Week Fifteen: Fishes and Sharks

The Age of Fishes , some 400 million years ago, marked a watershed in evolution. From these days forward, vertebrates existed on Earth. From Fishes arose reptiles and land mammals, thus marking the dawn of the Age of Dinosaurs and the Age of Mammals. The appearance of fishes soon displaced the shelled cephalopods  from the top of the food web and have remained there ever since. Today, more than 12,000 species of fish roam the oceans. Many of these are of vital importance to humans, supplying nutrition to millions of people. The many strategies fish have developed to exploit the oceanic environment is the subject of this section.

15.1 Three Kinds of Fishes

All fishes belong to the Phylum Chordata  in the Subphylum Vertebrata . You may recall that the sea squirt also belongs to the Phylum Chordata, but, not having a backbone, is placed in the Subphylum Urochordata . Other chordates include the marine reptiles , such as the Galapagos iguana , sea turtles , sea snakes , and marine crocodiles  (such as the estuarine crocodile).

The first fish to appear in the fossil record were the Agnathans , the jawless fish. These fish, which appeared about 430 MYA, were likely preceded by other soft-bodied cartilaginous chordates, but the fossil record has little to tell of early vertebrate evolution. Soon after the rise of Agnathans , the cartilaginous fishes arose, paralleled by the appearance of the largest class of fishes in the oceans, the bony fishes.

Members of these three classes of fish still exist today. The jawless fishes, the Class Agnatha  (a=lacking, gnathos=jaw), include hagfish  and lampreys . The cartilaginous fishes, Class Chondrichthyes  (chondres=cartilage, ichthys=fish), includes sharks, rays, skates , sawfishes , and ratfish . The Class Osteichthyes  (osteum=bone, ichthyes=fish), the "true" or bony fishes, includes pelagic fish, deep-sea fish, bottom fish, coral reef fish, coastal fish, surf fish, and any other bony fish that may be found swimming around in the sea.

15.2 The Agnathans

The first Agnathans  found in the fossil record had small sucking mouths with rows of gill openings along the bottom of their heads. Their living relatives, the hagfish  and lamprey aren't much different. These fish, which are eel-like in appearance have simple boneless mouths adapted for sucking, not biting. Their gills  are exposed and not covered by an operculum  (see below) as with other fishes. They also lack paired pectoral fins , a feature shared among all fishes, including the eel. They also lack paired pelvic fins  which is standard issue in most fishes. Hagfishes and lampreys  can reach lengths up to 3 feet, but most are smaller.

All of the 32 species of known hagfish  are marine and many live in the deepest depths of the ocean. They typically feed on polychaete  worms or scavenge on weak or dead organisms, but occasionally they will burrow into a living organism and devour it alive from the inside out. Hagfish eat flesh by attaching their mouths to the carcass and tying their body in a knot. The knot slips down towards its head allowing the hagfish to brace against its prey and get a better grip. Hagfish also produce copious amounts of slime, which it uses to protect itself and its food. These fish are perhaps best known as the deep-sea fishes that appeared in some of the first pictures of abyssal benthic organisms lured by buckets of dead fish. Whatever their notoriety, they are certainly among some of the most disgusting fish in the ocean and have little to recommend them except that they make good fake "eelskin" leather goods, a popular item in South Korea .

The lampreys , the other group of fishes comprising the Agnathans , possess a toothed mouth which they use to bore a hole into their prey. They typically live as parasites on other fishes, but they have also been found sucking the juices out of porpoises and whales. Fortunately, lampreys will usually detach before they kill their prey.

15.3 The Chondrichthyes

The second class of fishes need no introduction. The cartilaginous fishes in the Class Chondrichthyes  have prowled the world for 280 million years and some have changed little in that time. The most famous fishes in this class, the sharks, have been around for twice as long as the dinosaurs. Members of this class are generally divided into two groups, the elasmobranchs, which include the sharks, rays, and sawfishes , and the chimaerids , a small group which consists of the ratfishes. About 350 species of sharks and 320 species of rays are known to exist.

