The effects of exposure to wave action on the distribution and morphology of the epiphytic hydrozoans Clava multicornis and Dynamena pumila

The spatial distribution patterns of two species of epiphytic hydrozoans, Clava multicornis and Dynamena pumila, on the intertidal alga Ascophyllum nodosum were studied in adjacent wave-sheltered and wave-exposed areas. Clava were more abundant on the wave-sheltered algae than on the wave-exposed fronds, and in both areas occupied the basal and middle sections of the algae. There was no difference in the abundance of Dynamena between the wave-sheltered and wave-exposed areas, but in both areas Dynamena were most abundant on the basal and apical sections of the algae than on the central sections. The number of hydranths per colony of Clava was higher in the sheltered area than in the exposed area. The hydrocauli of Dynamena on the wave-exposed algae bore fewer hydrothecae than those on the sheltered algae, The proportion of Dynamena hydrothecae that contained hydranths was close to 100% in the sheltered area, but only 70% in the exposed area. It was concluded that variations in distribution and morphology could be caused by the direct or indirect effects of one or more variables, including wave action, feeding rates, and exposure to solar radiation. Experiments are required to elucidate the specific effects of these variables.