Low dissolved oxygen levels reduce anti-predation behaviours of the freshwater clam Corbicula fluminea

1. When dissolved oxygen levels decline in aquatic systems, prey may be unable to maintain behaviours protecting them from predators. We examined how oxygen availability affected anti‐predator responses in the freshwater clam, Corbicula fluminea. 2. When attacked, bivalves protect their soft tissues by closing their protective valves. This reduces vulnerability to small predators, but ventilation and oxygen uptake are suspended. We found that after a simulated attack, clams under low oxygen conditions reopened their valves sooner than clams under high oxygen conditions, suggesting that hypoxia increases vulnerability to predation. 3. Bivalves may also evade predators through burial into the substratum. Deeper burial confers greater refuge from predators, but increases the costs of ventilation. In a second experiment, we studied how burial depth of C. fluminea is affected by oxygen availability. Additionally, we examined whether clams changed burial depth following a simulated attack by a small predator, and whether this response was affected by oxygen availability. Our results offered partial support for the hypothesis that burial depth is reduced under hypoxic conditions, but the simulated attack did not affect burial depth in any oxygen treatments.     

Foraging behaviour influences the outcome of predator–predator interactions

Abstract.  1. Interactions among predators may influence the total efficiency of a predator complex. The effect of intra- and interspecific interactions of the generalist predators Orthotylus marginalis (Heteroptera: Miridae) and Anthocoris nemorum (Heteroptera: Anthocoridae) was investigated in a laboratory experiment. Outcomes of the interactions were determined by comparing predation rates on eggs and larvae of the blue willow beetle Phratora vulgatissima of single individuals with those of two individuals of the same or different species.
2. A non-additive, antagonistic effect on predation rates due to intraspecific interactions was found between individuals of A. nemorum . No such effect was found in O. marginalis . These results are as expected as a consequence of differences in behaviour of the two predator species: A. nemorum is a much more active and mobile predator than O. marginalis .
3. Contrary to expectation, interspecific interactions between A. nemorum and O. marginalis did not affect the total predation rate.
4. An observation from the field corroborated the results obtained in the laboratory study; there was no negative relationship between the densities of the two predator species, indicating that the two species do not interact negatively in the field at their natural densities.
5. It is concluded that the additive effect of multiple predator species is of potential value in biological control.     

Location and distribution of symbiotic bacteria during floral development in Ardisia crispa

Abstract. The location and distribution of symbiotic bacteria during floral development in Ardisia crispa (Thunb.) A.DC., a species characterized by bacterial leaf nodules, has been studied using light, scanning and transmission electron microscopy. During early floral development, bacteria in mucilage derived from host plant trichomes, become enclosed in a small conical chamber on top of the placenta, as a result of the closure and fusion of the carpel initials. The placental epidermal cells, which appear to be secretory in nature, become detached apically in places forming a network of grooves which traverse the placental surface. The symbiotic bacteria are preferentially located in these grooves. As growth and development of the placenta proceed, the grooves widen and deepen to form channels. The cells lining these channels secrete a mucilaginous material. The network of channels covers the entire placental surface and terminates at the placental margins surrounding the ovules. Bacteria are found within the channels, at the ends of the channels near the margin of the placenta, on the surface of the ovules and in the micropyle. It is suggested that these mucilage-filled channels are responsible for, and a prerequisite of, ensuring that the bacterial partner is efficiently transmitted from one host generation to the next by providing a mechanism by which the bacteria arc accurately placed within the developing seed.     

Gramicidin A-phospholipid model systems

Gramicidin A forms ion-conducting channels which can traverse the hydrocarbon core of lipid bilayer membranes. The structures formed by gramicidin A are among the best characterized of all membrane-bound polypeptides or proteins. In this review a brief summary is given of the occurrence, conformation, and synthesis of gramicidin A, and of its use as a model for ion transport and the interaction of proteins and lipids in biological membranes.     

Effects of essential divalent metals on carcinogenicity and metabolism of nickel and cadmium

Interactions between the physiologically essential metals calcium, magnesium, and zinc and the carcinogenic metals nickel and cadmium were investigated to help elucidate the mechanisms of action of the carcinogenic metals. Bioassay studies revealed several significant findings, including: (1) the ability of magnesium and calcium to inhibit nickel-induced elevation of pulmonary adenoma incidence in strain A mice; (2) the ability of magnesium, but not of calcium, to prevent cadmium-induced subcutaneous sarcoma formation; and (3) the ability of magnesium, but not of calcium, to inhibit nickel-induced muscle tumor formation. Biochemical studies indicated a direct relationship between the antitumorigenic potential of magnesium and the capacity of this metal to: (1) inhibit nickel and cadmium uptake by the target tissues in vivo; (2) inhibit nickel-induced disturbances in DNA synthesis in vivo; (3) inhibit nuclear and cytosolic uptake of nickel by the target tissue cells in vivo; and (4) inhibit nickel and cadmium binding to DNA in vitro. Calcium, which in most cases did not prevent carcinogenesis, had no consistent influence on the uptake of carcinogenic metals or their biochemical effects in the target tissues. Magnesium and zinc, but not calcium, were also found to attenuate the acute toxic effects of nickel, indicating a possible correlation between prevention of acute effects and reduction in tumorigenicity. Zinc, which antagonizes cadmium tumorigenicity in the rat testis, was found to reduce markedly cadmium uptake into isolated testicular interstitial cells. Also, zinc was found to inhibit strongly cadmium binding to DNA in vitro.     

