Prey behaviour and size-selective predation by fish

SUMMARY 1. We investigate the role of differential activity of chiro-nomids as an explanation for the size selection of small larvae by fish.
2. In the laboratory, pumpkinseed sunfish (Lepomis gibbosus L.) selectively consumed large larvae of Chironomus tentans Fab. when no tube-building materials were available. Small larvae were selectively consumed when a mud substrate was provided but there was no difference in predation rates on large and small larvae in sand.
3. Small larvae spent more time out of their tubes than large larvae in the presence of fish, which may explain the selection for small larvae in mud. Large larvae apparently compensated for decreased foraging activity in the presence of fish by increasing activity when fish were absent. Visibility of large larvae inside tubes may account for their increased mortality in sand.
4. Our results suggest that differential activity is important in explaining the size-selective mortality observed in the field.     

The Physiological Ecology of the Zebra Mussel, Dreissena polymorpha, in North America and Europe

SYNOPSIS. The zebra mussel, Dreissena polymorpha (Pallas), wasintroduced into North America in 1986. Initial North American(N.A.) studies suggested that physiological responses variedbetween N.A. and European populations. However, literature reviewindicates agreement on most aspects of physiological adaptationincluding: respiratory responses; hypoxia/anoxia tolerance;salinity limits; emersion tolerance; freezing resistance; environmentalpH limits; calcium limits; starvation responses; and bioenergeticpartitioning. The main differences among N.A. and European musselsappear to be elevated upper thermal limits and temperaturesfor optimum growth among N.A. populations. N.A. zebra musselsprobably originated from the northern shore of the Black Seain the warmest portion of the mussel's European range. However,most European physiological data come from northern Europe wherepopulations may be adapted to colder temperatures. Alternatively,N.A. research suggests that mussels may have a capacity forseasonal temperature acclimatization such that responses recordedin warmer N.A. waters may be different from those recorded innorthern Europe even after short-term laboratory acclimation.Studies of genetic variation and physiological response amongEuropean and N.A. D. polymorpha populations are required toelucidate the basis for physiological differentiation. Recentlyevolved D. polymorpha has poor resistance adaptations comparedto unionacean and sphaeriid bivalves with longer freshwaterfossil histories. Poor resistance adaptations make it less suitedfor stable habitats, instead, its high fecundities, early maturity,and rapid growth are adaptations to unstable habitats whereextensive resistance adaptations are of little value.     

Recent freshwater oscillatoriacean analogue of the Lower Palaeozoic calcareous alga Angulocellularia

Recent freshwater cyanophyte oncoids from Canandaigua Lake, New York, consist almost entirely of minute irregular, bush-like, micritic masses comparable with the calcareous alga Angulocellularia Vologdin from Lower Cambrian algal-arehaeocyathan bioherms and biostromes of the Siberian Platform and western Mongolia. The Recent specimens arc radially orientated within the oncoids and occasionally contain moulds of axial filaments 15 μm in diameter. Acid-residues of the oncoids yield abundant Schizothrix calcicola. The micritic bush-like masses are interpreted to have formed by the calcification of the sheaths of these cyanophytes. This analogue allows Angulocellularia to be reinterpreted as a calcified oscillatoriacean cyanophyte, rather than a rhodophyte as previously suggested. It indicates that filamentous cyanophytes can produce apparently solid calcareous fossils, not only porostromatc tubes. The occurrence of Angulocellularia has been overlooked in some previous studies of Cambrian and Ordovician bioherms. The generic diagnosis is emended.     

Distribution of Fibronectin and Collagen during Mouse Limb and Palate Development

Indirect immunofluorescence has been used to study the distribution of fibronectin and collagen types I, II, and III in the developing primary and secondary palatal processes and forelimb buds of the Swiss Webster (NIH) mouse. In the palatal processes fibronectin and types I and III collagen are distributed throughout the mesenchyme. Fibronectin is present in the basement membrane, while types I and III collagen are localized in a linear, discontinuous fashion beneath the basement membrane. Fibronectin is not observed in the epithelium, including the presumptive fusion areas. In the forelimb bud these components show a similar distribution prior to chondrogenesis (early day 11). When chondrogenesis commences (late day 11 or early day 12) fibronectin and, to a lesser degree, types I and III collagen are apparently concentrated in the core mesenchyme, suggesting that fibronectin has a role in initiating chondrogenesis, perhaps by increasing cellular aggregation. Type II collagen is observed only in chondrogenic regions. The codistribution of fibronectin and types I and III collagen supports in vitro studies which indicate that cells use fibronectin to bind to collagen in the matrix. The developing chondrogenic regions appear to lose fibronectin gradually, concomitant with the appearance of type II collagen, suggesting that fibronectin is not involved in the maintenance of functional chondrocytes in their matrices.     