Fishes in this class are entirely boneless. Instead, their internal skeleton is comprised of cartilage, the same material that makes up our ears and nose. However, they do have teeth and paired fins, like other fish. Most have two dorsal fins , a pair of pectoral fins  immediately behind the gills , a second pair of pelvic fins  near the tail, a single anal fin , and a tail or caudal fin . In sharks, the tail may be tilted upwards with most of the fin on the bottom, called a heterocercal tail , or the two lobes of the tail may be equal, called a homocercal tail . Heterocercal tails thrust the animal forward and creates lift in combination with the pectoral fins. These kinds of tails are common on bottom-feeding sharks, such as the nurse shark, and on mid-water sharks, such as the blue shark . Homocercal-tailed sharks are the fastest sharks and appear on the most famous of sharks, the great white shark .

The skin of chondrichthyes is very abrasive, composed of small tooth-like denticles or scales. In fact, the teeth of shark happen to be enlarged scales located in the mouth. As you probably know, shark teeth grow in rows one behind the other that move forward to replace teeth that are lost during feeding.

Elasmobranchs that feed on fast-moving prey, such as sharks, tend to be torpedo-shaped and have mouths with teeth designed for grabbing and tearing. Elasmobranchs that feed on the bottom, such as rays and skates , tend to have a flattened shape with enlarged pectoral fins  that work like wings. Skates and rays feed on shellfish and other bottom-dwelling invertebrates and they like to bury themselves in bottom sediments to hide. Some species of rays, known as stingrays , have a sharp barb on the end of their tail that lashes out at any organism that disturbs them, including humans. Getting lashed by a stingray is not a pleasant experience and they can inflict a serious wound. To avoid such an occurrence, shuffle your feet on the bottom as you walk in shallow waters where stingrays are known to occur (such as Florida ). Usually, the stingray will be startled and swim away before you step on him.

The largest elasmobranchs include the basking shark , the whale shark , and the giant manta ray . Not surprisingly, these animals are filter-feeders, feeding at lower trophic levels , i.e. on plankton rather than fish or squid. The whale shark can reach sizes of more than 60 feet and weigh in excess of 90,000 pounds! Although they are a shark, humans are not part of their diet, and these docile organisms swim slowly at the surface with their mouth gaping, filtering enormous quantities of seawater (up to 2,500 tons per hour) through their gill rakers , a fine mesh structure located on their gills . By backflushing water through the gill rakers, accumulations of plankton can be concentrated and swallowed.

While some species of sharks are known to leap into the air, such as the spinner sharks , few can boast the acrobatic capabilities of the manta rays. The largest of the manta rays, the giant manta ray , has a wingspan more than 20 feet in length. Armed with pectoral wings that allow it to glide through the water with the greatest of ease, these animals can also leap from the water for 20-30 yards. A more subtle acrobatic feat of the manta is its feeding "swim." Mantas slowly beat their pectorals and perform loops in the water, scooping the water with their cephalic fins, filling their gill arches with rich and nutritious plankton. They rise to the surface, swim upside down, and glide downwards, completing loop after loop like a "barnstorming pilot who never tires."

Elasmobranchs have adapted a number of interesting methods for obtaining food. Another type of manta, the torpedo ray , shocks its victims with a 1,000-watt jolt of electricity. Sawfishes, which reach lengths up to 18 feet, slash their saw-tooth proboscis into schools of fish and leisurely round up the dead and wounded. Yet another elasmobranch, the cookie-cutter shark , rams its favorite food, whales and tuna, with its sharp O-shaped mouth (like a cookie cutter) and extracts a plug of flesh. Victims of the cookie-cutter shark are henceforth tagged with an O-shaped scar on their bodies for the rest of their lives.

The final group of chondrichthyes are the chimaerids , or the ratfish . These interesting fish live at mid-water and deeper depths and get their name from the rat-like appearance of their heads. These chondrichthyes do not have exposed gills  or gill slits ; rather they have an operculum  that covers their gills, much like bony fish.

15.4 The Osteichthyes

The third class of fish is the Class Osteichthyes . The "true" bony fish are certainly the most diverse vertebrates in the ocean. Over 12,000 species  live in the marine environment with another 6,800 species in freshwater environments. Among the bony fishes, the largest group, the teleosts, which belong to the Order Teleostei (teleos=perfect, osteon=bone), make up about 90% of all the fish species in the sea. Among marine species, fish rank third in number; molluscs  (nearly 60,000 species) and crustaceans  (more than 30,000 species) comprise the largest groups in the marine environment.

