Orientation in the sea fan Eunicella cavolinii related to water movement

Abstract

1. In situ studies were performed on orientation of the planes of branching in colonies of the gorgonian Eunicella cavolinii growing in locations of different topography and water movement. Employing frequency analysis, orientation patterns were revealed which demonstrated the influence of water movement on these orientations. 2. The importance of substratum morphology was demonstrated by staining individual water bodies with fluorescein sodium. 3. A typical modification in growth pattern of E. cavolinii is characterized, indicating zones of strong whirls and turbulence: the second critical depth (Riedl 1964, 1971). 4. The second critical depth can be defined by projected circumscription of fan-top views plotted against depth as well as the flowing zone and oscillation zone. In zones of stable water movement directions, the projected area of fan-top views show only slight variations, expressed by the standard deviations. 5. In situ experiments demonstrate clearly the effect of water movement on growth pattern and fan orientation in E. cavolinii. Colonies of various sizes were turned from their natural position perpendicular to the direction of water movement until the fan was parallel to this direction. During structural re-adjustment, the following responses could be noted: (a) Inhibition of growth for the first 5 months; (b) new growth perpendicular to the original fan plane; (c) compensation in branches whose tips grew in new directions; (d) occasional loss of the two-dimensional shape under the new conditions of water movement. 6. Growth rate was compared in experimental and undisturbed colonies. Growth rate was in most cases lower in experimental than in undisturbed colonies. 7. The mechanisms of reorientation of E. cavolinii under the new conditions of water movement are discussed in the light of comparable responses shown by other organisms. © 1973 Biologischen Anstalt Helgoland.