Ontogenetic Increase Research Articles

Growth, production and bioenergetics of brown trout in upland streams with contrasting riparian vegetation

Summary1. A series of laboratory‐based equations on trout growth and bioenergetics developed by J.M. Elliott were applied to data collected for brown trout (Salmo trutta L.) under field conditions in Co. Mayo, Western Ireland. Fish were collected by electrofishing eight upland streams with contrasting riparian vegetation; grassland, open canopy and closed canopy deciduous.2. Stream temperatures, one of the main influencing factors on fish growth and energetics, did not differ significantly between riparian types.3. Observed growth rates were lower than the predicted maximum growth rates and were not influenced by riparian vegetation type. Growth ranged between 0.66% day−1 for 0 + trout to 0.08% day−1 for 2 + trout.4. Production estimates showed no clear difference between riparian vegetation types over the growing season.5. Fish densities and biomass tended to be greater in closed canopy streams particularly in summer.6. Actual ration sizes calculated for trout were similar to the ration required for maintenance metabolism and were only 45–63% of the maximum potential rations. Although there was an ontogenetic increase in ration size with increasing fish age, the proportion of ration available for growth (i.e. the difference between actual and maintenance rations) did not differ between age classes but was greatest in summer. 1+ and 2+ trout show greatest ration available for growth in grassland streams.7. Trout growth did not differ between riparian vegetation types but did vary seasonally with greatest attainment in summer. Growth was limited in the present study possibly due to combined effects of reduced prey available to fish and low stream temperatures reducing metabolic requirements. In such food limited systems, terrestrial invertebrate energy subsidies could have significant benefits to brown trout growth, production and bioenergetics.

Osmoregulation and immunolocalization of Na+/K+-ATPase during the ontogeny of the mitten crab Eriocheir sinensis (Decapoda, Grapsoidea)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 329:169-178 (2007) - doi:10.3354/meps329169 Osmoregulation and immunolocalization of Na+/K+-ATPase during the ontogeny of the mitten crab Eriocheir sinensis (Decapoda, Grapsoidea) Ude Cieluch1,*, Klaus Anger2, Mireille Charmantier-Daures3, Guy Charmantier3 1Alfred-Wegener-Institut für Polar- und Meeresforschung in der Helmholtz-Gemeinschaft, Columbusstraße, 27568 Bremerhaven, Germany 2Biologische Anstalt Helgoland, Alfred-Wegener-Institut für Polar- und Meeresforschung in der Helmholtz-Gemeinschaft, Meeresstation, 27498 Helgoland, Germany 3Equipe Adaptation Ecophysiologique et Ontogenèse, UMR 5171 GPIA, Université Montpellier II, 34095 Montpellier cedex 05, France *Email: ude.cieluch@awi.de ABSTRACT: The ontogeny of osmoregulation was studied in laboratory-reared early developmental stages of the Chinese mitten crab Eriocheir sinensis from the Elbe estuary (North Sea, Germany). At salinities ranging from 0.16 to 44.3‰, survival rate was quantified, hemolymph osmolality was measured, and osmoregulatory capacity was calculated as the difference between osmolality of the hemolymph and that of the external medium. Zoea larvae hyper-regulated in dilute media, but osmoconformed in seawater and at higher salinities (≥32.2‰). Megalopae and Stage I–II juveniles hyper-regulated at low salinities and hypo-regulated at ≥32.2‰, with an ontogenetic increase in osmoregulatory capacity. Survival at ca. 10 to 32‰ was generally high (90 to 100%), while complete mortality occurred in all zoeal stages (except for zoea I) at 0.16 to 5.3 ‰. By contrast, nearly 50% of the megalopae and all juvenile crabs survived at such low salinities. The expression of Na+/K+-ATPase and the development of transporting epithelia were studied by means of immunofluorescence light microscopy (ILM) and transmission electron microscopy (TEM). In early (stage I & II) zoeae, fluorescence staining was observed along the inner epithelium of the branchiostegites, and epithelial cells showed typical features of ionocytes. In the megalopa and juvenile crab stage I, ionocytes and immunolabeled Na+/K+-ATPase were located in the filaments of the most posterior gills, while no immunolocalization occurred in the anterior gills. Comparison of histological and physiological results shows a close relationship between the ontogeny of osmoregulation and the expression of Na+/K+-ATPase within the transporting epithelia of the branchial chamber. In conclusion, the adult pattern of osmoregulation develops in E. sinensis through 2 molts, (1) from a moderately hyper-iso-regulating zoeal phase to the moderately hyper-/hypo-regulating megalopa, (2) from the megalopa to a strongly euryhaline, hyper-/hypo-regulating first juvenile crab stage. The results of this study are consistent with an export strategy in this holo-euryhaline crab species. KEY WORDS: Ontogeny · Osmoregulation · Ionocyte · Export strategy · Crustacea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 329. Online publication date: January 11, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

