This gorgeous nasty creature is a colonial siphonophore—which means it is a complex organism made up of multiple individuals, and in this case the individuals are cnidarians, or jellyfish. A more familiar colonial siphonophore would be the Portuguese Man O' War, the dangerous surface water predator with potent stinging tentacles. This one, of the genus Erenna, is found only in the deep sea and was discovered with a submersible.

Like the Man O' War, it is also a major predator, living on fish. In the sparsely populated depths, though, it wasn't clear on how a slow moving animal like this caught prey. A closer look has shown that they have some interesting appendages.

Erenna tentacles have numerous side branches (tentilla), each consisting of a large cnido-band (an array of 3000 stinging cells) attached to a central stalk. The transparent stalk terminates in a bulb containing white spots, historically called "ocelli" (4). We found that, when ruptured in CaCl₂, these spots produced luminescence, indicating that they are in fact photophores filled with Ca²⁺-regulated photoproteins. Unlike typical cnidarian photocytes, these terminal photophores did not readily flash upon direct stimulation.

They glow red, which was a surprise—red bioluminescence is unusual. The nifty thing they do with these red glowing bulbs dangling from their tentacles is best seen in action:

Errena, (1.2MB MP4 movie)

They twitch and flick the red lures back and forth to attract fish…and then they fire their cnidocytes to harpoon and poison their prey.

Haddock SHD, Dunn CW, Pugh PR, Schnitzler CE (2005) Bioluminescent and Red-Fluorescent Lures in a Deep-Sea Siphonophore. Science, 309(5732):263.