The effects of salinity changes on the endogenous circa‐tidal rhythm of the amphipodCorophium volutator(Pallas)

The endogenous rhythm of swimming activity shown by Corophium volutator has been studied in sea water of different concentrations, and after subjecting freshly collected animals to pulses of different salinities. The free‐running period of the rhythm shows no significant change in different ambient salinities but a significant correlation exists between the phase of the rhythm and the concentration of the recording medium. These phase shifts are evident in the first activity peak following transfer, lower salinities effecting a phase advance whereas higher salinities bring about a phase delay. The rhythm is not equally susceptible to high salinity pulses at all stages of the tide and a phase response curve has been constructed for 3 h pulses of 40‰ S. Maximum delay follows pulses given around high water while similar pulses applied later in the tidal cycle cause a phase advance. Pulses of increased salinity, given at a tidal frequency are effective in rephasing the activity rhythm but are insufficient to sustain the pattern of swimming activity.