ECOLOGICAL TRAITS OF THE COLUBRID SNAKE CONOPHIS LINEATUS CONCOLOR (GUARDA CAMINO) IN THE YUCATÁN PENINSULA

Abstract Aspects of ecology are described for the Yucatan colubrid snake Conophis lineatus concolor, as inferred from examinations of museum specimens and a review of published literature records. Adult males of this distinctive subspecies are larger than females (> 890 mm versus < 760 mm SVL) and at the same body size males also have longer tails, although sexual dimorphism in other proportional features of morphology is negligible. Food habits are diverse; of 29 prey items identified to main taxonomic group, the most important in terms of relative composition (lx) were lizards (0.394) and arthropods (0.391), followed by squamate eggs (0.061) and thereafter in equal measure by frogs, snakes, and mammals (0.044). More than 90% of the arthropods were engorged cattle ticks, Boophilus microplus, and several snakes had fed exclusively on this prey type. An ontogenetic increase was evident in the proportion of larger prey consumed, although samples were inadequate to allow an assessment of the relationship bet...

Seasonal variations in water relations in current-year leaves of evergreen trees with delayed greening

We investigated seasonal patterns of water relations in current-year leaves of three evergreen broad-leaved trees (Ilex pedunculosa Miq., Ligustrum japonicum Thunb., and Eurya japonica Thunb.) with delayed greening in a warm-temperate forest in Japan. We used the pressure-volume method to: (1) assess the extent to which seasonal variation in leaf water relations is attributable to leaf development processes in delayed greening leaves versus seasonal variation in environmental variables; and (2) investigate variation in leaf water relations during the transition from the sapling to the adult tree stage. Leaf mass per unit leaf area was generally lowest just after completion of leaf expansion in May (late spring), and increased gradually throughout the year. Osmotic potential at full turgor (Psi(o) (ft)) and leaf water potential at the turgor loss point (Psi(w) (tlp)) were highest in May, and lowest in midwinter in all species. In response to decreasing air temperature, Psi(o) (ft) dropped at the rate of 0.037 MPa degrees C(-1). Dry-mass-based water content of leaves and the symplastic water fraction of total leaf water content gradually decreased throughout the year in all species. These results indicate that reductions in the symplastic water fraction during leaf development contributed to the passive concentration of solutes in cells and the resulting drop in winter Psi(o) (ft). The ratio of solutes to water volume increased in winter in current-year leaves of L. japonicum and E. japonica, indicating that osmotic adjustment (active accumulation of solutes) also contributed to the drop in winter in Psi(o) (ft). Bulk modulus of elasticity in cell walls fluctuated seasonally, but no general trend was found across species. Over the growing season, Psi(o) (ft) and Psi(w) (tlp) were lower in adult trees than in saplings especially in the case of I. pedunculosa, suggesting that adult-tree leaves are more drought and cold tolerant than sapling leaves. The ontogenetic increase in the stress resistance of I. pedunculosa may be related to its characteristic life form because I. pedunculosa grows taller than the other species studied.

Dynamics of activity of monoamine oxidase B and antioxidant defense enzymes in human brain during postnatal ontogeny

During postnatal development, activity of monoamine oxidase B in human brain structures increases, while activity of Cu,Zn-dependent SOD decreases with compensatory increase in catalase activity and ceruloplasmin content. Under these conditions, the resistance of the pontobulbar structures and thalamus to oxidative stress decreases at the age of 1-12 years and returned to the prenatal level during adolescence. The increase in catalase activity is most pronounced in structures of the forebrain (cortex and neostriatum), cerebellum, and hypothalamus; it is accompanied by ontogenetic increase in oxidative stress resistance, which is maximum at the age of 12-21 years. The maintenance of resistance to oxidative stress (depends on monoamine oxidase B) can be considered as an important condition of structural and functional maturation of cerebral structures during the postnatal ontogeny.

