This is a blue ocean slug (Glaucus atlanticus) approaching the venom-filled tentacles of a blue bottle (Physalia physalis). Both were washed ashore as part of a mass, multi-day stranding of thousands of blue bottles, which are colonial siphonophores that are also called Portuguese man-of-wars. The nudibranchs are part of the rarely seen blue community of the open ocean. They prey on other animals in that community, such as Physalia physalis, by-the-wind-sailors (Velella velella), blue buttons (Porpita porpita), and violet snails (Janthina janthina). Glaucus nudibranchs are immune to the venom of the blue bottles that they consume. They store their prey’s nematocysts and deploy them in specialized cnidosacs at the tips of their ornate cerata. Because these nudibranchs concentrate the venom of their prey, they can produce a more powerful sting than Physalia.
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