Photo exhibition brings new light to ocean depths

(Image: Sandra J. Raredon, Division of Fishes, NMNH)

ON 17 January 1978, this tropical hatchetfish was minding its own business about 850 metres below the surface of the North Pacific Ocean when it was captured, killed and taken to become part of the Smithsonian Institution's fish collection.

Now its portrait is part of the "X-ray Vision: Fish inside out" exhibition at the Smithsonian National Museum of Natural History in Washington DC, which runs until 5 August. Specimens from the museum's collection, including this Argyropelecus lychnus, were imaged using a digital radiographic machine that doesn't need film. Specimens are placed on a digital sensor and zapped with X-rays. Other species to feature include the winghead shark, the pancake batfish and the coelacanth, often referred to as a "living fossil".

Seeing the hatchetfish as a ghostly X-ray image seems appropriate, given that in life the species has bioluminescent camouflage. Hatchetfish elude deep-sea predators by using counterillumination. Photophores on the underside of the body emit light that matches the feeble illumination penetrating from the surface, so when looked at from below the fish merges with the surface.

The fish also has very large eyes. "This is typical of many such fish that live in the ocean's twilight zone where light penetration is diminished," says Lynne Parenti, curator of fish at the museum. In the mouth can be seen needle-like teeth that curve inwards. "Marine hatchetfish migrate towards the surface at night to feed on plankton, small fish and invertebrates," she says.

The X-ray has one last secret to reveal: those white chunks in the centre of the image are the undigested remains of the fish's last meal.