Ontogeny of muscle mechanical advantage in capuchin monkeys (Cebus albifrons and Cebus apella)

AbstractTerrestrial locomotion requires that animals maintain postural stability against the flexing force of gravity. To counteract volume/area relationships that limit muscle force and may compromise stability, larger animals increase the effective mechanical advantage of their extensor musculature by walking on more extended limbs and showing allometric increases in extensor muscle lever arm lengths. Assuming these size‐related principles are uniform, similar adaptations should characterize ontogenetic increases in size. Previous research on non‐primate mammals has shown, however, that extensor muscle mechanical advantage is greater early in ontogeny and decreases with negative allometry during growth. This study extended this work by investigating patterns of relative elbow lever arm growth in two capuchin monkeys, Cebus albifrons and Cebus apella. Unlike previously studied mammals, growing capuchin monkeys face unique anatomical constraints, including relatively long limb bones and positively allometric limb growth. These constraints could augment the flexing torque of gravity at the limb joints and require compensatory increases in extensor muscle mechanical advantage as size increases through growth. Forearm length and the length of elbow extensor and flexor muscle lever arms were measured in longitudinal radiographic series of growing capuchin monkeys. In contrast to other mammals, all lengths scaled to body mass with positive allometry. Anatomical mechanical advantage (lever arm length/forelimb length) scaled with negative allometry, however, matching the non‐primate mammalian trend. Greater muscle mechanical advantage during early locomotion may help young mammals overcome relatively weak extensor muscles and other growth‐related limits on locomotor performance. Additional data on ontogenetic changes in gait dynamics and musculoskeletal growth are required to validate this hypothesis.

Low Dose Effects in Psychopharmacology: Ontogenetic Considerations

Low doses of psychoactive drugs often elicit a behavioral profile opposite to that observed following administration of more substantial doses. Our laboratory has observed that these effects are often age-specific in rats. For instance, whereas moderate to high doses of the dopamine agonist apomorphine increase locomotion, suppressed locomotor activity is seen following low dose exposure, with this low dose effect not emerging consistently until adolescence. A somewhat earlier emergence of a low dose "paradoxical" effect is seen with the 5HT(1a) receptor agonist, 8-OH-DPAT, with late preweanling, but not neonatal, rats showing increases in ingestive behavior at low doses but suppression at higher doses. In contrast to these ontogenetic increases in expression of low dose drug effects, low dose facilitation of social behavior is seen following ethanol only in adolescent rats and not their mature counterparts, although suppression of social interactions at higher doses is seen at both ages. This hormesis-like low dose stimulation appears related in part to overcompensation, with brief social suppression preceding the subsequent stimulation response, and also bears a number of ontogenetic similarities to acute tolerance, a well characterized, rapidly emerging adaptation to ethanol. Implications of these and other ontogenetic findings for studies of hormesis are discussed.

Developmental changes in eyeblink conditioning and neuronal activity in the pontine nuclei.

Neuronal activity was recorded in the pontine nuclei of developing rats during eyeblink conditioning on postnatal days 17-18 (P17-P18) or P24-P25. A pretraining session consisted of unpaired presentations of a 300-msec tone conditioned stimulus (CS) and a 10-msec periorbital shock unconditioned stimulus (US). Five paired training sessions followed the unpaired session, consisting of 100 trials of the CS paired with the US. The rats trained on P24-P25 exhibited significantly more conditioned responses (CRs) than the rats trained on P17-P18, although both groups produced CRs by the end of training. Ontogenetic increases in pre-CS and stimulus-elicited activity in the pontine nuclei were observed during the pretraining session and after paired training. The activity of pontine units was greater on trials with CRs relative to trials without CRs in rats trained on P24-P25, but almost no CR-related modulation was observed in the pontine units of rats trained on P17-P18. The findings indicate that pontine neuronal responses to the CS and modulation of pontine activity by the cerebellum and red nucleus undergo substantial postnatal maturation. The developmental changes in pontine neuronal activity might play a significant role in the ontogeny of eyeblink conditioning.

