The Aquatic Eye David G. Heidemann Foreword by Ivan R. Schwab
Introduction It is with great pleasure that I have the opportunity to share some of my passions with you: marine biology and underwater photography. Being an ophthalmologist, I pay close attention to the eyes of ocean creatures. While many spectacular books have illustrated the beauty of ocean life, none have emphasized the eyes. I found myself asking many questions about the eyes of the ocean creatures that I was observing. I wondered, for example: Why is the lens of fishes so large and spherical compared to the lens of humans? Why is the pupil of fishes pear shaped? Why do some fishes have a shimmering cornea? The evolution of functional eyes was a huge factor in the explosion of life in the Cambrian period. Aquatic animals have evolved an amazing variety of ocular structures and functions in order to thrive in their particular environments. This book illustrates and describes the beauty and diversity of the eyes of these ocean animals. The text was kept short and accessible. For a wealth of additional information, please refer to the two excellent books and other articles listed in the references. The emphasis of this book is on the eyes of teleosts (bony fishes), elasmobranchs (sharks and rays), and cephalopods (squids and octopuses). A few examples of compound eyes in arthropods (crabs, lobsters, and shrimp) are included at the end. All images in this book show marine life in its natural setting and were taken while diving or snorkeling in the Atlantic Ocean, the Caribbean, and Hawaii. Great care was taken never to harm or stress any of these wonderful animals. Many thanks to my friend and colleague, Dr. Ivan Schwab, Professor of Ophthalmology at University of California, Davis and author of the outstanding and definitive book on eye evolution Evolution s Witness: How Eyes Evolved for educating me on the aquatic eye. I hope you find as much joy reading this book as I did assembling it. Dave Heidemann, 2014 2 The Aquatic Eye
Grey Angelfish, Curacao, 2014 The Aquatic Eye 3
What are the parts of an eye? CORNEA the front, clear part of the eye analogous to a crystal over a watch. It covers and protects the inner structures of the eye including the iris, pupil, and lens. FOVEA a small depression in the retina where visual acuity is highest. IRIS the thin, circular tissue behind the cornea which controls the shape and size of the pupil. In humans, the color of the iris gives the eye its color. LENS the solid, round structure behind the cornea that refracts (focuses) light rays entering the eye onto the retina. The lens is attached to the eye wall by various ligaments. PUPIL the opening in the iris that allows light to pass through the cornea and the lens, then on to the retina. PHOTORECEPTOR the individual cells in the retina that turn light into a chemical impulse. Rods and cones are the two basic types of photoreceptors. Cones allow color perception and rods have better sensitivity to light OPTIC NERVE the structure composed of retinal cell processes which exits through the back of the eye and transmits visual information to other regions in the brain. RETINA the light sensitive layer of tissue that lines the inner surface of the back of the eye. The optics of the eye (cornea and lens) focus light to create an image on the retina. Eye of bony fish 4 The Aquatic Eye
How does the eye work? There are two basic types of eyes compound and camera. Most invertebrates, for example insects and crabs, have a compound eye that is composed of many individual optical units, with light entering though multiple openings. This book will focus on the camera eye where light is transmitted and focused by a single opening (cornea and lens) as it passes through the eye and is focused on the retina. The retina then transmits information through the optic nerve to the brain. The iris and pupil act as a diaphragm to control the amount of light which enters the eye. A camera eye is found in vertebrates (mammals, reptiles, birds, and fishes) and cephalopods (squids and octopuses). Cephalopods are invertebrates, but they have also evolved a sophisticated camera eye that is amazingly similar to the eye of vertebrates. Other invertebrates with camera eyes include spiders, scallops, conchs, and even some jellyfishes. Vision helps animals survive by allowing them to find food, avoid predators, seek shelter, and find suitable mates. Two basic and important aspects of an eye are the ability to see fine detail (resolution) and ability to see in low lighting. Resolution depends on the nature and quality of the optics (cornea, lens), size of the eye, and the size and density of individual light-sensitive units (photoreceptors) in the retina. Sensitivity depends on the size the eye, the size of the lens and pupil, and on the number and quality of photoreceptors. The images in this book will demonstrate how the eyes of various aquatic animals in different environments have evolved to meet their unique visual needs. Compound Eye Camera Eye Spiny Lobster, Curacao, 2011 Barracuda, Florida Keys, 2006 The Aquatic Eye 5
A few more definitions: ACCOMMODATION the ability of the eye to change its focus from distant to near object (and vice versa). FOCAL LENGTH the distance between a convex lens and the point where parallel rays of light converge (or are focused). INDEX OF REFRACTION refers to the ability of a lens to refract light when passing from one medium into another. The higher the refractive index of the lens compared to the refractive index of the surroundings (air or water), the more refractive power. REFRACTION the bending of waves when they enter a medium. For example, the lens refracts incoming light so that it is focused on the retina. SPHERICAL ABERRATION an optical effect that occurs due to increased refraction of light rays when they strike a lens near its edge compared to when they strike a lens near its center. A spherical lens with high refractive power has more spherical aberration than a flatter lens with low refractive power. Snapper, Grand Cayman, 2007 6 The Aquatic Eye
