Differential morphological features of two Dendronephthya soft coral species suggest differences in feeding niches
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Octocorals are characterized by pinnate tentacles and internal sclerites. Their feeding ability is determined by the morphological features of the polyps. Capture of their food by these corals is also affected by the flexibility of the colony, which in turn is determined by the features of the sclerites. We studied the morphological features of two azooxanthellate octocorals, Dendronephthya hemprichi and D. sinaiensis, whose depth distribution partially overlaps at Eilat (northern Red Sea). Following Gause’s Law, such coexistence is considered to be possible if each species is adapted to utilize different food items. In order to examine this Law, the features of the polyps of the two species and their sclerites were studied. Each side of their tentacles displays 11–13 pinnules, which are longer at the tentacles’ distal end compared to its median section and proximal end, with the distal pinnules of D. sinaiensis being longer than those of D. hemprichi. At the proximal end of the tentacles of D. sinaiensis, the pinnules emerge perpendicularly, unlike in D. hemprichi, where they emerge from the lateral sides; the distance between the rows of pinnules is, therefore, shorter for the former. These findings imply that the filtered phytoplankton by the two species may differ in size. Their sclerites also differ in size and shape, and therefore the expansion and contraction abilities of their polyps also differ, and may thus affect their respective feeding abilities. The findings indicate that D. hemprichi and D. sinaiensis are adapted to utilize different food items, and therefore support Gause’s Law and explain the coexistence of the two species.