissima in a Danish embayment (56°03.793N, 10°16.148E) were investigated through an annual cycle (March 1999–March 2000) and related to patterns found in previous studies covering the distribution range of the species. The kelps exhibited meristematic growth as well as distal tissue loss all year around. Elongation rate peaked in spring (March–May, 0.75 cm day^?1), whereas maximum biomass growth occurred in May–July at higher insolation. S. latissima accumulated nitrogen (N) in November–March, when ambient N levels were high and the fast growth in March–July was followed by a depletion of these reserves. Light regimes and seasonal fluctuations of nutrients were, thus, major factors explaining the seasonal growth pattern of S. latissima in this temperate bay. Differences in light and exposure along depth gradients affected the growth, loss and storage product dynamics. High losses of N and carbon (C) through summer abscission of distal tissue question the efficiency of translocation, especially at shallower depths, where losses are accelerated by, e.g., high temperature. A large-scale comparison further highlighted that warming advanced the timing, increased the level of peak growth and also seemed to expand the growth period. Growth rates of the studied Danish population fell in the low end of the range for similar latitudes and temperatures, probably due to sub-optimal salinities (avg. 23.3 psu) in combination with periodically high summer temperatures (max. 21.1 °C).     

Influence des conditions hivernales sur les productions phyto-et zooplanctoniques en Méditerranée Nord-Occidentale. III. Caractérisation des eaux de surface au moyen de cultures d'algues

Bioassays were performed on surface-water samples, collected during the Médiprod I cruise of the R.V. “Jean Charcot” in the northern part of the Mediterranean Sea. Physiological responses of axenic algal cultures were measured in order to characterize the different water masses. The responses obtained are specific to each algal strain: Phaeodactylum tricornutum grows in all waters sampled during the first part of the cruise, while Chaetoceros lauderi does not develop because of its different nutritional requirements; Chlamydomonas magnusii does not grow in any of the tested samples. No clear relationship exists between the results obtained in vitro and those obtained in situ but, on the whole, the waters collected during the second part of the cruise allow a higher growth of the test algae in vitro. This higher fertility is probably related to the algal bloom which occurred between the first and the second part of the cruise. In the authors' opinion the great difference in uptake rates observed between the high-turbulence area (low production) and the adjacent area (high production), may be related more to biological conditioning than to any physical process. However, physical mixing seems to be responsible for inhibition of primary productivity at the two stations where mixing attains maximum values.     

Pre-natal investment in reproduction by female Antarctic fur seals

Summary Maternal investment, in terms of pup birth mass, in gestation by Antarctic fur seals (Arctocephalus gazella) was related to the date of birth in two consecutive years. There were significant differences in birth mass between years and between the sexes within years. Birth mass was used to calculate the mean energetic cost of producing a pup to parturition. The cost for a male pup in 1986 was 173 MJ while it was 191 MJ in 1987. For female pups the cost was 152 and 166 MJ in 1986 and 1987 respectively. Given the probable pattern of foetal growth, this constitutes a minimum of 5–15% of the maternal energy budget in the final months of gestation. Birth mass varied inversely with date of birth, but more strongly for male than for female pups. The sex ratio at birth was unity in both years and this did not vary with time through the birth season. In a subsample of mothers (n=79) which were captured on the day of birth, there was a decline in the body mass and standard length with date of birth. Male birth mass was directly related to maternal mass and maternal condition (mass/length) but there was no significant relationship for females. These results suggest that the growth of male foetuses is limited by maternal resources while female foetuses do not exploit fully maternal resources.     

