(responses of Nymphon Gracile (pycnogonida) to Pressure Cycles of Tidal Frequency.

Within recent years laboratory experiments have shown that a variety of animals, including representatives of at least six phyla, can perceive fairly small changes in hydrostatic pressure (Hardy & Bainbridge, 1951; Knight-Jones & Qasim, 1955, 1959; Qasim & Knight-Jones, 1957; Baylor & Smith, 1957; Digby, 1961; Rice, 1962; Bayne, 1963). Many are sensitive to changes equivalent to less than a metre of water. The usual response to a pressure increase is to swim more strongly, generally upwards and/or towards the light. A reduction in pressure induces passive sinking, or sometimes swimming downwards and/or away from the light. The responses, in fact, are generally such as would help the animals to regulate their depth. There have also been field experiments and observations which indicate that copepods and siphonophores have some sense of depth (Hardy & Paton, 1947; Moore & Corwin, 1956), which is presumably a pressure sense. Behaviour of a rather similar kind is thought to help the littoral amphipod, Synchelidium, to migrate regularly up and down surf-swept beaches as the tide floods and ebbs (Enright, 1961, 1962). Mysids also move inshore with the flood tide and Rice (1961) has collected data indicating that they are led to do so by pressure responses, for these responses are depth-regulatory, though the mysids are bottom-living, and the responses must therefore tend to keep them in shallow water at the edge of the sea. It has for some time seemed likely that various animals of shallow seas can perceive tidal pressure changes (see, for instance, Korringa, 1947). Rice (1961) showed that his mysids responded to pressures changing at an overall rate similar to tidal changes, but his pressure changes were made in a series of sudden steps, each equivalent to about 50 cm. of water. It is still not certain that the mysids are able to perceive very slow changes such as would be caused by the tides. Indeed, there has been no clear demonstration of such sensitivity in any animal as far as we are aware. We therefore thought it particularly worth while to study the behaviour of the pycnogonid Nymphon gracile Leach, for this shows a response to pressure changes which cannot be concerned with depth regulation, being opposite in sense (Rice, 1964).