Abstract
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.
Highlights
The polyp stages of marine hydrozoans may be found on a wide variety of natural and artificial substrata, including seagrasses and intertidal algae (Seed and O’Connor, 1981; Boero et al, 1985; Gili and Hughes, 1995)
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
Often delicate, animals that may require special adaptations, either of morphology or habit, to withstand the potential damage caused by wave action
Summary
The polyp stages of marine hydrozoans may be found on a wide variety of natural and artificial substrata, including seagrasses and intertidal algae (Seed and O’Connor, 1981; Boero et al, 1985; Gili and Hughes, 1995). Some species are found mainly, or only, on specific plant substrata, often in extreme environments, such as the rocky intertidal zone. The epiphytic habit requires the ability to select specific substrata difficult for other species to colonise (Calder, 1991; Hughes et al, 1991a,b; Genzano and Rodríguez, 1998) and may be an adaptation to reduce interspecific competition, so prevalent in epifaunal communities. Epiphytism may lead to selective pressures to adapt to special circumstances associated with that host and its environment. Seagrasses, for example, are usually found in sheltered conditions, but the leaves are short-lived and the specialist epiphytic hydroids have special growth strategies that help maintain their position on the plants (Hughes, 1991a,b; Rossi et al, 1997). The problems faced by fucoid epiphytes are associated with the highly hydrodynamic environment of the rocky intertidal. Often delicate, animals that may require special adaptations, either of morphology or habit, to withstand the potential damage caused by wave action
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