Bacteria Light Up Squid
One of the more amazing symbioses in nature is that of the squid Euprimnes scolopes with its light-producing bacteria, Vibrio fischeri. A recent paper by Margaret McFall-Ngai details the chemical language that maintains the relationship between the two. The squid light organs have a ciliated-cell appendage (left, confocal microscopy) with three pores that admit V. fischeri (and no other bacterial species) into the light organ. When the baby squid hatches, the bacteria enter the three pores (marked p, at right), then grow until their population density peaks; at which point the bacteria all tell each other to luminesce (pale blue).
Why light up? Opposite to what you might think, the light mimics moonlight, shining downward to prevent the squid casting its nocturnal shadow that alerts prey. The light hides the predator.
Within the symbiosis, there are two amazing kinds of communication going on: between the squid and the vibrios (allowing exclusive colonization), and amongst the vibrios (measuring their population, called quorum sensing). We don’t fully understand either, but it turns out that many human pathogens do similar quorum sensing, for example Mycobacterium tuberculosis.
As a final wrinkle, the squid light organ expels 90% of its bacteria every morning, to swim off and find a new host.
What’s your favorite story of marine luminescence?
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Ultraphyte 
I’m becoming increasingly fond of the whole marine luminescence world, from giant squid whose eyes are thought to be optimized to detect whales (and likely prey) through the luminescence tracks they leave in deep water, to the Vibrio story here.
Although I’ve long read descriptions of how the deep ocean is more like the night sky than pitch black, I’d never fully processed it. It’s really a world of light, where animals use lights to communicate, to expose predators, and to hide, both by turning the lights off, turning them on, and by moving slowly and without turbulence to avoid triggering anti-predator reactions. It’s a world of light down there, as much as a world of darkness.