Morphologische und ökologische Anpassungen von Acabaria-Arten (Octocorallia) im Roten Meer an verschiedene Formen der Wasserbewegung

Abstract

The influence of turbulent current in comparison to regularly unidimensionally flowing current on the gorgonians Acabaria pulchra, A. erythraea and A. biserialis has been studied in the merchant harbour of Eilat. Their distribution on the concrete pillars supporting the harbour pier was restricted to shadow areas (2-0.5% of the illumination above the free surface). Only in the surf-influenced surface layer with oscillating and pendular currents of changing intensity and direction (down to 2 m), A. pulchra and the very similar but rare species A. erythraea were found. Corresponding to these findings, A. pulchra could be observed in the natural reef only in the dark channels and crevices of the reef edge where currents are irregular according to the varying surf intensity. In contrast, A. biserialis settled on harbour pillars below the A. pulchra region in a zone of a regularly unidimensionally flowing long shore current. The rigid colonies of A. pulchra are small (up to 6 cm high) and thus capable of resisting surf pressure. They are branched in all directions; the polyps are extended on all sides of the branches. A. biserialis reached a hight of 20 cm at the harbour pillars and 30 cm at the slope base of various reefs. Colonies were always two-dimensionally branched. Fans grew from the lateral sides of the harbour pillars into the prevailing long shore current. Corresponding conditions were found on the reef. Perpendicularly positioned to the current, they achieve a maximum filter effect. The polyps of A. biserialis are situated in two lateral rows along the branches. The different growth forms of A. pulchra und A. biserialis seem to be species specific. Both species exclude themselves in their distributional patterns, depending on water movement:A. pulchra (together with A. erythraea) is the single gorgonian in the Red Sea, settling on the reef edge and outer flat exposed to surf influence;A. biserialis grows in deeper water, like a typical sea fan, in the zone of unidimensionally flowing long reef current. © 1973 Biologischen Anstalt Helgoland.