By Ivan R. Schwab M.D. F.A.C.S.
Sharks always draw a crowd. We have a macabre fascination with these creatures because of their commanding presence and predatory lifestyle. Such a lifestyle demands high quality sensory systems, something sharks have had millions of years to develop.
The class Chondrichthyes arose and separated from other fish in the Silurian, over 400 million years ago. It includes all fish having a cartilaginous skeleton. Evolutionarily, these fish followed a different path by separating from bony fish and evolving into two different groups: elasmobranchs (including modern sharks, skates, and rays) and a much smaller group of fish called holocephalans (including elephant fishes and chimeras).
Some of the elasmobranchs, and in particular the sharks, have perhaps the best olfactory system of any group on the planet. Hammerhead sharks are particularly interesting in this regard; they have one of the largest olfactory bulb (collection of sensory cells for smelling) to brain ratio of any species and must rely heavily upon this sense. The scalloped hammerhead has an olfactory bulb that occupies 7% of its total brain mass as compared to approximately 3% for sharks in other families. Additionally, much of the forebrain in these sharks is devoted to the intrepretion of odors.
The unusual flattened head of hammerhead sharks, termed a “cephalofoil,” probably evolved for improved sensory perception although it isn’t clear for which sense. This unusual cephalofoil design allows for improved stereo-olfaction. Hammerheads have special grooves leading to the wide-spaced nares (essentially the nasal passages at the distal tips of the cephalofoil), which lead to enormous olfactory rosettes. Thus these sharks have true stereo-olfaction. These olfactory abilities, almost certainly lead this cartilaginous fish to its prey since hammerheads can detect one part per 25 million of blood in seawater. Other species of sharks have an excellent sense of smell, but hammerheads have to be among the best.
The cephalofoil of the hammerhead also houses electroreceptors, called the “ampullae of Lorenzini,” unique to elasmobranchs and the chimera. This unique organ senses low-level electric current in water with sampling done via pores distributed along the dorsal and ventral surface of the cephalofoil. The utility of the electrosensory abilities is poorly understood although prey location and migration have been proposed. While scything its way through the water, a hammerhead processes odors, weak electric currents, and visual inputs, although it is not clear how these signals are reconciled and integrated.
What role does the morphology of the eye and vision play in the sensory abilities of hammerheads or other sharks?
Sharks and other elasmobranchs have different visual capabilities depending on their niche. Shark eyes have a rather typical vertebrate pattern and closely resemble fish eyes. Most, if not all sharks, have eyeshine, which is also called a tapetum, to increase light capture in dimmer environments. Most sharks have rods (night vision) and cones (daytime and color vision), although some sharks that live almost exclusively in dim or dark environments have only rod retinas. But those sharks, such as reef sharks, live in brightly sunlit environments will have a relatively high ratio of cones and probably have color vision. For example, the lemon shark (Negaprion brevirostris) and the silky shark (Carcharhinus falciformis) have rods and cones in roughly the same ratio, or even a higher ratio when compared to humans. In contrast, sharks that live in dim environments, such as benthic (ocean floor) sharks, may have an all-rod-retina and use vision for some purposes but rely on other senses for prey capture or predator avoidance.
Color vision in sharks is controversial. There is behavioral evidence that some coastal reef sharks have a form of color vision somewhat like color deficient male humans, but there is no universal agreement on this. Some sharks, especially those that live in darker environments certainly don’t have color vision and would have no reason for it. Some of the rays, however, definitely have color vision, so it is quite possible that at least some sharks do too.
Similarly, visual abilities and integration of the various sensory input into the brain in sharks is poorly understood, and very difficult to study. In hammerheads at least, a portion of the brain called the tectum receives input from the visual system as well as the auditory, mechanoreceptive, electroreceptive, somatosensory, and trigeminal nerves. It isn’t clear how these inputs are integrated, but the high degree of sensory input suggests that these creatures are tuned in to their surroundings with magnificent sensory perception.
Empirical evidence, though, tells us that sharks are inherently robust, vigorous, clever, and ultimately survivors. Sharks are ancient, inhabit all the oceans, and remain highly successful predators. Although the lineage has changed and radiated in many different directions, it has survived many global extinctions including the great dying of the Permian when 96% of all species were terminated. If it weren’t for shark–fin soup and massive fishing vessels, few sharks would be threatened by anything. These are magnificent animals with sensory mechanisms we are only beginning to understand.
Ivan R. Schwab M.D. is currently a professor at the University of California, Davis where he has worked as an Ophthalmologist for over twenty years, and was on the faculty at West Virginia University for seven years before coming to UCD. He is the author of Evolution’s Witness: How Eyes Evolved. His strong interest in biology and natural history has led him to investigate a diverse range of topics including ocular stem cells, bioengineered tissues for the eye and comparative optics and physiology. He has published extensively in these fields, with three previous books to his credit, and he was the winner of the 2006 IgNobel for Ornithology. He has combined those interests with one in evolution to produce this text on the evolution of the eye. If you are interested in more information on sharks or visual perception, go to www.evolutionswitness.com.