ALBERT

Notes: Summary Deuterium oxide lengthens the period of the endogenous tidal activity rhythm of the sand-beach isopod, Excirolana chiltoni. Heavy water has also been shown to retard the circadian rhythm of deer mice, when added to the animals' drinking water. The average dosage dependence of the effect can be estimated with high precision for both isopod and mouse, and the two values are indistinguishable. A similar slowing of circadian rhythms, due to D2O, has also been reported for an alga, a higher plant, two species of birds and three other rodents. Although data permitting reliable estimates of dosage dependence have not been published for these latter cases, the effect is apparently also of about the same magnitude. This evidence suggests fundamental similarities in the rhythmic mechanisms. Heavy water also produces a reversible slowing of several biological rhythms with periods in the millisecond range: the electric-organ discharge of a gymnotid fish (Stenarchus albifrons); the respiratory cycle of goldfish, as well as of an amphipod (Paraphoxus) and an isopod (Excirolana); and the cardiac cycle of a clam (Donax) and a crab (Emerita). Since these high-frequency rhythms originate in pacemakers dependent on diffusion processes, the experimental results suggest the possibility that long-period biological clocks are also based on diffusion-dependent pacemakers.

Notes: Summary The stimuli which normally synchronize the endogenous tidal rhythm of the isopodExcirolana chiltoni arise from turbulent waves moving across the beach. A phase-response curve for two-h pulses of similar stimuli has been derived from experiments in which individual isopods were treated with vigorous intermittent shaking in a flask of seawater. This response curve differs qualitatively from all results previously obtained by administering pulse stimuli to ordinary circadian rhythms: it is bimodal per circadian cycle, with two intervals of about 6 h duration, during which phase advance of up to 4 h results from treatment, separated by two other 6 h intervals during which phase delay of up to 3 h is evoked. This kind of responsiveness to entraining stimuli is of clear adaptive value for synchronization of a circadian rhythm to the mixed semi-diurnal tidal regime of the isopods' habitat. In addition to inducing phase shifts, this same treatment can strongly modify the persistent pattern of activity, and such effects also depend upon phase of treatment: when administered shortly before or shortly after onset of activity, shaking tends to increase the amount of activity in subsequent cycles; when administered in antiphase (6 to 18 h after activity onset), it tends to decrease the activity in the dominant activity peak, and to transform a unimodal pattern of activity into a bimodal pattern. Such induction of a persistent secondary peak of activity in a previously unimodal circadian pattern demonstrates a plasticity which has not been previously reported in those circadian rhythms which are synchronized to the day-night cycle.

Notes: Summary An extremely elaborate performance, involving endogenous timing with both tidal and lunar frequencies, has been recorded in the locomotor activity of an adult specimen of the intertidal isopod,Excirolana chiltoni. During two months of observation, under constant, non-tidal conditions, this animal showed a persistent tidal rhythm in its swimming activity. Bursts of activity were initially well synchronized with times of tide crest on the shore. The average free-running period of the tidal rhythm was about 24 h 55 min, i.e. about 5 minutes longer than the average period of the tides; thus, the loss of synchrony with the concurrent tides was very gradual. The amount of activity per burst showed a conspicuous pattern of variation, a periodic amplitude modulation which paralleled, in detail, the complex lunar cycle of changes in height of high tide. The free-running period of the bimodal, circa-lunar rhythm of amplitude modulation was one or two days longer than the natural 29-day lunar cycle of tide heights. Each feature of this recording has been qualitatively replicated in activity records from other individuals of this species. Freshly-collectedExcirolana generally show spontaneous bursts of activity at times of tide crest, bursts which are repeated as a persistent tidal rhythm, the period of which commonly departs by only a few minutes from that of the natural tidal cycle. Superimposed on the tidal rhythm is an endogenous monthly pattern of amplitude modulation, which alters the amount of activity per burst. This circa-lunar rhythm has a free-running period between about 26 and 33 days, and generally leads to maximum activity on days of highest of high tides. The net result of the tidal and lunar rhythms is an activity pattern which permits the isopods to recapitulate, in great detail, certain significant ecological aspects of the mixed, semi-diurnal tidal regime of California. The experimental data are not compatible with the hypothesis that uncontrolled environmental factors, such as vibrations from waves, were responsible for the rhythmic behavior of the animals. Neither do the data support the hypothesis that “beats” between the observed tidal rhythm and a hidden daily or circadian rhythm were responsible for the observed circa-lunar rhythm. Furthermore, the pattern of the circa-lunar rhythm cannot be accounted for by a single monthly oscillation in the “excitability” of the animals, such as might be mediated by changes in the level of an excitatory or inhibitory hormone in the circulatory system.

Notes: Summary Dilute solutions of ethyl alcohol lengthen the free-running period of the endogenous tidal rhythmicity of the isopod, Excirolana chiltoni. One-half percent alcohol increases the period, on the average, by about an hour, and the dosage dependence is apparently linear up to 1% alcohol. Since alcohol has also been shown to slow the circadian leaf-movement rhythm of the bean plant, Phaseolus, to about the same extent, the effects of this substance on endogenous biological clocks may represent a general phenomenon. Both this influence of alcohol, and previously reported effects of D2O on biological clocks are consistent with the hypothesis that the fundamental and general process underlying these endogenous rhythms involves an oscillation in cellular electrochemistry. Attempts to produce period values greater than about 26 hours in the isopods' rhythm, by simultaneous administration of alcohol and heavy water (which also slows the rhythm), were unsuccessful because these two substances interact to produce heavy mortality, at doses which, when given separately, permit good survival. Cycloheximide, which can, under some circumstances, slow the circadian rhythm of Euglena, had no significant effect on the tidal rhythm of Excirolana, at doses up to 3 μg/ml. MS 222, an anaesthetic which slows the high-frequency electric-organ rhythm of the fish, Stenarchus, was similarly without effect on the rhythm of Excirolana, at doses up to 150 μg/ml. Variations in total salinity of the aquarium water, ranging from 50% to 150% that of seawater, were also without a measurable effect on the period of the rhythm, as were variations in the external calcium ion content between 50% and 500% that of seawater, when the osmotic strength was held approximately constant.

Notes: Summary Based upon a conceptual dichotomy between those environmental influences on a population which vary in their effect with population density, and those which do not, it is widely held that only density-dependent factors can determine equilibrium abundance or “carrying capacity” for a population under field conditions. Since the direct effects of climate on mortality and birth rates are commonly thought to be independent of density, that interpretation means that correlations between climatic factors and long-term average population density are presumed to reflect only indirect effects of climate, acting, for example, on resource availability. When density-dependent and density-independent processes are carefully defined, however, and their expected consequences derived, it becomes clear that the equilibrium abundance for a species can usually be altered by any environmental factor which has a consistent effect on either birth rate or mortality rate, regardless of whether that action itself depends upon density of the species. This implies that the concept of “carrying capacity” must be extended, to include possible direct, density-independent effects of climate on birth and death rates; these conclusions follow from a very simple model which appears to be extremely robust. Hence, geographic trends in average population density, which are so familiar to the biographer, may well be due in a straight-forward manner to the influences of climate, even if climate were to act only in a densityindependent manner.