What are the predators of Glaucus atlanticus?
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Frick et al. (2009)1 found that about 42% of logerhead sea turtle (Caretta caretta) stomachs contained remnants of G. atlanticus.


  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. 



Citations:

 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.