FACTORS CONTROLLING EMERGENCE OF PINK SHRIMP (PENAEUS DUORARUM) FROM THE SUBSTRATE

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Abstract
1. A close relationship exists between the day-night cycle and the times of activity of pink shrimp. The shrimp bury beneath the substrate during the day but emerge at the time of sunset and are active at night.2. Emergence from the substrate is markedly synchronized in all members of the population. This study elucidates the mechanisms whereby this synchrony is maintained.3. The persistence of the pattern of nocturnal activity for several days under conditions of constant low light intensity confirmed that emergence and subsequent activity were under rhythmic control.4. Resynchronization of the phase of the rhythm controlling emergence to a shift in the light-dark cycle indicated that the light-dark cycle itself, or some component of it, is responsible for maintaining the relationship between emergence and the day-night regime.5. The close association between emergence and the time of light-dark transition indicated the possibility that the latter was the important component of the Zeitgeber responsible for the control of the time of emergence. This was supported by experiments in which a changing responsiveness was shown to light-dark transitions imposed at various times during the light period. Maximum response (in terms of the rate and degree of emergence from the substrate) occurred when a light-dark transition was imposed at a time (18:00) to which the shrimp had previously been entrained to receive the onset of the dark period.6. Experiments in which shrimp, which had been fed 24 hours previously, emerged from the substrate and carried out food-searching movements, in the absence of food and despite high intensity illumination, which would normally suppress emergence, indicated the presence of a feeding rhythm with an approximate 24-hour periodicity.7. The feeding rhythm probably accounts for the fact that the phase of the rhythm of emergence was resynchronized to a shifted light-dark cycle in three days among shrimp fed immediately following the light-dark transition of the shifted cycle, as opposed to six days in those deprived of food during this time of entrainment. The phases of the two rhythms, being almost identical, probably supplement each other.8. Shrimp smaller than 4 cm. (total length) emerged significantly earlier than larger individuals. This possibly reflects the lesser dependence of the early juveniles on inherent rhythms and consequently their greater receptivity to exogenous stimuli.9. The most obvious advantage of the circadian rhythm controlling emergence and subsequent activity is to confine the times of activity of shrimp to the hours of darkness when predation by fish is minimal. The proximity of the phase of the 24-hour feeding rhythm to the circadian rhythm of emergence and activity probably serves to strengthen the entrainment properties of the latter such that the time of emergence is further synchronized between all members of the population. The synchrony of the time of emergence is probably important for the maintenance of cohesion between aggregations of shrimp.

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