I'm researching the Glaucus atlanticus, also known as the blue glaucus, among various other names, but after reading quite a few articles and searching on the Internet as many different ways as I can think of, I can't find any of its predators, or any indication it has no natural predators. Everything I've read is just silent on the issue. Does anyone know what, if any, predators this has? Thanks!
Try out using Google Scholar to take a more scientific literature-based approach to your search.
We were able to identify G. atlanticus in the diet of loggerheads because whole or nearly intact specimens were present.
Loggerhead turtle mouth and throat with sharp papillae. Source NCSU CMAST (Photo by Craig Harms)
Bieri (1966)2 observed G. atlanticus frequently attacking one another and biting off pieces of each other. In one case, one individual ate all but the head and trunk of a smaller specimen.
1, Frick, M.G., Williams, K.L., Bolten, A.B., Bjorndal, K.A. and Martins, H.R., 2009. Foraging ecology of oceanic-stage loggerhead turtles Caretta caretta. Endangered Species Research, 9(2), pp.91-97.
2. Bieri, R., 1966. Feeding preferences and rates of the snail, Ianthina prolongata, the barnacle, Lepas anserifera, the nudibranchs, Glaucus atlanticus and Fiona pinnata, and the food web in the marine neuston.