While similar in appearance to sharks, bony fishes differ in fundamental ways. Bony fishes have skeletons made of solid bone and their teeth arise independently, not as an extension of their scales, as in sharks. Bony fishes also have a flap, called an operculum , that covers their gills . Fishes also have a swim bladder , an air-filled sac used to maintain buoyancy , and different methods for dealing with osmotic challenges in seawater.

Within the basic fish body plan, which includes typically two dorsal fins , paired pelvic fins , paired pectoral fins , one anal fin , and one caudal fin , a seemingly infinite variety of fishy forms have evolved. The variety of fin types evolved by fishes relates largely to how they feed and where they live. In fast-swimming fishes, such as the bluefin tuna , have added finlets  behind the dorsal to give them better control and speed. Other species, such as snappers  and groupers  have fused the dorsal fins into one long fin, which provide them to quickly ambush their prey, or, in the case of butterflyfish  and surfperch , allow them to quickly escape their predators .

Fish living on the bottom have become more eel-like, like moray eels , or flattened, like flounders . These fish have fused their dorsal fins  and anal fins  and lost their pectorals or pelvic fins . The modification of their fins into these shapes allows them to hide in rocks or sandy bottoms, and helps them catch prey. Still other fish have decided to use their fins instead of their tail for propulsion. The use of fins provides low-speed maneuverability, like a Harrier jet, for species such as the triggerfish  or pufferfish . Fishes such as sea horses , pipefish , and trumpetfish  have elongated bodies propelled by their fins.

Some fish have even taken to flying. The flying fish  has modified pectoral fins  that allow it to glide through the air above the surface of the water to escape predators . Flying fish can easily cruise tens of yards in a single bound, floating on their silvery wings, skimming the waves. Other fish have taken to hitchhiking. The remora  has modified its dorsal fin into a sucker, which allows it to hitch a ride from sharks, whales, mantas, or other large swimmers. Still other fish have taken to walking along the bottom. The batfish , who lives in tropical waters, walks on his modified pectoral fins and pelvic fins  to traverse the terrain.

Certain fishes have also modified their fins to lure prey. As we learned earlier in the semester, the angler fish has modified a spine on his dorsal fin into a fishing rod, designed to lure prey into his mouth. Other fish have modified their fins to discourage predators . Perhaps the best example of this adaptive strategy is expressed in the lionfish , whose sharp and flamboyant pectoral spines contain poisonous venom.

Finally, at least one fish has made such a job of his fins that he has become a plankton, no longer able to swim against the strong ocean currents. The giant sunfish, Mola mola , has virtually lost his caudal fins, and relies on one enlarged dorsal fin, one enlarged anal fin , and two feeble pectoral fins  for locomotion. Despite their awkward appearance and sluggish nature, the giant sunfish is a sight to behold and a rare treat for anyone lucky enough to see these shy fishes.

There are thousands of other interesting fishes that we could review. We haven't even mentioned the marlin  or the stonefish  or the clownfish  or the swordfish , all with their own unique set of adaptations for making a living in the sea. We could spend an entire semester studying fish and never be tired of their flavor. Unfortunately, we only have time to skim the surface here. Fortunately, we can gain a general appreciation for the problem-solving skills of fishes by looking at the broad categories of adaptations that they have achieved.

15.5 Adaptations of Marine Fishes

What are the problems of a fish? Though this may appear a rhetorical question, a brief consideration of the characteristics of the marine environment will help you appreciate what a fish has to go through.

First, water is about 1000 times denser than air. Have you ever tried running through waist-high water? If so, then you can appreciate the difficulty of moving through this medium. Fishes have been forced to adapt locomotory mechanisms for propelling themselves through water and hydrodynamic mechanisms to streamline their bodies.

Have you ever tried to float in a lake or the ocean? Maintaining yourself at a constant depth requires some kind of buoyancy  regulation or at least tissues with similar densities to seawater. Carrying around a set of bones almost certainly insures that you will sink without some kind of buoyancy mechanism.

Have you ever tried to breathe underwater? Probably not. Yet fish, like all heterotrophic organisms, require oxygen  to live. Thus, some means for extracting oxygen dissolved in seawater must be found.

