An Adaptable Design
In terms of real estate, we do not rule this planet. Sharks do. For their kingdom is far greater than ours. More than 70% of the globe is covered by water. The ocean is the single largest interconnected living space on Earth. Sharks are the dominant predators in virtually every marine habitat — and in a few freshwater ones, as well. In their liquid realm, we are almost helpless. We can barely move or see and we cannot breathe at all. Yet here, sharks glide with the effortless grace of a magician’s silk scarf. But even sharks cannot glide anywhere they want.
From our terrestrial perspective, the ocean may seem remarkably constant, but nothing could be further from the truth. The ocean is an incredibly dynamic, ever-changing environment. There are constant variations in water temperature, concentrations of dissolved oxygen and salts, ambient light levels, and movement of water masses.
Great oceanic gyres stir both water-borne nutrients and marine life. The rotation of the Earth induces currents to arc clockwise in the Northern Hemisphere and counterclockwise in the Southern until they collide with land. Currents then flow along irregular coastlines, shaping and being shaped as they go. At irregular intervals, upwellings bring cold, nutrient-rich water to the surface, fostering vigorous ecosystems. Yet during El Niño events in the eastern Pacific, these upwellings periodically cease, causing large-scale changes in current and weather patterns. Bottom topography and the underlying geomagnetic field change continually, sometimes violently. In response to these changes in the physical environment, an astonishing diversity of living things swarm and pulse, ebb and flow.
Sharks, too, are part of this restless system, facing an endless procession of potential predators and prey against a writhing chemical and geologic backdrop. To some extent, sharks must interact with all the multitudinous players in this great oceanic drama.
The basic design of sharks is remarkably adaptable. This adaptability allows these animals to respond to the challenges imposed by their complex, dynamic marine environment. All modern sharks were derived from a single common ancestor, from which they inherited certain basic features. The extent to which a given shark species can adapt to a particular aquatic environment depends largely on the plasticity of their basic design.
System after system, sharks are a marvel of elegant design. Of all shark characteristics, the best known and most feared is their teeth. Shark teeth are unique in that they are serially replaced. New teeth are continually produced, used, and shed, riding forward on the tooth bed as though on a dental conveyer belt. Shark teeth feature a structure that renders them extraordinarily resilient and they come in an amazing range of forms.
Each tooth form is adapted to capturing and consuming a specific range of prey types. Stiletto-like teeth are well suited to grasping slippery-bodied prey that is small enough to be swallowed in a single gulp. Multi-cusped and molar-like teeth are ideal for crushing hard-shelled prey. And triangular, serrated teeth are fearsomely efficient at slicing hunks of flesh from prey too large to be swallowed whole.
The jaws wielding these teeth are a splendid bit of bioengineering. Being slung loosely beneath the skull allows the jaws to be protruded from the head with astonishing dexterity. Protrusion of the jaws creates a partial vacuum in the throat region that helps suck in prey and also extends the reach of those terrifying teeth.
Shark skin is no less amazing than are the teeth. The skin is remarkably tough and armor-like, studded with tiny, thorny scales called “dermal denticles”. Each of these dermal denticles has the same basic structure as shark teeth, featuring an outer coating of a hard, enamel-like material over a layer of dentine which, in turn, surrounds a pulp cavity with a blood and nerve supply (in fact, shark teeth are actually highly enlarged and modified dermal denticles). Collectively, these denticles are sculpted to reduce drag and enable sharks to stalk their prey in ghost-like silence.
The internal skeleton of sharks is mostly cartilaginous, making it tough but light in weight. In regions where strength is particularly important — such as the jaws and vertebrae — the skeleton is fortified with unique hexagonal plates of calcium salts called “tesserae”. Sharks’ muscles attach directly to their armor-like skin, which serves almost as an external skeleton. Mechanically, this is a very efficient arrangement, minimizing energy loss as muscle contraction is translated into explosive action.
