Food preferences of two nudibranch species from the South China Sea revealed by fatty acid trophic markers

Nudibranchs are mostly predators preying on a variety of invertebrates. The dietary preferences of tropical nudibranchs were studied by the method of fatty acid trophic markers (FATM) in order to for better understanding of their trophic ecology. For this, the fatty acid profiles of two nudibranch species from the South China Sea, Doriprismatica atromarginata (Cuvier, 1804) and Jorunna funebris (Kelaart, 1859), were analyzed and trophic markers were identified. The high level of very long chain fatty acids (from C24 to C28), which are characteristic of sponges, in nudibranchs was evidence of their predation on sponges. However, the distribution of these components differed significantly between the species. The acids 24:2Δ5,9, 25:2Δ5,9, 26:2Δ9,19, and especially 26:2Δ5,9 dominated in D. atromarginata, but were not found in J. funebris that was rich in 28:2Δ5,9 and 28:3Δ5,9,19. The significant differences in the profile of these demospongic acids indicate that these nudibranchs consumed different species of sponges. The similarity between the FATMs of J. funebris and its potential prey, the sponge Xestospongia, confirmed their predator–prey relationship. Doriprismatica atromarginata from different sites along the Vietnam coast had different FATM profiles, which showed this nudibranch as having no any strict food specialization and feeding on various Demospongiae species. The abundance of bacterial FATMs in the nudibranchs suggests the importance of bacteria in their diet. Thus, the FATM method has proven to be useful for identifying the feeding specialization and assessing the effect of food availability on the diet of these tropical nudibranch species.