Have you ever tasted seawater? Obviously, it's salty, and a person who drinks large quantities of seawater runs the risk of renal (kidney) failure and death. Only through slow acclimation can a human drink seawater safely. Yet marine fish require freshwater just like you and me. Their bodies are less saline than seawater and to prevent water loss due to osmosis, they must continually replenish their internal supply of water. Freshwater fish have the opposite problem; their bodies are more saline that the environment they live in so they must find a way to get rid of water as it is constantly absorbed through their bodies as a result of osmosis. Thus, fishes have developed osmoregulatory mechanisms to cope with changes in salinity.

Finally, the watery environment offers poor light penetration for seeing and provides little protection in the way of solid objects to hide behind. As such, fishes have evolved elaborate sensory systems for detecting objects in their environment and found a multitude of waters to blend in or camouflage  themselves.

Let's take a look at how fishes have adapted to the oceanic environment.

15.6 Buoyancy

Living in the oceans, particularly in pelagic environments, demands some means to stay afloat. To accomplish this, most fish have evolved a gas or swim bladder , an air-filled structure that regulates the buoyancy  of fishes. This structure forms from 5-10% of the body volume of the fish and has different means for exchanging air with the environment. As fish dive deeper, the increasing pressure of water reduces the volume of air in the swim bladder and fishes must "pump" in more air. Alternatively, as fish rise to the surface, the air in the swim bladder expands, and the fish must remove air from the swim bladder. If the fish rises too quickly, the swim bladder may explode. If you've ever seen the bloated remains of a fish pulled too rapidly from the depths, you'll know what I mean.

Fast-moving fishes, such as bonito  and mackerel , have dispensed with swim bladders  altogether because of the slow response time to changes in pressure. Swim bladders in these fishes would not be able to respond quickly enough to rapid changes in depth and would be more of a liability than an asset. These fast-moving fishes probably maintain buoyancy  by constantly moving and having a greater proportion of neutral or positive density tissues in their bodies. This latter means to achieve buoyancy is accomplished by increasing the fat content or creating pockets of oily lipids . These compounds, which are less dense than water, contribute to buoyancy. Lipids may be stored in various body parts, such as muscles, internal organs, and the body cavity. In sharks, lipids are concentrated around the liver. Of course, many marine mammals store lipids in blubber  to create buoyancy.

In addition to internal flotation devices to maintain buoyancy , nektonic fish have also adapted hydrodynamic mechanisms for staying afloat. One of these is the heterocercal tail , as discussed above, which provides lift in combination with the pectoral fins . In this sense, the pectoral fins act a lot like airplane wings, propelled by the "rear" engine, the tail. Sturgeon  and sharks, along with other primitive fish, use this mechanism to maintain buoyancy. The tradeoff, however, is that these fish have to be constantly moving to stay afloat, an attribute of sharks that is well known.

15.7 Locomotion

Fish also have adapted various strategies for propelling themselves through the watery medium. In fact, the adaptations of fishes to moving through the oceans makes them among the fastest animals on Earth. Short-bursts of 75 miles per hour  have been recorded for swordfish  and marlin !

To move through water, two things must be accomplished. First, the animal must have some means to propel itself. In fishes, this is accomplished by undulatory movements of the entire body or side-to-side motions of the tail and posterior portion of the body. Second, to achieve high speeds, the animal must be streamlined and employ hydrodynamic principles to reduce the amount of drag on its body. Drag  is the resistance of a body to movement in air or water. Drag increases as the speed of the fish increases, and body features such as contour, frontal area, and skin texture affect the amount of drag on a fish.

Fishes that use eel-like movements of their body for propulsion are not as efficient as fishes that swim using their tail. S-shaped movements expose more of the body to the water and increase drag. This type of swimming also requires more energy and eel-like fish are not known for their long-distance swimming abilities.

Fast-moving fishes attain high speeds both by increasing the efficiency of their tail strokes and by reducing drag on their bodies. In swordfish  and tuna, their caudal fins (tails) tend to be lunate  and the region of their body just before the tail, called the caudal peduncle, tends to be very narrow. In this way, propulsive forces are generated just by the tail, which works like a rapidly flapping hinge to propel the fish through the water. Whales also employ this strategy, except that their tail motions are up and down instead of side to side.

