Experiments on the tidal rhythm ofBlennius pholis

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Abstract
The endogenous tidal rhythm of the littoral fishBlennius pholisL. was studied in the laboratory. It was found that there was no difference in the mean level of activity of fish caught on spring and neap tides. The time-interval between activity peaks (the period length) gradually increased in constant darkness. A similar, although not significant, increase was also observed in continuous illumination.There was no evidence of any diurnal component in the rhythm, but freshly caught fish subjected to alternating periods of light and darkness, equal in duration to the natural period, responded to the onset of illumination by greatly increased activity only when the light period began between activity peaks.Fish which had been kept in the laboratory for several weeks and had lost their natural rhythm still responded to light by increased activity. This increase was not as great as that shown by freshly caught fish, however, and the response decreased with time. Evidence of re-entrainment of the natural rhythm after 7 days in alternating periods of light and darkness was not found. It was shown that feeding the fish half-way through the 12 h light-period resulted in a reduction of activity after feeding. This did not occur when the fish were fed half-way through the dark period.A 4h cold shock of 10° C resulted in partial re-entrainment of rhymthic activity inB. pholis.B. pholis, which alone appears to have a persistent rhythm, differs from certain other fish examined in its strictly littoral habitat and its possible need for activity periods synchronized with the tide.

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