More asymmetric tree competition brings about more evapotranspiration and less runoff from the forest ecosystems: A simulation study

The present paper reports how stand size-structure dynamics due to competition between different-sized trees affect long-term forested water balance in Japanese cool-temperate planted stands (evergreen coniferous Cryptomeria japonica and deciduous coniferous Larix kaempferi stands) using a fully coupled multi-layered meteorological surface physics—terrestrial ecosystems model. The simulation captured the well-known annual variation in leaf area index (LAI) accurately with stand age in monocultured and even-aged stands; the occurrence of maximum LAI during the early growth stage and then a gradual decline followed by a steady state after the maximum LAI. The simulations also detected a high dependency of annual evapotranspiration (AETr) on LAI with stand age that is well known by prior observational researches. In the C. japonica (shade-tolerant late-successional species) stand, the relationship between annual net primary productivity of an individual tree (NPP_ind) and individual tree mass (w) changed from linear to a convex curve during self-thinning, indicating that the degree of asymmetric tree competition intensified with forest stand development. The higher degree of competitive asymmetry characterized by the convex-shaped NPPind-w relationship produced greater size inequality, i.e., the formation of trees stratified by height. Under such conditions, AETr and annual transpiration (ATr) were mainly regulated by larger trees. On the other hand, the NPPind-w relationships in the L. kaempferi (shade-intolerant early-successional species) stand were linear throughout the simulated period, indicating the lower degree of competitive asymmetry. Under such conditions, the growth of intermediate-sized trees was enhanced and these trees became a dominant source of AETr (and also ATr) during self-thinning. Furthermore, the sensitivity analysis of the effects of ecophysiological parameters such as foliage profile (i.e., vertical distribution of leaf area density) of an individual tree (distribution pattern is described by the parameter η), the maximum carboxylation velocity (V_cmax0) and biomass allocation pattern of individual plant growth (μ₁) on AETr, ATr and annual runoff (AR_off) showed that the temporal trends of AETr, ATr, AR_off and NPPind-w relationships were completely the same as those in the control simulations. However, the NPPind-w relationship during self-thinning indicated higher degrees of competitive asymmetry when η or V_cmax0 were greater than those in the control simulation and generated greater AETr and ATr and thus smaller AR_off. We found that more asymmetric tree competition brings about greater size inequality between different-sized trees and thus more evapotranspiration and less runoff in a forest stand. Overall, our simulation approach revealed that not only LAI dynamics but also plant competition, and thus size-structure dynamics, in a forest ecosystem are essential to long-term future projections of forested water balance.     

Trends in space occupation by the encrusting sponge Crambe crambe: variation in shape as a function of size and environment

The relationship between sponge size, habitat and shape was studied in the encrusting sponge Crambe crambe (Schmidt, 1862), which is distributed widely throughout the shallow Mediterranean littoral. Examination of sponge patches in shaded and well-illuminated habitats showed that the degree of peripheral irregularity of the edges of a patch is directly related to patch size. This relationship is valid only for sponges of >100 mm² in area. Photophilic and sciaphilous sponges display different growth forms. The pattern of growth is interpreted in terms of competition for space. The directional growth of sciaphilous sponges may be due to the presence of dominant neighbours that are good space competitors, and the irregular growth of photophilic sponges to the absence of such neighbours.     

Growth and production of<Emphasis Type="Italic"> Ophiocten gracilis</Emphasis> (Ophiuroidea: Echinodermata) on the Scottish continental slope