Another way that fish move faster is to reduce their width relative to their length  to an optimal value that creates the least turbulence. Width to length ratios for swordfish , tuna, Greenland  sharks, dolphins , and the blue whale  average near 0.25, which is ideal for teardrop-shaped objects moving through water. As a result of this streamlining, these fish deliver between 60% and 80% of their muscle force to the tail. These high efficiencies of swimming allow these animals to achieve high speeds over short periods (such as the swordfish and marlin , mentioned above), or to maintain constant speeds over long periods of time. The bluefin tuna , a fish without a swim bladder , swims at a near-constant speed of 9 miles per hour for its entire lifetime, a journey of 1 million miles.

15.8 Gas Exchange

Fish need oxygen  to live. They also need to remove waste products, such as carbon dioxide, generated as a result of metabolism. For this reason, fish have evolved blood-enriched membranes called gills . These membranes are highly convoluted and packaged in a series of thin filaments or plates. By passing water through their gill membranes, fish accomplish the gas exchange that is so necessary for metabolic processes.

Dissolved oxygen  is seawater readily diffuses through the gill membranes and into the bloodstream of fish because concentrations of oxygen in seawater are greater than they are in the blood of fish. Similarly, the concentrations of carbon dioxide dissolved in the blood of fish is greater than seawater, so carbon dioxide readily diffuses out of the fish and into the seawater. Helping this process is the way that blood and water flow across the gills ; blood typically flows in a direction opposite to that of the water flow in a countercurrent flow that assists gas exchange.

Fast-swimming fish require large amounts of oxygen  to maintain the high metabolic rates required for fast swimming. As a result, the surface area of their gills  may be as much as 10 times the surface area of their bodies. For slow-moving fishes, the gills may be much smaller. Gas transfer efficiencies are also very high for active fishes, who are able to extract as much as 85% of the oxygen dissolved in seawater. In comparison, air-breathing vertebrates only extract 25% of the oxygen in air that enters their lungs.

15.9 Osmoregulation

Because the bodies of marine fishes are less salty than seawater (i.e. they contain smaller concentrations of dissolved constituents than seawater), water tends to leave their bodies as a result of osmotic pressure. Freshwater fish, whose body "salinities" are greater than freshwater, tend to gain water for the same reasons. Organisms, such as marine fish, whose bodies are less salty than medium in which they live are said to be hypotonic. Organisms, such as freshwater fish, whose bodies are more salty than the medium in which they live are hypertonic. The process by which organisms maintain their internal salt balance is known as osmoregulation .

The skin of both marine fishes and freshwater fishes is nearly impermeable to water and salts. However, across the gills  where gas exchange takes place, water and salts may readily diffuse. To deal with this problem, marine fishes drink plenty of seawater (up to 25% of their body weight per day in some species), secrete the excess salt using salt glands in the gills, and excrete very small amounts of urine to conserve water. Freshwater fish, on the other hand, drink no water (water is absorbed by the fish through osmosis), absorb salt in their gills, and excrete copious amounts of urine to remove excess water.

Some fish, such as salmon, who spend part of their lives in seawater and some of their lives in freshwater, have raised osmoregulation  to an art form. These fishes must be able to tolerate both hypertonic and hypotonic environments.

15.10 Sensory Systems and Camouflage

Competition for food has certainly placed considerable adaptive pressure on the fishes. As a result, they have developed not only elaborate mechanisms for finding prey, but they have also developed elaborate mechanisms to avoid becoming prey.

Because light penetration in the oceans is limited, most fish have well-developed eyes. Sight is very important for finding prey or avoiding predators  and fish respond readily to moving shadows. In fact, most humans that have spent any time underwater often express their "concerns" over shadows lurking just beyond their vision. Good vision also allows fish to respond to visual cues emitted by photophores  for communication or reproduction .

Fish also make use of lateral line systems, a low-frequency response nervous system that covers their head and body. The lateral line system  of fishes allows them to sense vibrations in the water, changes in water pressure, changes in current direction, and possibly, changes in temperature and salinity. Sharks use their lateral line systems to find prey. This capability is especially advanced in blue sharks , which live off the coast of southern California and elsewhere. Fishes also use lateral line systems to detect prey, but this detection system is also useful for avoiding predators .

Perhaps the most interesting adaptations of fishes concerns their use of camouflage . Nowhere in the animal kingdom are the lines drawn so distinctly between those who want to hide it and those who want to flaunt it. Consider the strategic differences between a flounder, who uses chromatophores  to match the background pattern of the bottom on which it rests, with that of the highly ornate and decorated lionfish , who uses color and flamboyance to warn predators  that "I am not to be eaten." In this way, fishes use a kind of reverse psychology, letting predators know where they are so that they are not mistakenly eaten, an adaptation known as warning coloration .

