The activity patterns of New Zealand mud crabs under field and laboratory conditions

Abstract

The activity of the intertidal mud crabs Helice crassa Dana and Macrophthalmus hirtipes (Jacquinot) was studied concurrently in the field and under constant laboratory conditions. In the field both species are active on the surface at L.W. but pitfall trap sampling showed that exploratory excursions away from the burrow are undertaken mainly at H.W. Macrophthalmus hirtipes unlike Helice crassa is affected by the day/night cycle with greatest catches in the traps occurring during night high tides. The size of the catch at H.W. for both species is directly related to tidal amplitude but the effect is more marked in H. crassa. Under constant laboratory conditions both species show persistent rhythmicity with peak activity coinciding with the times of high tide. At times this circatidal rhythmicity is modulated by a persistent day/night rhythm. In addition in H. crassa the amplitude of the persistent rhythm and the clarity of the peaks vary with the stage in the synodical monthly cycle. The relationship between the activity in the field and that recorded under constant laboratory conditions is discussed.