North Queensland Naturalist 49: 47-64
Andrew C. Mitchell
Wedge Shells or Pipis (Donax cuneatus) are abundant on the Cairns Northern Beaches and are the dominant macrofauna present in the swash zone. They migrate up and down the beach to remain within the swash zone as tides of up to 3 m rise and fall. Waves are used to ‘swash ride’ to a new location. The ability to select waves suitable for swash riding appears to be critical to survival as errors in wave selection could be fatal. If Pipis are swept up the beach above the active swash zone or swept into deeper waters below the toe of the beach, as far as is known, they have no ability to move themselves back into suitable habitat. This paper records how Pipis swash ride in variety of wave conditions in order to reveal aspects of wave selection. In north Queensland, wind-driven, choppy wave conditions prevail and provide a contrast with previous studies of swash riding in Donax spp., which have taken place on beaches subject to ocean swells. Beaches exposed to wind-driven chop are steeper and are subject to very wide fluctuations in wave conditions. Pipis manage to swash ride in a variety of conditions including very still weather, long shore winds with waves striking the beach at low glancing angles and when multiple waves travelling in different directions are superimposed at the moment of encounter with the beach. Often Pipis can be seen ejecting from the sand en masse just prior to the arrival of a suitable wave. The variety of wave conditions and the localised responses of Pipis to approaching wave fronts allow investigation of swash riding behaviour at a high level of detail. Small groups of Pipis approximately a metre across will often eject to swash ride when elsewhere on the Pipi-loaded beach there is no visible Pipi activity. These localised responses suggest that Pipis can determine, with a spatial resolution of a metre, whether the incoming waves will combine in a way that provide the conditions they need for swash riding.
Reasons why Pipis may need to swash ride in such a difficult environment are also investigated. Being within the swash zone may provide shelter from predators and the ability of predators to obtain Pipis from within and adjacent to the swash zone is described. Beach geomorphology may also drive tidal migration in Pipis, as just below the swash zone is the beach step, a feature which also migrates up and down the beach with the tides. With each tide, waves impacting the beach step churn and rework the sand to a depth of tens of centimetres, probably making the beach step a very hostile environment for in-fauna, including Pipis.