ALBERT

Notes: Flicker noise in c-axis oriented long YBCO bicrystal grain boundary junctions was characterized as a function of temperature, biasing conditions, and magnetic field applied perpendicular to the a–b plane over a wide range of temperatures from 15 K to over 70 K. Aperiodic variations, as a function of magnetic field, were observed in both the junction voltages, VJ, and the flicker noise magnitude under constant current bias as the magnetic field was scanned from 0 to 8 G. The noise magnitudes were found to peak at the minima of VJ. Analyses of the field dependencies of the magnitudes and the functional form of the voltage noise power spectra show that the noise did not arise from thermally activated flux motion. Based on the dependencies of the noise power spectra on the bias current and the dynamic resistance of the junction, we conclude that the noise originates from the fluctuations of the critical current of the devices most likely due to trapping of carriers or defect motion within the grain boundary. © 1995 American Institute of Physics.

Notes: [Auszug] Intraspecific competition should result in an increase in the range of a resource spectrum used by a species, as at high population levels the advantages to any individual of being at the competition-free optimum of a resource gradient are offset by the intense intraspecific competition found there ...

Notes: Summary This paper examines the reproductive cycles of three ecologically important marine bivalves-Modiolus modiolus (L.), Cerastoderma edule (L.), and Mytilus edulis L. in Strangford Lough, Northern Ireland over a period of almost five years. Whilst Modiolus does not appear to become sexually mature until it is several years old, Mytilus and Cerastoderma can reproduce in their first and second years of life respectively. In Cerastoderma and Modiolus sexual maturity is preceded by a period of rapid somatic growth. The subtidal Modiolus population remained in a more or less fully ripe condition virtually throughout the period of this investigation suggesting that this particular population lacked any marked cyclical reproductive activity. We interpret this as evidence of slow but almost continuous release of gametes throughout much of the year, a suggestion which is supported by recruitment data. A small intertidal population of Modiolus in Belfast Lough monitored over a period of two years exhibited a much more seasonal cycle. Here spawning occurred mainly during the autumn and winter. These data suggest that localised environmental factors are exceedingly important in controlling the annual reproductive cycle of this species. Cerastoderma from the mid-tidal sand flats ripened rapidly during the spring and spawned over a relatively restricted period in the summer. In Strangford Lough Mytilus occurs predominantly in the low-shore and while it spawns mainly in the spring and summer the annual cycle is considerably more protracted and variable than in Cerastoderma. Variations in the duration of the spawning periods in these bivalves can perhaps be explained in terms of both environmental stability and the immediate physical conditions experienced by these particular populations. The reproductive strategies exhibited by Cerastoderma, Modiolus, and Mytilus in Strangford Lough are considered in relation to population stability and to the different patterns of mortality which characterise these species in their respective local habitats.

Notes: Summary The population structure and apparently erratic distribution patterns exhibited by M. edulis on many open exposed coasts, are discussed in the light of findings relating to the breeding and settlement of this species. In the absence of previous literature concerning open coast mussels, the reproductive cycle has been described in some detail. These mussels do contribute to the spawning stock, and in the three years from October 1964–December 1967, the period of spawning was shown to be particularly extended. Spawning occurred mainly from early spring to late summer, though individuals could be found in the spawning condition more or less throughout the year. No marked or consistent differences in the spawning periods of mussels from different local habitats or amongst animals of different size (=age) were recorded. Sexual maturity was attained in the first year of life. The appearance of larvae in plankton samples confirmed the times of major spawnings recorded from histological preparations of the gonad. Settlement of young mussels (=plantigrades) on existing beds was not direct, there being a period of temporary attachment spent especially upon filamentous substrates. This occurred after about 3–5 weeks of planktonic existence. The majority of plantigrades remained on these primary sites for a period of 4 weeks (i.e. until they measured from 1–2 mm in length), although those settling later in the year often remained there overwinter before migrating on to adult beds. The stimulus to migrate is due to changes in the thigmotactic requirements of small mussels. Whilst filamentous substrates were ideally suitable to early plantigrades, somewhat older individuals preferred the niches and crevices provided on adult beds (especially amongst the byssus threads) and by the small cracks and pits in the rock surface. Since the thigmotactic requirements of all plantigrades were not satisfied at exactly the same time, some migration to adult beds occurred throughout the year, though periods of maximum settlement occurred from 8–10 weeks after spawning. Migration from these extensive “reservoirs” of temporary attachment could account for the sporadic outbursts of settlement recorded on many shores at certain times of the year and which often could not necessarily be predicted on a knowledge of the breeding cycle alone. Although the extended settlement period is an important contributory factor, it is not thought that this alone could account for the distinctive population structure in this species. At settlement, mussels are particularly gregarious, attraction of plantigrades to adult beds being essentially a thigmotactic response, aided by their ability to attach and detach themselves until favourable situations are encountered. Settlement is favoured by roughened, scarred or pitted surfaces and the distribution patterns on many shores could partly be attributed both to the surface texture or topography of the shore (smooth, rapidly draining shores being particularly unsuitable) or the amount of local wave splash. In a subsequent publication, the population structure and distribution patterns will be examined in the light of growth and mortality rates of this species on exposed shores.

Notes: Summary Growth studies in M. edulis L. have shown that rates vary considerably according to age, size and environmental conditions. This may in part be attributed to its sessile habit, being unable to move away from the variable external conditions. The use of modal length frequency distributions is somewhat limited, since with three or more year groups represented, the growth of the majority of the population is so slow that individual year classes lose their identity. Growth boxes, containing marked animals of different ages, and set out in a variety of habitats gave information regarding local and seasonal growth rates. Disturbance rings were shown to be annual, and from them growth curves have been constructed. Growth is particularly seasonal, little or none occurring during the winter. Growth rates varied considerably with a variety of environmental factors (both biotic and physical) and some of these are discussed. Variable individual growth rates, together with slow growth of the majority of animals in mixed populations, are perhaps the major factors in producing population structures typical of this species on open shores. Survivorship curves for mussels in a variety of habitats have been constructed by following the survival of groups of marked animals. Whilst high mortalities occurred in the mid and low shore, survival in the upper shore in the absence of major predators, was greatly enhanced, resulting in established populations of considerable age. Periods of maximum mortality during spring and summer could be correlated with the abundance of major predators. The almost cosmopolitan distribution of M. edulis in the N. hemisphere has been made possible by virtue of its high reproductive capacity, successful larval dispersion and wide tolerance of environmental conditions, Its patchy and apparently erratic distribution both from one shore to annother and even on the same shore, is greatly influenced by the local and seasonal abundance of major predators. Whilst the upward extension of mussels is prevented, ultimately, by physical factors (e.g. temperature, dessication), its lower limits (and absence from the shallow sub littoral on many shores) are governed essentially by predators. The upward extension of many predators such as crabs or starfish, resulting in locally intense predation, may in turn be partly influenced by the actual topography of the shore itself.