Sharks are absolute masters of the low-energy lifestyle. They typically have very low metabolic rates, which enable them to wait long periods between meals. This quality enables them to pick and choose their food items very carefully and often allows prey to be lulled into a false sense of security. It also enables sharks to store energy in their tissues for use during long migrations. The vast majority of shark species are cold-blooded, relieving them of the lavish energy demands we mammals accept as a fact of life.
Sharks further reduce their energy demands by increasing the efficiency of their buoyancy, digestion, and osmoregulation (the physiological mechanisms that maintain an optimal salt-to-water balance inside the body). Shark livers are typically very large and oily. This oil imparts near-neutral buoyancy over a wide range of depths without sustained effort. Shark digestive and osmoregulatory systems are likewise remarkably efficient. The intestine is internally partitioned, which maximizes food-absorbing surface area in a compact space. Retention of certain nitrogenous waste products — principally urea and trimethylamine oxide (or TMAO for short) — reduces the effort of osmoregulation. Shark form and function is thus wonderfully spare and economical, each feature a veritable touch of genius from the drawing boards of evolution.
Sharks are particularly sophisticated in their mode of reproduction. Fertilization in all modern sharks is internal. Intromission is affected by paired, sausage-like copulatory organs called “claspers”, which are developed along the inner margin of the pelvic fins of male sharks. Internal fertilization significantly reduces waste of eggs (which, unlike sperm, constitute a substantial parental energy investment) and allows at least part of gestation to take place in the relatively safe environment of the mother shark’s body. About a third of all shark species lay eggs, the remainder giving birth to live young. The more ‘advanced’ requiem sharks nourish their young via an organ very similar to a mammalian placenta. The sophisticated reproductive mode of sharks allows these fishes to produce exceptionally large and well-formed young. Such precocious young stand an excellent chance of surviving to maturity simply by being too large and mobile to be eaten by most potential predators.
As generalist predators in dynamic but relatively stable ecosystems, sharks feature low genetic diversity and are extremely slow to change. But that does not mean that they are slow-witted. Most sharks grow very slowly and mature rather late in life, typically allowing many years to hone hunting and social skills. Many sharks also feature a relatively large, well-developed brain, an exquisite battery of senses, and surprisingly complex behavior. Experiments have demonstrated that some sharks have brain mass to body mass ratios on par with some mammals and that they learn and remember at least as well.
Shark sensory systems render them keenly attuned to every sight, sound, vibration, chemical, and electrical cue in their liquid environment. The sensory acuity of sharks is legendary, making them among the most sensitive and responsive animals with whom we share our planet. The mental complexity and sensory talents of sharks have undoubtedly contributed to their long-term success.
Sharks possess a remarkable constellation of features that has demonstrated itself to be extremely successful, enabling them to survive and flourish through more than 425 million years of environmental change. Today, some 390 species of sharks are known. Collectively, they represent an astonishing diversity of forms and lifestyles that exploit a remarkable range of aquatic habitats.
But not all shark species are found in all aquatic habitats. Some species enjoy the freedom of very broad distribution, while others are restricted to very limited ranges. Where each shark species lives represents a dynamic compromise between the adaptability of shark design and firm physical and chemical living conditions imposed by the environment. Each aquatic habitat presents sharks with a distinct suite of opportunities and challenges. To a very large extent, how well an individual shark can take advantage of the former and cope with the latter dictates where it can survive and flourish.
Since the ocean is regionally variable and evolutionarily related groups of sharks share similar requirements and tolerances, distributional patterns are bound to occur. Defining and explaining these patterns falls under the scientific discipline of zoogeography. For the purposes of this book, I have divided the World Ocean into 26 zoogeographic regions and 10 habitat types. The zoogeographic regions are illustrated and defined on the endpapers. Each habitat is defined and characterized in a separate chapter, followed by an exploration of how selected shark species make a living in that habitat.
This section, "Shark Ecology", has three main purposes. It will help you anticipate what kinds of sharks are likely to be seen in a given region or habitat. It will help you recognize the most commonly seen shark species. And it will explain how selected shark species thrive in each habitat type. In this way, it is hoped that you will better understand and appreciate why a given shark species lives where it does and not somewhere else.
For sharks, as in human real estate, location is everything.