Diet of deep-sea octocorals from the Emperor Seamount Chain inferred by fatty acid trophic markers

Despite the important role that octocorals (Cnidaria: Octocorallia) play in creating and maintaining deep-sea communities known as coral gardens, little is known about their diet. We analyzed the fatty acid compositions of deep-sea octocorals collected from Koko Guyot, located in the southern Emperor Seamount Chain, to identify their main food sources. The data suggests that octocorals feed mainly on zooplankton. However, the contribution of zooplankton to their diet varies greatly among different species and is determined by different zooplankton groups (herbivores, carnivores, or omnivores). This is evidenced by the increased content of some monounsaturated fatty acids (FA) and the different ratios of polyunsaturated fatty acids in corals. In bamboo coral Isidella, we observed the maximum enrichment in calanoid copepod markers (20:1n-9 and 22:1n-1). In addition to the copepod markers, most corals showed a high level of the carnivore marker (18:1n-9), which indicates a significant contribution of other (non-calanoid copepods) items to the diet. The high level of n-3 polyunsaturated fatty acids (PUFA) such as the diatom marker 20:5n-3 suggests that corals of the family Primnoidae take up more freshly produced material. However, the high level of n-6 PUFA (especially, 20:4n-6), found in Paramuriceidae and Keroeides corals may indicate the involvement of detrital links in their diet. FA analysis is useful for identifying the diet of cold-water octocorals and provides a basis for prediction of future potential changes of the bottom ecosystems in the Emperor Seamount Chain.