The Fish That Cracked the Mystery of Mirror Vision

Biological Strategy

Javelin spookfish

Mary Hoff

Image: Florida Atlantic University / Science Daily / Some rights reserved

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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Auxiliary eyes of the brownsnout spookfish create a clear image using mirrors to reflect and focus light.

Introduction

Deep in the ocean, above the region of total darkness, but below the point where 99% of sunlight is filtered out lives a species of barreleye fish called the brownsnout spookfish (Dolichopteryx longipes). Like many other fish that inhabit this mesopelagic (or “middle of the ocean”) zone, it has two eyes that point upward.

This arrangement makes it possible to see shadows of predators, prey, or potential mates swimming above, but it doesn’t help the fish to see in other directions or detect bioluminescence, a special light given off by other sea creatures.

The mirror reflects light and creates sharp images from a wide field of vision. This allows the brownsnout spookfish to have a clear view of what’s below it and also to detect bioluminescent light produced by other organisms.

The Strategy

Enter the brownsnout’s optical innovation: it has outgrowths from its upward-facing eyes that point downwards. While other mesopelagic fish have similar outgrowths, they lack lenses to reflect and focus incoming light, and as a result they can only see blurry shapes at best. Lenses are useful because they can bend light to focus the rays on the retina and create an image. The brownsnout, too, lacks lenses, but it does have stacks of plates that act as a mirror. The mirror reflects light and creates sharp images from a wide field of vision. This allows the brownsnout to have a clear view of what’s below it and also to detect bioluminescent light produced by other organisms.

Additionally, the plates of the mirror are angled to create a curved shape that allows the maximum amount of light to be reflected, creating the sharpest possible image. It’s also hypothesized that the fish can alter the position of the mirror to focus on objects at different distances.

Brownsnout spookfish (Dolichopteryx longipes) eye diagram

Image: Egmason /

The brownsnout spookfish’s downward facing eye (1) focuses light on the retina (a) using a mirror made of reflective crystals (b). The main eye (2) more conventionally uses a lens (c) to focus light on its retina (d).

Image: Florida Atlantic University / CC BY NC - Creative Commons Attribution + Noncommercial

In addition to the upward-pointing eye shown here, the brownsnout spookfish has additional eye structures that allow it to see what's below as well. (Credit: Florida Atlantic University)

The Potential

This strategy provides the spookfish a valuable advantage as it competes with other animals to find food and avoid becoming food in the almost-total darkness of the mesopelagic zone. And it could provide equally valuable advantages to specialty cameras and other human-made systems that would benefit from the ability to form crisp images over a broad field in extremely dim light.