You Are What You Eat
Large filter-feeding marine animals, like the whale shark and manta ray, are most commonly observed feeding near the ocean surface on blooms of zooplankton. Recent techniques make it possible to determine the precise food sources of these animals, without relying on what can be limited observation. Certain components of an animal’s prey are incorporated into the predator’s tissues, and so tissue analysis of sharks and mantas can used to identify their diet. Prey species of different sizes and those living at different depths have distinct profiles of components such as fatty acids. Fatty acids are key components of cellular structures and sources of energy for the body. Fatty acid analysis of predator tissues can identify the sources of the fatty acids, i.e. the shark’s prey. These authors collected muscle tissue biopsies from the giant manta ray (formerly Manta birostris, now Mobula birostris) off the coast of Ecuador and analyzed fatty acid profiles. Surprisingly, the surface zooplankton that mantas are often observed feeding on constituted only a small proportion of their prey. Instead, the manta tissues contained a fatty acid profile consistent with feeding primarily on deep-water species - specifically, mesopelagic zooplankton that inhabit depths below 200 meters. So mantas may feed on surface zooplankton during the day, but at least off coastal Ecuador this feeding is minimally important compared to mesopelagic sources.
Does this mean that at some point in their feeding mantas leave the surface to feed in the mesopelagic zone? Perhaps, but the answer may not be quite that simple. Mesopelagic zooplankton perform a diel migration, spending daytime hours at depth, but rising nearer the surface at night. It may be that mantas dive to feed at depth during the day, or they may feed unobserved during the night on rising mesopelagic plankton. Further studies are needed to determine whether these animals are targeting mesopelagic food sources at mesopelagic depths, or nearer the surface at night.
The paper is: Burgess KB, Guerrero M, Marshall AD, Richardson AJ, Bennett MB, Couturier LIE. (2018) Novel signature fatty acid profile of the giant manta ray suggests reliance on an uncharacterised mesopelagic food source low in polyunsaturated fatty acids. PLoS ONE 13: e0186464. DOI: 10.1371/journal.pone.0186464