Prey selection by greenback flounder Rhombosolea tapirina (Günther) larvae

The potential for greenback flounder ( Rhombosolea tapirina), as a new species for aquaculture in temperate Australia, is currently under investigation. Isolation of an endemic temperate rotifer, Testudinella sp., provided the opportunity to assess this genus against standard live prey species used in aquaculture. Greenback flounder larvae were offered a range of rotifer prey species ( Testudinella sp., Brachionus rotundiformis, Brachionus plicatilis) and the brine shrimp Artemia, either in isolation or combination, in order to test the hypothesis that factors besides prey size affect prey selection. From first-feeding to approximately day 13 post-hatching, greenback flounder larvae preferentially consumed the rotifer Testudinella sp. over the larger width rotifer B. plicatilis and to a lesser extent the rotifer of similar width dimension, B. rotundiformis. From day 15 post-hatching, consumption (number of prey consumed per larva per unit time interval) and selection ( α) of B. plicatilis was significantly higher than of Testudinella sp. and Artemia, but by day 18 post-hatching, there was a shift of feeding preference to Artemia. This indicated that size of prey ingested increased with age, likely reflecting an ontogenetic increase in larval sensory function, mouth gape and prey capture and handling ability. Significant selection of Testudinella sp. over the larger rotifer B. plicatilis occurred despite greenback flounder larvae being able to consume B. plicatilis. This along with significant selection (day 11 post-hatching) and consumption (days 11 and 12 post-hatching) of Testudinella sp. over the rotifer of similar width B. rotundiformis, indicates that prey size is not the sole determinant of prey consumption and suggests that greenback flounder larvae exhibit both species-specific and size-specific prey selection. Understanding the basis of prey selection by larval fish predators is essential for enhancing prey consumption in culture, especially in the context of ontogenetic shifts in prey species preference.

Distribution, extent, and evolution of plant consumption by lizards

AbstractAnimal diets crucially affect fitness, yet many aspects of their ultimate determinants are unknown. The distribution and extent of herbivory in lizards, its evolutionary history, and ecological factors that may favour it are discussed. Most lizards are exclusively or primarily carnivorous, yet many species eat some plants and a few are almost exclusively herbivorous. Based on a literature survey of diets of over 450 lizard species, the distribution and degree of omnivory and herbivory are described. Some plants occur in the diets of slightly over half of lizard species, and plants formed 10% or more of the dietary volume of 12.1% of species, and 90% or more of the diet of 0.8% of species. The greatest percentage of omnivorous species (&gt; 10% plant diet), over 30% in each, and highest mean percentage plant matter in the diet are in Iguanidae, Corytophanidae, Gerrhosauridae, Agamidae, Xantusiidae, and Tropiduridae. Numerous other omnivores occur in Lacertidae and Scincidae and fewer in several additional families. Herbivorous lizards (&gt; 90% plant volume) tend to be folivorous and to possess adaptations for processing leaves, including specialized dentition for cutting or reducing leaves, elongated intestines, colic valves that slow passage of food, and intestinal flora that digest cellulose. Omnivorous lizards lacking such specializations may eat some leaves, but consume much more fruit, flowers, and seeds, plant parts that are easy to digest, likely to be very abundant seasonally, and may be highly nutritious. Some lizards eat nectar and pollen; even sap is eaten by at least one gecko. Ontogenetic increase in plant consumption and decrease in prey consumption is known, but its generality has been controversial. Such ontogeny has been demonstrated in three iguanid species, a skink, a lacertid, two tropidurids, a phrynosomatid, and two corytophanids, but it does not occur in some other species. The importance of specific foods may vary with age. Omnivory and/or herbivory have originated in many lizard families, with at least nine origins in Iguania and 23 in Scleroglossa. Origins have been rare in Gekkonoidea and Anguimorpha and common in Scincomorpha, especially in Lacertidae and Scincidae. Losses of omnivory have been much less frequent than gains. Only a few origins can account for all the herbivory in lizards. Concentrated changes tests show that there is a significant association in Lacertidae, Lacertiformes, Lacertoidea, Scincidae, and Scleroglossa between insularity and omnivory. Insular lizards may broaden their diets to compensate for limited availability of prey. Addition of other factors that reduce availability of prey, i.e. extreme aridity and cave‐dwelling, to insularity, strengthened the relationship to omnivory in Lacertidae and Lacertoidea. We were unable to demonstrate a role of aridity independent of insularity, but present anecdotal evidence suggests that it may promote evolution of plant consumption. Large body size in lizards has long been associated with herbivory, and more recently, with omnivory in lacertid lizards. Using a conventional regression approach in which each species is considered to supply an independent data point, this relationship was confirmed for all lizards. Although larger species have diets with more plants, plant consumption accounts for only 9% of the variation in body length, which is not surprising given that other factors such as predation, competition, and sexual selection affect body size. The frequency of transitions body size associated with transitions to omnivory or carnivory was also examined. In Iguania, Scleroglossa, and all lizards, transitions supporting the hypothesis that omnivory favours increase in body size were significantly more frequent than non‐supporting transitions. This suggests that substantial plant consumption favours evolution of larger size, probably because of the energetic considerations first presented by Pough (1973). Because actively foraging lizards move widely through the habitat to locate prey and tongue‐flick to locate prey by chemical cues, we hypothesized that they may be more likely to evolve omnivory than ambush foragers, which wait motionless for prey and do not tongue‐flick to locate or identify prey. The basis of this prediction is that the wider seaching of active foragers predisposes them to contact with a greater variety and quantity of plants and that chemosensory tongue‐flicking used by omnivores to identify plant food might be easier to evolve in active foragers that already use pre‐chemical discrimination. The prediction is supported by a significantly greater per species frequency of origins of omnivory by active foragers than by ambushers. A scenario for the progressive evolution of omnivory and herbivory from ancestrally carnivorous lizards is discussed.