Growth and production of the bathyal ophiurid brittle star Ophiocten gracilis was studied from skeletal growth bands and disc size frequencies of specimens collected in sled and trawl samples taken on the continental slope off Scotland. Growth bands showed up in SEM examination as ring-like zones in surface relief and texture of the stereom microstructure of the intervertebral muscle insertions on the arm ossicles. Seasonal variability in somatic growth, presumed to underlie this growth pattern, may reflect reproduction and/or a possible non-feeding period during gonad maturation. Disc size-at-age was back-calculated from size-at-age interpreted from growth-band series on the vertebral ossicles from arms of O. gracilis. Pooled growth-band frequency data and normal-distribution mixtures based on size-at-age data were used to test for overgrowth of early growth bands on the ossicles from larger individuals. Von Bertalanffy and Gompertz growth models were fitted to the finalised back-calculated disc size-at-age data. These were used along with the modal structure of the observed disc size frequencies to develop a demographic model based on normal-distribution mixtures constrained by the growth model. These and other defining parameters were fitted by non-linear regression to size structure observed in a sample of the breeding population from 997 m depth on the Hebrides Terrace. Recruitment was estimated according to available data from sediment-trap time series. A ratio of somatic production/biomass, P_S/B, in the range of 0.43–0.54 was estimated using a fitted size/mass relationship and the increment summation method (ISM) applied to the fitted growth models. A narrower, but otherwise similar, range in estimated P_S/B ratios (0.48–0.49) was obtained in a parallel approach using the mass-specific growth rate method (MSGRM), whereby the same size/mass relationship was applied to the observed frequencies and growth parameters fitted to growth banding. Using previously obtained data on population density, a standing crop of 4.8 g wet weight (~0.58 mg AFDW) m^?2 would provide annual wet weight production in the range of 1.9–2.4 g (~0.23–0.29 mg AFDW) m^?2 in the population between ca. 700–1000 m depth. Somewhat greater production estimates (P_S/B=0.73–0.98) were obtained from MSGRM by pooling the sample with size frequencies from other large samples in which postlarval sizes were more numerous, but larger sizes less numerous. Similarly high production was estimated by MSGRM from a box corer sample from the Wyville-Thomson Ridge. Explanations for variability in size structure are discussed, but even the lower estimates are comparable to boreal shallow-water brittle stars. The high rate of growth and production by accepted deep-sea standards may be related to a capability for interface feeding.     

Sex differences in mass loss rate and growth efficiency in Antarctic fur seal (Arctocephalus gazella) pups at Macquarie Island

We investigated the relationship between the mass gained by a pup during a period of maternal attendance (as an index of milk intake) and the duration of the preceding foraging trip in relation to the mass-specific rate of mass loss during fasting periods and the growth rate of Antarctic fur seal (Arctocephalus gazella) pups at Macquarie Island. We found that (1) serially weighed male pups grew significantly faster than females pups and that (2) fasting female pups lost mass at a significantly higher rate (2.55% day⁻¹) than male pups (2.12% day⁻¹) of the same mass; (3) during periods of maternal attendance, there were no intersexual differences in the amount of mass gained by pups of the same size, hence (4) female pups required a higher daily mass gain to grow at the same rate as male pups. Our results show that intersexual differences in growth rate may be accounted for by intersexual differences in mass-specific rate of mass loss, because females lost 0.42% more of their total mass per day (i.e. 4.2 g kg⁻¹ day⁻¹) compared with male pups of the same body mass. Despite intersexual differences in growth rates, our results indicate equality of maternal expenditure between the sexes. Intersexual differences in the rate of mass loss may be due to differences in the metabolic rate, activity level and/or body composition of male and female pups. Received: 19 August 1998 / Received in revised form: 19 April 1999 / Accepted: 19 April 1999     

Extending the leaf economics spectrum to decomposition: evidence from a tropical forest

I investigated the relationship between leaf physiological traits and decomposition of leaf litter for 35 plant species of contrasting growth forms from a lowland tropical forest in Panama to determine whether leaf traits could be used to predict decomposition. Decomposition rate (k) was correlated with specific leaf area (SLA), leaf nitrogen (N), phosphorus (P), and potassium (K) across all species. Photosynthetic rate per unit mass (Amass) was not correlated with k, but structural equation modeling showed support for a causal model with significant indirect effects of Amass on k through SLA, N, and P, but not K. The results indicate that the decomposability of leaf tissue in this tropical forest is related to a global spectrum of leaf economics that varies from thin, easily decomposable leaves with high nutrient concentrations and high photosynthetic rates to thick, relatively recalcitrant leaves with greater physical toughness and defenses and low photosynthetic rates. If this pattern is robust across biomes, then selection for suites of traits that maximize photosynthetic carbon gain over the lifetime of the leaf may be used to predict the effects of plant species on leaf litter decomposition, thus placing the ecosystem process of decomposition in an evolutionary context.     