Seed dispersal in a cycad,Macrozamia riedlei

The influence of animals on patterns of seed dispersal has not been well explored, despite the importance of seed dispersal in plant population biology. Furthermore, the exploitation of animals as dispersal agents has been considered one reason for the replacement of gymnosperms by angiosperms in the Cretaceous. We studied seed dispersal in a cycad, Macrozamia riedlei, and showed that in the absence of animals, ripe seeds drop directly beneath the parent not more than 40 cm from the base. Several animals eat the fleshy outer integument of the seeds, and possums (Trichosurus vulpecula), transport seeds up to 24 m from the parent. The postdispersal positioning of seeds gave a log density-log distance relationship of y = 7.20 – 2.85x (r²= 0.96, N = 17). Effective local dispersal may have contributed to the persistence of cycads in the world flora for the last 200 million years.     

Inotropism and Contracture of Aplysiid Ventricles as Related to the Action of Neurohumors on Resting and Action Potentials of Molluscan Hearts

The postsynaptic actions of neurohumors on molluscan musclemay be exerted through control of force as well as by meansof excitation and inhibition. The control of force may appearas potentiation of the response to excitation, as increasedinotropism in spontaneous contractions, or as an increase intonus. We have directed our attention to the alterations offorce induced in aplysiid ventricles by applied postulated neurohumors.The results are interpreted in terms of the known effects ofneurohumors on resting potential and action potentials of molluscanhearts. We recorded compound membrane potentials of aplysiidventricles extracellularly, using a single sucrose gap apparatustogether with a force-displacement transducer to measureforcein contractions or contracture. Aplysia californica ventriclehas a membrane potential of –52.5 ± 9.4 mV. Ventriclesof Aplysia dactylomela or Aplysia californica are depolarizedby increased concentrations of external potassium ion, withan accompanying contracture. After incubation in calcium-freemedium, KCl contracture-force is directly dependent on calciumion concentration. A depolarization of 8.3 ±2.14 mV inpotassium-free medium is blocked by substitution of lithiumfor sodium in the medium, suggesting an electrogenic sodiumpump. There is a sustained depolarization in low chloride medium,which suggests a significant chloride contribution to the restingpotential. Ventricles of Dolabella auriculana or Aplysia dactylomelaare depolarized by acetylcholine (ACh). Threshold for depolarizationis lower than threshold for contracture-force. The ventricleof A. dactylomela is depolarized by 5- hydroxytryptamine (5HT)with threshold at 10^–9 M and a maximum depolarizationof 30 mV at 10^–4M. Depolarization by 5HT may induce beatingbut does not induce contracture. Ventricles of Aplysia californicaare not depolarized by ACh although beating ventricles are inhibited,and a depolarized ventricle in a tonic contracture may be hyperpolarizedand relaxed by low concentrations. The force of contractionof the ventricle of Dolabella auricularia is dependent on theduration of the plateau phase of the cardiac action potential.The plateau is lengthened by 5HT with an accompanying increasein force of beat, and shortened by ACh, with an accompanyingdecrease in force of beat. The action potential in the ventricleof Aplysia californica is not differentiated into spike andplateau phases, and neither ACh nor 5HT has any marked effecton the form of the action potential. Nevertheless, isolatedventricles of Dolabella auricularia, Aplysia dactylomela, andAplysia californica are all excited by 5-hydroxytryptamine,with a threshold at about 10^–M. Both spontaneous beatingand excitation induced in A. californica ventricles by 5HT areblocked by lack of the sodium ion, which may be responsiblefor pacemaker potentials in molluscan hearts.     

MDMP Action: Degradative Effects on Polyribosomes from Wheat Roots and the Inhibition of Protein Initiation

The rapid action of MDMP in promoting the conversion of polyribosomesinto monoribosomes in wheat roots suggests that the compoundmay interfere with the attachment of ribosomes to messengerRNA. The action is stereospeeffic: the L-lsomer has no effecton polyribosomes and does not compete with D-MDMP with respectto polyribosome degradation or the inhibition of initiation-dependentamino acid incorporation in vitro.