Diet and endogenous carbohydrases in the temperate marine herbivorous fish Kyphosus sydneyanus

Stomach content analysis showed that Kyphosus sydneyanus collected by spearing from coastal reefs near Leigh, north‐east New Zealand, is herbivorous throughout its post‐settlement life. All size classes collected consumed a wide range of macroalgae, predominantly the phaeophytes Carpophyllum maschalocarpum and Ecklonia radiata, and the rhodophytes Gigartina macrocarpa and Caulacanthus ustulatus. Multivariate analysis of variance and discriminant function analysis showed an ontogenetic diet shift, with gut contents dominated by rhodophytes and chlorophytes in juveniles and by phaeophytes in adults. No significant seasonal variation in diet was apparent for any size class. Amylase and amylopectinase activity was highest in the smaller size classes. Laminarinase activity was relatively low throughout all size classes, and did not correlate with the ontogenetic increase in phaeophyte consumption. The activity of endogenous enzymes hydrolysing starch broadly correlated with rhodophyte consumption in adult specimens of three other species of marine herbivorous fishes: Odax pullus, Girella tricuspidata and Parma alboscapularis; laminarin hydrolysis, however, was negatively correlated with phaeophyte consumption. Overall, the diet and enzyme results suggest that juvenile K. sydneyanus rely mainly on the endogenous digestion of rhodophytes and chlorophytes, while exogenous (microbial) digestion is used to digest the phaeophyte‐dominated diet of adults.

Diet and endogenous carbohydrases in the temperate marine herbivorous fish Kyphosus sydneyanus

Stomach content analysis showed that Kyphosus sydneyanus collected by spearing from coastal reefs near Leigh, north-east New Zealand, is herbivorous throughout its post-settlement life. All size classes collected consumed a wide range of macroalgae, predominantly the phaeophytes Carpophyllum maschalocarpum and Ecklonia radiata, and the rhodophytes Gigartina macrocarpa and Caulacanthus ustulatus. Multivariate analysis of variance and discriminant function analysis showed an ontogenetic diet shift, with gut contents dominated by rhodophytes and chlorophytes in juveniles and by phaeophytes in adults. No significant seasonal variation in diet was apparent for any size class. Amylase and amylopectinase activity was highest in the smaller size classes. Laminarinase activity was relatively low throughout all size classes, and did not correlate with the ontogenetic increase in phaeophyte consumption. The activity of endogenous enzymes hydrolysing starch broadly correlated with rhodophyte consumption in adult specimens of three other species of marine herbivorous fishes: Odax pullus, Girella tricuspidata and Parma alboscapularis; laminarin hydrolysis, however, was negatively correlated with phaeophyte consumption. Overall, the diet and enzyme results suggest that juvenile K. sydneyanus rely mainly on the endogenous digestion of rhodophytes and chlorophytes, while exogenous (microbial) digestion is used to digest the phaeophyte-dominated diet of adults.

Photosynthetic differences between saplings and adult trees: an integration of field results by meta-analysis.

