Responses of Nymphon Gracile (Pycnogonida) to Pressure Cycles of Tidal Frequency

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Abstract
The responses of Nymphon gracile to changes in hydrostatic pressure are described. (M, the unit of pressure used here, denotes 1 metre sea water.) A sudden increase in pressure tended to inhibit swimming, while a reduction promoted it, a response opposite in sense to that normally associated with depthregulation. Nymphon showed no adaptive response to cyclical pressure changes similar to those produced by onshore waves. Pressure changes of similar range (1M) but with a 2 sec. period usually induced swimming in previously sluggish individuals. Pressure cycles of 8M range and periods ranging from 14 to 160 min. evoked a depth-regulatory type of response in several individuals, but the majority responded in the opposite way. Nymphon responded to cyclical pressure changes of approximately tidal range (8M) and frequency, by swimming most actively during the late ebb or low water of the artificial cycles. The data of Fage (1932) support the hypothesis that Nymphon swims most often when stimulated by decreasing pressure as the tide ebbs.