Using three-dimensional surface area to compare the growth of two Pocilloporid coral species

Many facets of coral research require coral colony surface area estimates. This study developed a relationship between the two-dimensional (2D) projected area and the three-dimensional (3D) whole colony surface area for two commonly studied Indo-Pacific coral species: Pocillopora damicornis and Stylophora pistillata. The surface index function was used to measure the growth of colonies in situ around Heron reef on the southern Great Barrier Reef. The results show that while growth between the two species was not significantly different when measured in two dimensions, the 3D area showed significantly different growth rates with S. pistillata growing at almost double the rate of P. damicornis. The study demonstrates that it is possible to make reliable estimates of the 3D surface area of entire colonies of these complex branching coral species, using the plan view of the coral and a pre-determined surface index function. In addition, this study shows that the 3D surface area provides a more useful measure of colony growth than the traditional methods of either 2D area or longest dimension.     

Modelling coupled peach tree-aphid population dynamics and their control by winter pruning and nitrogen fertilization

Nitrogen fertilization and winter pruning are commonly used to control crop production in peach [Prunus persica (L.) Batsch] orchards. They are also known to affect the dynamics of Myzus persicae (Sulzer) (Homoptera: Aphididae) aphid populations via bottom-up regulation processes. Interactions between crops and pests can cause complex system behaviour in response to management practices. An integrated approach will therefore improve the understanding of the effects of these two cultural practices on aphid and peach performances.We developed a simulation model that describes the cultural control of interacting peach tree and aphid population dynamics. It uses the principles of common trophic models while gathering available knowledge and explicit assumptions on peach and aphid functioning and the effects of cultural practices.The model was able to qualitatively reproduce the system behaviour observed in the field. It accounted for actions and feedback such as stimulation of foliar growth by winter pruning, consecutive aphid population increase, subsequent damage to foliage, and partial compensatory growth of foliage. The model also reproduced low losses in fruit production due to aphid infestations. However, it called for further integration of ‘long-term’ effects. Analysis of the model showed the complexity of peach tree and aphid responses to leaf N × winter pruning interactions. Simulations indicated that fruit production losses remained low within a range of realistic values of leaf N and pruning intensity, whereas manipulating peach and aphid dynamics, their interactions and their relationships to practices could result in higher losses.The model is useful to evaluate the relevance of cultural practices for a bottom-up regulation of aphid dynamics in crop-pest management. After considering other control methods and fruit quality, it can be used to find a combination of practices that optimises trade-offs between fruit production and environmental conservation goals. A modelling approach that links crop growth and pest population dynamics and integrates management practice effects has strong potential for improving crop-pest management in an integrated crop production context.     

Bioenergetics of ephyra larvae of the scyphozoan jellyfish Aurelia aurita in relation to temperature and salinity

The body mass of Aurelia aurita ephyrae was better correlated with the diameter of the central disc than with the distance between opposite rhopaliae or distance between opposite lappet tips. Body dry weight (y, in μg) related to the disc diameter (x, in mm) through the equation y = 22.33 x ^1.99. The exponent 1.99 was significantly lower than that for the medusa stage, indicating a tendency to grow in diameter rather than in weight through the ephyra stage. The average ash-free dry weight (AFDW) of ephyrae was 38.0% of the dry weight. The AFDW/diameter relationship was used to convert measured diameters to body AFDW and calculate succession in body mass, daily ration, daily growth rate and gross growth efficiency. Effects of temperature (6, 9.5, 12, 15 and 18 °C) and salinity (17.5, 22, 26, 30.5 and 35 PSU) on these parameters and feeding were studied at saturated prey concentration (222 Artemia nauplii l⁻¹ initial concentration) by daily measurements over 10 d. There was a strong effect of temperature for total ingestion, growth rate, growth efficiency and final body mass of individual ephyrae, whereas the daily ration was not significantly different between the different temperatures. The experimental group kept at the highest temperature (18 °C) diverged the most, and ephyrae at this temperature ingested 2.7 times more and increased in weight 5.4 times more than at 6 °C. The average daily growth rate and gross growth efficiency of these ephyrae were 34.5% and 25.1%, respectively, significantly higher than at 6, 9.5 and 15 °C. Significant effects of salinity were shown for total ingestion, daily ration, daily growth rate and final weight, although only total ingestion and daily ration diverged sufficiently to show effects in a post-hoc test. This test showed that total ingestion was significantly different for all salinities except between 22 and 35 PSU and between 17.5 and 26 PSU. The daily ration for 35 PSU diverged from all other salinities, whereas none of the other salinities showed any significant differences. Thus, provided food in excess A. aurita can double its weight every 2 to 4 d, dependent on temperature and can therefore develop to the medusa stage in short time. Differences in environmental salinity in the range 17.5 to 35 PSU have little or no effect on growth rate and growth efficiency, whereas our results indicate that the full seawater salinity (35 PSU) causes significantly higher ingestion rate compared to lower salinities. Received: 11 January 1999 / Accepted: 11 May 1999     