Ontogenetic changes in gas exchange parameters provide both insight into mechanisms underlying tree growth patterns, and data necessary to scale environmental impacts on young trees to predict responses of older trees. We present a quantitative review and meta-analysis of field measurements of gas exchange parameters in saplings and mature trees of 35 tree species (seven conifers, seven temperate deciduous trees, and 21 tropical evergreen trees). Data for saplings were obtained in both understory environments and open areas or large gaps. We also present data on ontogenetic changes in photosynthesis for Pseudotsuga menziesii (Mirb.) Franco and Tsuga heterophylla (Raf.) Sarg., species of particular interest because of their large maximal heights and long life-spans. Among tree species, there is evidence for both ontogenetic increases and ontogenetic decreases in photosynthetic capacity on a leaf area basis (A(area)). Overall, A(area) is generally higher for upper-canopy leaves of adult trees than for saplings, especially in temperate deciduous trees. However, the pattern for photosynthetic capacity on a leaf mass basis (A(mass)) is the reverse of that observed for A(area). Saplings of both conifers and broad-leaved trees, even when acclimated to low-light conditions, characteristically have a higher A(mass) than adult trees. This pattern is driven largely by an ontogenetic increase in leaf mass per unit area (LMA), as found in 100% of studies reviewed. Data for Pacific Northwest conifers, although including measurements on some of the tallest trees studied, did not differ greatly from patterns found in other tree species. We conclude that ontogenetic changes in LMA are the single most consistent difference between saplings and adult trees, and that changes in LMA and related aspects of leaf morphology may be critical to understanding both variation in gas exchange during tree growth, and stage-dependent responses of trees to environmental change.

Number of cells in tomato fruit depending on fruit position and source-sink balance during plant development

Fruit sink strength or its ability to attract assimilates depends bothon sink activity and size. This study investigated one main component of sinksize, that is the number of fruit cells during tomato plant development. Plantswere grown in a controlled climate chamber under a limiting (LS, six fruits pertruss) and non-limiting (NLS, two fruits per truss and CO2enrichment) supply of carbon assimilates. Under NLS conditions, fruit cellnumber was homogeneous among successive trusses, and fruits contained onaverage1.2 × 106 more cells than under LS conditions,though differences were not significant on the first truss which underwent thelowest competition. Under LS conditions, an ontogenetic increase in cell numberwas observed in proximal fruits of the upper trusses attributed to theenlargement of the apical meristem during plant development. The decrease ofcell number from proximal to distal fruits within a truss, that was expectedfrom the literature, was generally observed in the LS experiment, with anaverage significant difference of about1.6 × 106cells between the first and fifth fruits. Nevertheless, whereas the gradient incell number from proximal to distal fruits was steep in the upper trusses, itwas not significant on the lower trusses indicating that this gradient largelydepended on the level of competition during floral development. Thus, under lowassimilate supply, cell division is a main limiting factor for fruit growth,although cell enlargement during further fruit development is also affected,butwas not measured in this work.

Inter‐ and intra‐annual variability in the phytoplankton community of a high mountain lake: the influence of external (atmospheric) and internal (recycled) sources of phosphorus

1. The inter‐ and intra‐annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice‐melting and atmospheric deposition (external).2. Inter‐annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P‐recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P‐recycling.3. Year‐to‐year variations in seston C : P and N : P ratios exceeded intra‐annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice‐out (C : P &lt; 100 and N : P &lt; 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios.4. Intra‐annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice‐out, when the zooplankton community was dominated by copepod nauplii, and high towards mid‐ and late‐season, when these were replaced by copepodites and adults.5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice‐melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (&gt; 1000%), zooplankton recycling (9–542%), and ice‐melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition.6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice‐melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load.7. The elemental composition of the zooplankton explains changes in phytoplankton taxonomic and elemental composition. The elemental negative balance (seston N : P &lt; zooplankton N : P, low N : P recycled) during the thaw, would promote a community dominated by species with a high demand for P (Cryptophyceae). The shift to an elemental positive balance (seston N : P &gt; zooplankton N : P, high N : P recycled) in mid‐season would skew the N : P ratio of the recycled nutrients, favouring dominance by chrysophytes. The return to negative balance, as a consequence of the ontogenetic increase in zooplankton N : P ratio and the external P inputs towards the end of the ice‐free season, could alleviate the limitation of P and account for the appearance of other phytoplankton classes (Chlorophyceae or Dinophyceae).