Nearly all fishes use some kind of body shape or coloration pattern to avoid being detected. Some fishes, such as the Sargassum fish  or kelp bass , closely resemble the environment in which they are found, in this case, Sargassum weed or kelp, respectively. This form of camouflage , known as cryptic coloration , allows animals to blend in with their environment. The flounder, described above, uses an active form of cryptic coloration to match his environment.

Pelagic fishes rely on a different method of color blending called countershading . By darkening their top side and lightening their underside, these fish match the background against which they are seen. The dark countershading seen from a predator above the fish resembles the bottom or abyss into which the predator is looking. The white underside as seen from a predator below the fish matches the well-lit surface waters .

Of course, safety in numbers has always been a good defensive posture and many fish spend their lives in schools. A large mass of fish grouped together as a school often resembles a larger fish. That "large shadow" seen in the distance by a diver, is it a shark or just a school of fish? Most organisms would be unwilling to chance that the shadow is just a school of fish. One other interesting tidbit about schools that was recently discovered. Scientists once thought that the safest place for a fish in a school was in the very middle, where it is seemingly protected by all the fish surrounding it. However, new studies reveal that fish in the center are more likely to be eaten, as predators , such as sharks, dolphins , killer whales , and barracuda , lunge for the center of the school where the density of meet is the thickest. Personally, I'd rather hide under a rock.

All in all, the best defense is a good offense and nothing appears to replace speed as a way to avoid being eaten. If you can swim faster than anyone else, then it is unlikely that you are going to be caught and eaten. Thus, tuna, swordfishes, and other fast-moving fishes take their positions at the top of the food web by virtue of their speed. Still, we can't discount the poor flying fish , who actually leaves the water to avoid predators . Unfortunately, many flying fish end up on the deck of a ship, a "predator" for which there is no adequate defense.

15.11 Reproduction of Fishes

As with all fish strategies, there is no lack of variety where their modes of reproduction  are concerned. Some species produce millions of eggs and hope for the best as their larvae struggle to survive as members of the plankton. Others care for their eggs and guard them ferociously until they are hatched. Still others give live birth to their young, a strategy that exposes their young to the least dangers. The table on the following page provides a good outline of the kinds of strategies employed by fishes to reproduce.

Strategy/Fish

True Cod

Lingcod

Surfperch

Amount of parental energy devoted to care/nourishment of the young

NONE
Parents abandon fertilized eggs

INTERMEDIATE
Males guard fertilized eggs

MOST
Females give live birth after nourishment by placenta

Number of eggs or young

MOST
up to 7,000,000

INTERMEDIATE
up to 500,000

LEAST
25

Size of egg

SMALL
1 mm

LARGE
3 mm

No Eggs

Development of young at hatching or birth

LEAST
4 mm

INTERMEDIATE
12 mm

MOST
78 mm

(Percentage of maximum adult size at birth/hatching

0.2%

0.8%

52%

Exposure of young to hazard of plankton community

MOST

INTERMEDIATE

LEAST

One other strategy widely employed by fishes is the ability to switch environments to take advantage of differences in food availability or protection from one environment to another. Typically, this behavior involves living part of their life cycle in oceanic water and part of their life cycle in freshwater, or vice versa.

Species who are born in freshwater but spend their juvenile life feeding and growing in the ocean are known as anadromous  species. Anadromous fishes, which include the salmon, are most common in temperature latitudes, where ocean waters tend to be more productive than freshwaters. More then 250 species of anadromous  fish live in temperate latitudes.

Species who are born in the oceans but spend their juvenile life in freshwater are known as catadromous  species. Mullets are a good example of a catadromous  fish. More than 100species of catadromous  fish live in the tropics, where freshwater food webs  are more abundant than oceanic food webs.

We would certainly be remiss if we did not at least mention the commercial importance of fishes to man. Man's relationship with fishes as a source of nourishment, pleasure, employment, and even spirituality has existed probably since the first time a human managed to catch a fish in some pond or stream. Even today, fisheries supply about 10% of the world's protein and the number is growing steadily.

