Movement and zonation of the intertidal anemone Actinia tenebrosa Farqu. (Cnidaria: Anthozoa) under experimental conditons

Abstract

Actinia tenebrosa Farqu, is viviparous and the juvenile anemones are ejected from the gastrovascular cavity ready for immediate attachment to the first solid substrate they contact. It is several days after the initial attachment before the juveniles have the potential to move, and then it is at a rate approaching that of the adults. This may be at least 2.5 cm/hr for 8 hr and often more. When these animals are in the preferred zone in the intertidal region movement is much less. The movement of Actinia may be resolved into three categories: firstly, movement in the vertical plane, where prolonged submergence initiates negative geotaxis and prolonged emergence initiates positive geotaxis; secondly, slight inherent activity which continues even in the absence of environmental gradients; thirdly, directed lateral movements in response to stimuli other than submergence, emergence, or gravity. In the tidal tank, both adult and juvenile Actinia could be induced to form a zone in the intertidal region which corresponded to that formed in the natural environment. The distributions formed by adults and juveniles under identical conditions were significantly different although the range of the distributions was very similar. The high mortality of juveniles observed in the laboratory is attributed to the effects of dehydration during emergence, and it is suggested that at this stage of development Actinia is very susceptible to comparatively low relative humidities. The response of juvenile Actinia to a humidity gradient is examined. The zonation of Actinia tenebrosa is discussed with reference to coelenterate behaviour patterns and to the zonation of intertidal molluscs.