As explained by Wired:
Portugese man-of-wars are not jellyfish, they’re siphonophores, which mean’s they’re actually a group of organisms, called zooids, that depend on each other to live. The creature gets its name from the upper-most organism which people say looks like the man-of-war warships that were first built by England in the 16th century. A different organism forms the tentacles, which usually hang 30 feet below the surface and have venom-filled nematocysts that are used to kill small fish. Man-of-wars are most commonly found in warm water and sting thousands of humans each year.The photos are taken by Aaron Ansarov on a light table. The translucent bodies of these creatures allows the light table to back-light them making all the colors and shapes shine through. Aaron and his wife collect the Man-of-War's on the beach near their home in Delray Beach FL, where the creatures often wash up . The creatures are alive when they're found, so they are transported in coolers filled with salt water and returned to the beach after the photo shoot.