Inter- and intraspecific trait compensation of defence mechanisms in freshwater snails.

Trait compensation occurs when mechanically independent adaptations are negatively correlated. Here, we report the first study to demonstrate trait compensation in predator-defence adaptations across several species. Freshwater pulmonate snails exposed experimentally to predation chemical cues from fishes and crushed conspecifics showed clear interspecific differences in their behavioural avoidance responses, which were negatively correlated with shell crush resistance. The type of avoidance response varied between species: thin-shelled species (Lymnaea stagnalis and Physa fontinalis) moved to the water-line or out of the water, while those with thick shells moved under cover or showed a mixed response. There were also intraspecific size-linked differences, with an ontogenetic increase in shell strength accompanied by a decrease in behavioural avoidance. Such trait compensation in response to predation has important implications for interspecific interactions and food-web dynamics.

GETTING TO THE CANOPY: TREE HEIGHT GROWTH IN A NEOTROPICAL RAIN FOREST

There is still limited understanding of the processes underlying forest dy- namics in the world's tropical rain forests, ecosystems of disproportionate importance in terms of global biogeochemistry and biodiversity. Particularly poorly documented are the nature and time scale of upward height growth during regeneration by the tree species in these communities. In this study, we assessed long-term height growth through ontogeny for a diverse group of canopy and emergent tree species in a lowland neotropical rain forest (the La Selva Biological Station, northeastern Costa Rica). Species were evaluated based on annual height measurements of large samples of individuals in all postseedling size classes, over a 16-yr period ( .11 000 increments). The study species were seven nonpi- oneers (Minquartia guianensis, Lecythis ampla, Hymenolobium mesoamericanum, Sima- rouba amara, Dipteryx panamensis, Balizia elegans,and Hyeronima alchorneoides ) and two pioneers (Cecropia obtusifoliaand Cecropia insignis). For each species, inherent height growth capacity was estimated as the mean of the five largest annual height increments (from different individuals) in each juvenile size class (from 50 cm tall to 20 cm in diameter). All species showed marked ontogenetic increases in this measure of height growth potential. At all sizes, there were highly significant differences among species in height growth potential. The two Cecropia species consistently showed the highest observed maximum height increments as smaller juveniles (#4 cm in diameter). Among the nonpioneers, Sim- arouba had the highest growth potential across all juvenile sizes. For all species, mean annual height increments in all juvenile size classes were very much lower than the species' potential growth rates and reflected the impacts of frequent periods of zero growth and major height losses from physical damage. Because of the same factors, maximum net height increments declined over increasing measurement intervals (1-15-yr periods). With only one exception (Simarouba amara saplings #1 cm in diameter that survived 10 yr), the annual height growth increments of these species showed no significant temporal au- tocorrelation. For the seven nonpioneer species, we estimated the minimum time required to grow from 50 cm tall saplings to 10 cm diameter trees, based on their greatest net height increments over increasing intervals. Estimated passage times increased from 7-33 yr, when based on maximum 5-yr height increments, to 37-83 yr, when based on maximum net height growth over 15 yr for the six longest studied (nonpioneer) species. Given the erratic height growth trajectories found for these juvenile trees in this 16-yr study, it is clear that much longer term observation is required. Still unknown are the actual growth trajectories that characterize successful regeneration in each of our focal species, how much time successful trees spend as suppressed juveniles, and the number of times they experience and recover from major physical damage.

Oxygen Diffusion Coefficient in Isolated Chicken Red and White Skeletal Muscle Fibers in Ontogenesis

Oxygen diffusion from medium to cultured isolated muscle fibers from red gastrocnemius muscle (deep part) (RGM) and white pectoralis muscle (WPM) of embryonic and postnatal chickens (about 6 months) was explored. The intracellular effective O2 diffusion coefficient (Di) in muscle fiber was calculated from a model of a cylindrical fiber with a uniform distribution of an oxygen sink based on these experimentally measured parameters: critical tension of O2 (PO2) on the surface of a fiber, specific rate of O2 consumption by a weight unit of muscle fibers (VO2), and average diameter of muscle fibers. The results document the rapid hypertrophic growth of RGM fibers when compared to WPM fibers in the second half of the embryonic period and the higher values of VO2 and critical PO2 during the ontogenetic period under study. The oxygen Di in RGM fibers of embryos and 1-day chickens was two to three times higher than observed for WPM fibers. For senior chickens, the oxygen Di value in RGM and WPM fibers does not differ. The Di of O2 in both RGM and WPM fibers increased from 1.4–2.7 × 10−8 to 90–95 × 10−8 cm2/s with an ontogenetic increase in fiber diameter from 7.5 to 67.0 μm. At all stages the oxygen Di values in RGM and WPM fibers are significantly lower than the O2 diffusion coefficient in water: for 11-day embryos they are 889 and 1714 times lower and for adult individuals 25 and 27 times lower, respectively. Why oxygen Di values in RGM and WPM fibers are so low and why they are gradually increasing during the course of hypertrophic ontogenetic growth are still unclear.