The coupling of biogeochemical cycles of nutrients

For any element which is incorporated into biomass, the biogeochemical cycle of that element in a given ecosystem will be coupled to that of any other element similarly incorporated. The mutual interaction of two such cycles is examined using a simple model in which each cycle is constrained into four compartments. In each cycle the assimilation rate (primary productivity) is related in a non-linear fashion to the two nutrients and to biomass. The interactions are represented by combining a hyperbolic dependence for each nutrient (involving a "Michaelis constant") with a logistic equation governing the dependence of rate on biomass (involving a "carrying capacity"). The response of the model to perturbation (e.g. mobilization of an abiotic reserve) is strongly governed by the values assigned to these constants. The coupled cycles can exhibit positive feed-back with anomalous responses of the steady state and time-dependent solutions may exhibit complex oscillatory behaviour. Both the steady-state sensitivity and the kinetic behaviour of such coupled systems are simplified if the range of atomic ratios permitted by the assimilation process is restricted. It will therefore be of importance to determine under what conditions the assimilation rates for different elements are governed by mass-action effects (Liebig's Law) or by stoichiometric constraints (Redfield ratios).     

Three-dimensional structure of deoxycholate-treated purple membrane at 6 Å resolution and molecular averaging of three crystal forms of bacteriorhodopsin

The three-dimensional structure of the deoxycholate-treated form of purple membrane has been determined to a resolution of about 6 Å. Using low temperature electron diffraction data, room temperature electron microscope images and improved methods of data analysis, higher resolution has been reached than was obtained using native membranes of the same size. Statistical analysis of the data shows that the new map is considerably better than earlier maps. The map indicates the probable sites for the lipid molecules that remain in the deoxycholate-treated membranes; some of these sites differ from those suggested by the projection map of Glaeser et al. (1985). Comparison of the bacteriorhodopsin structures now determined independently from three crystal forms shows that the monomer structure is independent of the detailed contacts with lipid molecules. The average of the three structures gives a picture with very little noise showing seven similar rod-like features which are clearly best interpreted as -helices; there is no indication that part of the structure is -sheet as suggested by Jap et al. (1983). Phases from the averaged structure at 6 Å resolution will enable better refinement of the parameters that will be required in the analysis of higher resolution images from tilted specimens needed to extend the projection map at 3.5 Å resolution (Henderson et al. 1986) to produce a three-dimensional atomic resolution map.     

Flower change in Oenothera drummondii Hooker as a response to pollinators' visits

Flower change in Oenothera drummondii Hooker as a response to pollinators' visits. The colour and shape of flowers of Oenothera drummondii change owing to senescence, pollination and/or nectar withdrawal. This phenomenon is interpreted as a signal emitted by the plant to 'inform' the potential pollinator of its new status as a rewardless flower. This method of signalling might be important for the pollinators in order to save energy in seeking the correct flowers and for the plant to ensure visits to those flowers that have not yet been visited.     

Effects of clonal variation of the host plant,interspecific competition,and climate on the population size of a folivorous thrips

Summary The effects of clonal variation, interspecific competition, and climate upon the population size of Apterothrips secticornis was assessed by a series of observations and experimental manipulations. Three clones of the host plant, Erigeron glaucus, consistently supported different numbers of thrips during monthly censuses. When rosettes of the three clones were transplanted to a common garden, relative numbers of thrips on the clones remained the same as those observed where the clones grew in situ. The presence or absence of other hervivores had no effect on thrips numbers in the common garden. Plume moth caterpillars and thrips were observed to co-occur less often than expected in the field but this was caused by differences in habitat selection by these two species rather than being the result of interspecific competition. Populations of thrips were affected by climate, but analyses suggest that the host clone was a more important factor.     

Experimental studies of exploitative competition in a grazing stream insect

Summary Field and laboratory experiments were conducted to determine whether intraspecific competition for food occurs during the larval stage of the periphyton-grazing caddisfly Glossosoma nigrior (Trichoptera: Glossosomatidae). Larvae were placed in field enclosures at densities less than, equal to, or greater than their natural densities. Most of these individuals began to pupate after 3 weeks, whereupon the mass of each individual was determined. Final mass declined significantly as larval densities increased, whereas neither developmental rate nor mortality/emigration rate was significantly affected by density manipulations. a supplemental experiment comparing the final mass of individuals grown at reduced densities in a laboratory stream with individuals from a natural stream bottom confirmed the results of the more extensive field experiment: reductions in density resulted in significant increases in final mass. Periphyton availability in field enclosures declined according to a negative exponential function as larval densities increased. Over the 25-fold range of larval densities used in these experiments, the final mass of individuals increased linearly with periphyton standing crops. This result suggests that Glossosoma larvae may compete for food even at densities below those employed in this study. Path analysis was used to explore the importance of indirect (i.e., exploitative) and direct (i.e, interference) mechanisms for the observed competitive effects. The analysis indicates that a model based solely on exploitation explains nearly as much of the variance in mass as a model incorporating both interference and exploitation.