SPOOKFISH AND BARRELEYES

A good Thursday to you. I am always amazed with discoveries in science. When you think new discoveries maybe slowing down, new data continues to emerge. Take the case of two deep sea fish that have been known for years. These fish live in the dark ocean depths and, as you see from the photos below, have strange eyes. The eyes have been a mystery as to their operation. We now know how these creature's eyes function.

The four-eyed spookfish above may have seemed strange enough. Now researchers say it doesn't really have four eyes. Instead, it is the first known vertebrate to use mirrors, rather than lenses, to focus light in its eyes.

“In nearly 500 million years of vertebrate evolution, and many thousands of vertebrate species living and dead, this is the only one known to have solved the fundamental optical problem faced by all eyes — how to make an image — using a mirror," said Julian Partridge from the University of Bristol.

While the spookfish looks like it has four eyes, in fact it has only two, each of which is split into two connected parts. One half points upwards, giving the spookfish a view of the ocean — and potential food — above. The other half, which looks like a bump on the side of the fish's head, points down. These diverticular eyes, as they are called, are unique among all vertebrates in that they use mirrors to make the image, Partridge and colleagues found.

Very little light penetrates the ocean's waters below a depth of about a half-mile (1 kilometer). Like many other deep-sea fish, the spookfish is adapted to make the most of what little light there is. The spookfish largely looks for flashes of bioluminescent light from other animals. The diverticular eyes image these flashes, warning the spookfish of other animals that are active, and otherwise unseen, below its vulnerable belly.

Although the spookfish was discovered 120 years ago, no one had discovered its reflective eyes until now because a live animal had never been caught.

A computer simulation showed that the precise orientation of the plates within the mirror's curved surface is perfect for focusing reflected light onto the fish's retina. The use of a single mirror has a distinct advantage over a lens in its potential to produce bright, high-contrast images. That must give the fish a great advantage in the deep sea, where the ability to spot even the dimmest and briefest of lights can mean the difference between eating and being eaten.

Above is a bizarre deep-water fish called the barreleye. It has a transparent head and tubular eyes. Since the fish's discovery in 1939, biologists have known the eyes were very good at collecting light. But their shape seemed to leave the fish with tunnel vision.

Now scientists say the eyes rotate, allowing the barreleye to see directly forward or look upward through its transparent head. Most existing descriptions and illustrations of this fish do not show its fluid-filled shield, probably because this fragile structure was destroyed when the fish were brought up from the deep in nets.

Recently researchers were fortunate to bring a net-caught barreleye to the surface alive. Over several hours in an aquarium on the ship, they were able to confirm that the fish rotated its tubular eyes as it turned its body from a horizontal to a vertical position.

The barreleye (Macropinna microstoma) use their ultra-sensitive tubular eyes to search for the faint silhouettes of prey overhead.