Glucocorticoid Receptor Regulation in the Rat Embryo: A Potential Site for Developmental Toxicity?

Glucocorticoids play a key role in controlling numerous cellular processes during embryogenesis and fetal development. Excess glucocorticoids during development have been linked to dysmorphogenesis and/or intrauterine growth impairment in rodents. The actions of glucocorticoids are mediated by interaction with their receptors. Negative feedback regulation of glucocorticoid receptor (GR) is important for limiting cellular sensitivity to the hormones. Hence, acute exposure of the adult rat to the synthetic glucocorticoid dexamethasone (DEX) reduced both GR mRNA and protein in a variety of tissues that include hippocampus and liver, in a dose- and time-dependent fashion. Reduction in GR mRNA and protein were observable when DEX was given repeatedly at doses as low as 0.05 mg/kg. In the control whole rat embryo, GR mRNA was low but measurable at as early as gestational day (GD) 10, but underwent rapid ontogenetic increase in the ensuring days. In contrast to the adult, neither GR mRNA nor protein in the whole rat embryo was affected by acute or repeated DEX administration to pregnant rats on GD10–13, even at doses as high as 0.8 mg/kg. Similar results were obtained in embryonic palate and liver, tissues known to be glucocorticoid targets. These data suggest that GR autoregulation does not occur during organogenesis in the rat. Accordingly, hormonal elevations from stress or chemical insults can be transduced unrestrictedly, ultimately leading to aberrant cell function and development. The unique mode of GR regulation seen in the embryonic cells may provide a potential common mechanism for developmental perturbation and toxicity for a variety of insults.

Ontogeny of G-protein expression: control by β-adrenoceptors

Cardiac cell homeostasis is maintained in the face of excessive β-adrenoceptor stimulation through the process of desensitization. Desensitization is not an inherent property of these cells but rather is acquired during development; neonates given β-agonists actually show heterologous sensitization, involving changes in the expression and catalytic activity of adenylyl cyclase (AC) as well as an increased receptor/G-protein coupling. The current study examines the role of specific G-protein components, G sα and G iα, in the ontogeny of β-adrenoceptor responses and in the transition from agonist-induced sensitization to desensitization. Between postnatal days (PN) 6 and 15 there was a significant decrease in the 52 kDa isoform of G sα with no accompanying change of the 45 kDa form; over the same period, G iα3 also declined substantially. In contrast, the 45 kDa isoform of G sα and G iα1,2 remained fairly constant over the same period and fluoride-stimulated AC activity increased. Treatment with isoproterenol on PN2–5 did not result in any significant changes in G sα expression but robustly decreased G iα1,2. These changes were accompanied by heterologous sensitization of AC activity at the level of AC itself, evidenced by equivalent increases in the enzymatic response to fluoride and forskolin–Mn 2+. Isoproterenol given to older animals (PN11–14) also caused specific loss of G i protein, in this case targeting G iα3, whereas G sα again was unchanged; in contrast to the younger group, the older animals displayed heterologous desensitization of AC at the level of G-protein function (specific loss of the fluoride response). These results indicate that the normal ontogenetic increase of cardiac β-adrenoceptor coupling to AC is not dependent on the absolute amount of G-proteins, nor on the relative balance of stimulatory (G s) and inhibitory (G i) subunits. However, the ability of receptor stimulation to downregulate G iα1,2, an event which is specific to immature cardiac cells, is likely to be an important component of the resistance of the fetal/neonatal heart to agonist-induced desensitization and hypertrophy. The maintenance of cardiac β-adrenoceptor signaling in the face of intense stimulation is likely to play an important role in the physiologic adaptations necessary to the perinatal transition.