1. The swimming activity of 6 specimens of the Pachon cave form of Astyanax mexicanus was tested with regard to its time control under various light-dark(LD)cycles and constant conditions, and it is compared to that of a river form.
2. In general, activity is entrainable by all applied LDs, but even if the amplitude of a forcing signal increases the signal energies are lower than in the river fish.
3. In case of entrainment the maximum values of surface activity correspond to the dark phases, those of bottom activity to the light phases of a LD. Flexible patterns -as often observed in the river form in the range of resonance about 24 h - are very seldom. Furthermore, disturbances of ten occur in the entrainment of one activity form, or one form runs arrhythmic while the other is still entrained.
4. The activity answers to changing environmental conditions are not as uniformly quick as in the river fish. But the system hardly needs a swing-in time to become entrained when a LD starts.
5. After transition from LD to DD (= constant darkness) the entrained rhythms disappear immediately.
6. In no LD with a period length differing from 24 h a circadian rhythm can be observed in addition to the entrained frequency.
7. These results show that the passive system of the river form has developped into an extremely passive one being unable to oscillate and thus has become simplified during regressive evolution. Concerning the circadian oscillator of the epigean ancestor, it was also subjected to regression, but it has not been completely lost. After a LD with a period length about 24 h the circadian oscillator is able to act as a stable system, clearly shown by the freerunning circadian rhythms of surface activity. But out of this range the oscillator is unable to control activity. In DD after all other LDs activity patterns are arrhythmic.
1. The swimming activity of 6 specimens of an Astyanax mexicanus' river population was tested with regard to its time control under various light-dark(LD)cycles and under constant conditions.
2. Activity is classified into three different forms according to the special experimental arrangement: surface activity, bottom activity and the sum of both (total activity).
3. All applied LD-cycles act as socalled forcing signals (Zeitgeber) and entrain the activity.
4. The maximum values of surface activity correspond to the dark phases of a LD-cycle, those of bottom activity to the light phases. This inversity causes a less strong entrainment of the total activity up to a loss of a significant oscillation in extreme cases.
5. This inverse pattern is kept the more stronger the more the period length of a LD deviates from 24 h. In the range of resonance about 24 h there is a greater flexibility with regard to the phase relation of the maximum values.
6. Activity reacts very sensitive to the differential parameter of the forcing signal. Therefore, no phaseangle difference occurs between forcing and forced signal. Moreover, the system needs no swing-in time to become entrained when starting a LD.
7. After transition from LD to DD (= constant darkness) the forced signal does not die away immediately, but damps out within one or a few cycles with decreasing amplitude and unchanged frequency.
8. In nearly all applied LDs a non-synchronized circadian rhythm can be observed in addition to the entrained frequency, which is dominant.
9. Also, in DD a freerunning circadian rhythm is detectable.
10. We suggest the model that first a passive system with a nearly unlimited range of entrainment controls activity. In contrary to extremely passive systems, it is able to oscillate. Moreover, it acts like a linear system with respect to frequency transfer: In the tested cases, output and input frequency are equal. In addition, activity is under control of an endogenous circadian oscillator. Its effects are overlapped under forcing conditions, but they become obvious under constant conditions. Furthermore, nonstationary processes are features of this circadian rhythm. The proper ties of a passive and a circadian system alone do not explain the flexible patterns in the range of resonance. Therefore, a time-dependent controller is demanded to control the phase relation of the maxima.
Rudiments of an ability for endogenous time-measuring are indicated a) by bimodal activity in the dark phase of LD-cycles of 16:16 or 24:24 h and b) by damped activity oscillations frequently following a transition from LD to constant conditions. These oscillations always have the same period length as the applied LD.
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