Occurrence and survivorship of azooxanthellate octocorals reflect recruitment preferences and depth distribution

Like many other environmental factors, light limits the distribution of hermatypic corals along depth. Octocorals, accordingly, respond to light intensity, featuring a depth-related distribution. Among others, competition for space, resources, and settlement processes may determine their distribution. The azooxanthellate octocoral Dendronephthya sinaiensis inhabits flow-exposed vertical habitats such as reef slopes. The current study engages with its distributional patterns on both the southern and northern faces of the vertical pillars of the oil jetties at Eilat (northern Red Sea). It examines the possible role of light intensity and competition in determining its spatial distribution during recruitment processes. The distribution of D. sinaiensis along depth (14–32 m) was studied and light intensity was measured at both the light-exposed and shaded faces. The colonies were found mostly on the shaded faces, suggesting that D. sinaiensis might be adapted to low-light intensity, where zooxanthellate corals may be more restricted. Translocation of D. sinaiensis fragments from deep-to-shallow waters to either lit or shaded faces revealed their survival on the shaded faces, where they do not naturally occur, whereas they did not survive on the lit ones. This finding suggests a preference for deep water, which may reduce competition with zooxanthellate species. The occurrence of D. sinaiensis thus appears to be determined by both a selective preference to inhabit deep water and, at least partially, by light intensity.