Catching and releasing the light

Lynn Savage, lynn.savage@laurin.com

The Hawaiian bobtail squid (Euprymna scolopes) is a nocturnal animal that spends its days buried in the sand, away from the gaze of diurnal predators. At night, the animal leaves its cozy resting place to search for food, but it still must avoid any creatures that are looking for their own meal. The squid is barely an inch long, not counting its limbs – but with a nickname like bobtail, there isn’t much extra length with which to defend itself.

E. scolopes does, however, have a secret weapon: Deep inside a specialized organ in the middle of its mantle, or main body, are entrenched hordes of Vibrio fischeri, bacteria that live in symbiosis with the squid. In exchange for a safe place to live, with plenty of food and oxygen, the bacteria emit a soft blue-green glow that, to the eyes of a hungry predator, blends the squid in with the background of the moonlight or starlight from above.

The bioluminescence provided by V. fischeri is activated at the will of E. scolopes: The squid controls the oxygen flow to the organ in which the bacteria reside, and it is the oxygen that fuels the glow. The more oxygen released, the brighter the light, allowing the squid to adjust its camouflage to ambient light conditions.

Recently, though, it has been found that the organ hosting the symbionts does double duty. Not merely a source of light, the organ also is a light sensor.

In the June 2 issue of PNAS, Margaret McFall-Ngai of the University of Wisconsin-Madison School of Medicine and Public Health and her colleagues reported that the organ has light-detecting tissue inside its roughly kidney-shaped lobes. Furthermore, this tissue includes genes that express the same proteins found in the retina of the animal – proteins that respond to light and evoke a physiological response.

According to McFall-Ngai, who has spent 20 years studying the relationship between E. scolopes and V. fischeri, the organ’s dual nature may have come about to help the squid sense the bacterial symbionts, to sense the light from outside the animal, or both.

Your eyes make a couple of fine light receptors, but unless you’re Superman (or one of a smattering of other fictional people), you don’t project light from them.

That makes the score: Squids, 1 – You and me, 0.