The way deep-sea animals use light is a mystery, but the majority carry light organs. Some squid carry as many as two hundred light organs. One brought up alive from deep water in the South Atlantic by the German research ship Valdivia presented a glorious sight. The body seemed to be adorned with a diadem of brilliant gems. Parts of the eyes shone with an ultramarine light and other parts with a pearly sheen. On the undersurfaces of the body there was a group of ruby-red lights in the front half, the middle was illuminated with sky-blue lights, and the group of lights at the back was white or pearly.

The firefly squid lives in the deep waters off Japan, but comes into shallower waters in spring to spawn. It also carries a living gem-display, and when caught in the nets it glows with a brilliant bluish light. There are five lights round each eye, others on some of the arms, and hundreds of smaller ones all round the body. The squid can even, it seems, switch these lights on and off, for each has a shutter of dark skin which can be drawn over it to shut in the light or drawn back to let the light shine out.

But even these remarkable displays are put to shame by one of the deep-sea prawns. It has 150 light-organs which can light up like an electric sign and be switched off as suddenly. The lights come on one after the other from head to tail. Each light goes out as the next one lights up, and the whole sequence takes several seconds.

Our second extract describes the sea-bed from a bathysphere.

There were no more bubbles now and the hissing had stopped. Outside there was a heavy blackness—as black as black velvet—except where the electric light pierced the empty water and showed the colour of it—a yellowgreen. Then things like shapes of fire swarmed into sight, following each other through the water. Whether they were little and near or big and far off he could not tell.

Each was outlined in a bluish light almost as bright as the lights of a fishing smack, a light which seemed to be smoking greatly, and all along the sides of them were specks of this, like the lighted portholes of a ship.