Balloonfish, Curacao, 2006 The Aquatic Eye 7
Bony fishes: Why is the cornea flat? The cornea of humans is steep (highly curved) and accounts for more refractive power than the lens. However, in water, the cornea would have no refractive power whether it was steep or flat because the index of refraction of water and cornea tissue is similar (Appendix B). Therefore, there was no selection pressure or advantage for fishes to evolve a steep cornea. In fact, a flat cornea would be helpful in aquatic animals in another way it would protrude less and create less drag in the water than a steep cornea. The cornea of humans must be extremely smooth to allow a clearly focused image. However, because of the similar refractive index of water and corneal tissue, a rough corneal surface in water does not result in a blurred image (Appendix C). In the images on these pages, while the cornea surface looks relatively smooth, it is actually quite rough and irregular compared to the cornea of humans and land animals. Porcupinefish, Bonaire, 2012 Porcupinefish, Bonaire, 2012 8 The Aquatic Eye
Balloonfish, Blue Heron Bridge, Florida, 2014 Balloonfish, Blue Heron Bridge, Florida, 2014 The Aquatic Eye 9
Checkered Pufferfish, Blue Heron Bridge, Florida, 2012 Checkered Pufferfish, Blue Heron Bridge, Florida, 2012 Why do some fishes have yellow corneas? Some shallow water fishes are able to change their corneas to become yellow or orange under direct sunlight. They do this by moving pigment granules (called melanosomes) in cells in the upper peripheral part of the cornea. These pigment granules reflect or absorb certain wavelengths of light and act as sunshades. The pigment-containing cells may be pear-shaped and have ribbon-like processes extending to the central cornea, forming a veil which reduces the amount of light entering the eye from above. The yellow or orange coloration helps the fish in bright light by: 1) limiting blurred vision which would result from focusing of different colors on different focal planes; 2) limiting glare by absorption of blue light; 3) improving contrast between different colored objects. Note the yellow cornea in the checkered puffer on this page and the facing page. In the upper two images on this page, note the parasitic isopod attached to the fish below the eye. The image on the facing page represents a close view of the eye from above. Note the yellow pigment and the pear-shaped processes originating from the upper part of the cornea. 10 The Aquatic Eye
Checkered Pufferfish, Blue Heron Bridge, Florida, 2013 The Aquatic Eye 11
Balloonfish, Curacao, 2005 Why do some fishes have shimmering or sparkling eyes? Iridescence is a type of structural coloration that applies to surfaces that change in color with viewing angle. These colors can appear rainbow-like, shimmering, or sparkling. Iridescence is produced when light encounters boundaries between media that differ in refractive index, creating colors that change with angle of view. We sometimes see iridescent coloration from films in soap bubbles and oil slicks. Iridescent coloration can be found throughout the animal kingdom, for example, in insect wings, bivalve shells, and bird feathers. Many fishes have corneal iridescence to limit the amount of light entering the eye like sunglasses. Bright sunlight from above is reflected without reducing the amount of light coming in from the side or below. In a darker setting, the coloration lessens or changes to allow more light into the eye. Note the marked corneal iridescence in the balloonfish eye viewed from the side on this page and viewed from above on the facing page. See references for a more detailed explanation of iridescence. Balloonfish, Blue Heron Bridge, Florida, 2013 12 The Aquatic Eye
Balloonfish, Blue Heron Bridge, Florida, 2014 The Aquatic Eye 13
Corneal iridescence Jawfish, Blue Heron Bridge, Florida, 2014 Jawfish, Blue Heron Bridge, Florida, 2014 14 The Aquatic Eye
Sea Robin and Diver, Blue Heron Bridge, Florida, 2014 Corneal iridescence Sea Robin, Blue Heron Bridge, Florida, 2014 The Aquatic Eye 15
Corneal iridescence Note that the pupil of the scorpionfish appears red. This represents light being reflected from the retina at the back of the eye. Flecks of corneal iridescence are visible within the red reflex. Scorpionfish, Blue Heron Bridge, Florida, 2014 Scorpionfish, Blue Heron Bridge, Florida, 2013 16 The Aquatic Eye
Striated Frogfish, Blue Heron Bridge, Florida, 2014 Corneal iridescence Striated Frogfish, Blue Heron Bridge, Florida, 2012 The Aquatic Eye 17
About the author Dave Heidemann has been hooked on the underwater world ever since snorkeling in Michigan inland lakes as a child. Dave has immersed himself in underwater photography and marine biology starting in the mid-1980s. He enjoys diving with friends and family and sharing the beauty and diversity of marine life with others. He is particularly interested in evolution and imaging of the aquatic eye. When not diving, Dave is a corneal transplant surgeon in the Detroit area. Please visit Dave at: www.theaquaticeye.com or contact him at: Dave@TheAquaticEye.com. The Aquatic Eye 75