Special Eyes Increase Photosensitivity

Biological Strategy

Javelin spookfish

AskNature Team

Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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The eyes of spookfish increase photosensitivity in deep water via accessory eyes.

“The six-eyed spookfish (Bathylychnops exilis) has an optical system that is unique in the animal kingdom. Unknown to scientists until as recently as 1958, and dwelling at depths of 330-3280 feet (100-1000 m), this slender pike-like fish has paired, downward-pointing, spherical organs housed within the lower half of its large eyes. These were once believed to be light-producing organs – bioluminescence is a common phenomenon among deep-sea fishes. Closer examination, however, exposed their much more extraordinary, true identity. In reality, these organs, now referred to as secondary globes, are accessory eyes. Each of these globes possesses its own lens and retina and probably serves to increase the spookfish’s sensitivity to light (photosensitivity) within its dimly lit undersea realm.

“But this is not the only anomaly of its optical system. Scientists subsequently unfurled a further surprise associated with the fish’s accessory eyes. Behind them is a third set of ‘eyes,’ even tinier than the secondary globes, but less sophisticated. These ‘eyes’ lack retinae. Instead, they serve merely to direct incoming light into the spookfish’s principal pair of eyes, thereby enhancing these latter organs’ powers of vision.” (Shuker 2001:13)