Density-dependent growth in the Atlantic surfclam, Spisula solidissima, off the coast of the Delmarva Peninsula, USA

Spisula solidissima (Dillwyn, 1817) is a large, suspension-feeding bivalve, whose range extends from Nova Scotia to South Carolina. This species is harvested commercially. Shell length and age data were collected for this species from 1980 to 1994 during surveys of population size and structure. These data were used to examine the relationship between the growth rate of S.␣solidissima and intraspecific density. The null hypothesis was that density (represented by number of individuals per tow) would have no effect on rate of growth. A negative relationship would support the alternative hypothesis, that intraspecific competition had taken place. This analysis focused on the surfclam population offshore from the Delmarva Peninsula, USA because: (1) a major recruitment event occurred in 1977, (2) clam fishermen had reported “stunted” surfclams in that area, (3) a wide range of local densities were available to examine, and (4) the existence of a closed area within the study area set up an interesting contrast with areas left open to harvesting. Maps of surfclam abundance across the Delmarva region demonstrate that areas of highest density have generally remained in the same location through time. The results suggested that intraspecific competition has been important in structuring this population. Based on data from 1980 to 1992, shell length was significantly reduced at high density, and a significant interaction between age and density was observed. Growth modeling indicated decreased asymptotic lengths and growth rates with increasing density. In nine out of ten pairwise randomization tests, fitted von Bertalanffy growth curves, representing different densities, were significantly different from each other. High densities of clams have persisted in the area that was closed to harvesting for 11 years (1980 to 1991). In 1994, length at age was significantly less in this closed area compared to that in the surrounding area. This effect was apparent in clams from 3 to 17 years of age, and most pronounced in the cohort that recruited to the Delmarva region in high numbers in 1977. Lower growth rates within the closed area have management implications for the optimal duration of closures. Received: 16 May 1997 / Accepted: 7 October 1997     

Relation of food preference to fitness for the green sea urchin,Strongylocentrotus droebachiensis

We maintained 30 to 35 mm sea urchins (Strongylocentrotus droebachiensis) on 17 different macroalgal diets in the field over a 130-d period and correlated the resulting changes in somatic and gonadal tissues with measures of food preferences. The varions algae fell into there distinct categories according to their ability to support growth. The most striking effect of diet was on gonadal mass, there being a 14-fold increase for the best algal diet, compared to initial gonadal mass, and nearly a threefold decrease for the worst diet. The relation of food preference to somatic growth was nonlinear. A sharp increase in the growth of test diameter, total mass and test mass occurred between the nonpreferred and intermediately preferred algae, but growth rates were similar for intermediate and preferred algae. Gonadal growth tended to increase exponentially with an important measure of food preference, feeding rate (g ingested d^–1). These observations suggested that urchins only channel food resources into somatic growth up to a given point, additional reserves being used for producing gametes. The positive relationship of food preference to growth, and especially the strong correlation with gonadal production, indicates that the highly selective feeding of the urchin contributes to ils fitness.