Still, man's exploitation of the sea and fishes in particular has had some very ugly and catastrophic consequences. Overfishing, use of drift nets, utter disregard for incidental catch, including dolphins , are just a few of the problems facing fisheries today.

15.12 Sharks

Feared by man since times ancestral, the shark evokes images of terror and horror to all who cross their path. Yet much of our hysteria over this animal is misdirected, and recent studies of sharks show them to be quite predictable and not-so-single-minded in their behavior. By observing sharks in their natural habitats and under natural conditions (i.e. without chumming and turning the water blood red), scientists have found several plausible reasons to explain shark attacks on humans. Unfortunately, our wholesale slaughter of these highly evolved animals threatens to remove them permanently from our planet. Only through careful scientific research and public education will the monstrous myths of these creatures be put to rest. In this section, we spotlight three local species, the Blue, the Hammerhead, and the Great White.

15.13 Shark Attacks

The first sharks to roam the ocean depths appeared more than 350 million years ago. While these ancestral sharks differ considerably from modern day sharks, the evolutionary success of sharks is little disputed. From the basic body plan of these ancestral sharks evolved the lean, mean, eating machine that characterizes sharks today, perfectly adapted with its torpedo-like body and hydrodynamic fins.

Interestingly, some of the earliest fossils of sharks  have been found in Ohio. Here, during the Upper Devonian, a broad sea extended southwest from the St. Lawrence Seaway to Arkansas. While the only hint of an ocean today are the amber waves of grain blowing in the wind, within the sediments along the banks of the Rocky River are the well-preserved 6-foot long bodies of Devonian sharks. Their sleek and streamlined bodies and their mouths with carnivorous teeth belie an efficient predator that fed on early fishes. Some specimens still contain the remains of a whole fish in their fossil bellies.

Today, sharks are among the most feared predators  in the ocean. While more than 350 species are known to exist, only 32 species have been documented as attacking humans. Of these, three sharks—the great white, tiger and bull sharks—appear most dangerous to humans. The smallest shark, the 6-inch cigar shark , lives at depths of 1500 feet in the Atlantic , Indian, and western Pacific  oceans, and feeds at night on squid and luminescent fishes. The largest shark (and fish, for that matter), the 60-foot whale shark , eats plankton, hardly the mark of a ferocious predator. Whale sharks are distributed worldwide in temperate and tropical waters.

Still, their reputation as killers is not totally undeserved. In California, twelve people have been killed by sharks since 1926. Less than 50 shark attacks have occurred in the last 25 years. Worldwide, reported shark attacks number in the low thousands, yet this number doesn't include the toll in areas of the world where shark attacks are rarely reported nor does it tell the story of thousands of sailors who have survived shipwrecks only to die at the jaws of a shark.

Consider these accounts from the annals of World War II:

November 28, 1942, a troopship carrying 900+ men was torpedoed by a German submarine. Most of the men successfully abandoned ship only to be ravaged by sharks while they drifted helplessly in lifejackets. Only 192 men survived and many of the bodies recovered had their legs chewed off.

November 11, 1943, a troopship carrying 1,429 men was torpedoed by a Japanese submarine. Only 448 survived while frenzied sharks climbed half out of the water onto life rafts to snatch survivors.

July 30, 1945, the U.S. cruiser Indianapolis,  the ship that delivered the nuclear warhead to the Philippines  that eventually destroyed Hiroshima , was torpedoed by a Japanese submarine 600 miles southwest of Guam . Although most of the 1,199 men aboard the ship succeeded in getting off the ship, only 316 survived. It took four days for rescue ships to reach the men and many of those who died were killed by sharks, "the blood spilling into the sea attracting wave after wave of these voracious killers." The recovered bodies were heavily mutilated and even some of the survivors carried home scars inflicted by sharks. The death toll of 883 men was the worst in American history.

Despite their voracious reputation, many species of sharks are in danger today. The recent popularity of sharks as food and medicines has caused their populations to be decimated in some parts of the world. Shark flesh has long been favored as food, as have the fins (for soup). The shark liver was once prized as a valuable source of vitamin A until synthetic vitamins became widely available and cheaper. Shark skin is widely used for leather goods and teeth are used in jewelry. Shark eyes have even been used for cornea transplant.

Among some fisherman, the practice of finning —catching a shark, removing its fin, and throwing it back in the water—is considered acceptable. One conscientious fisherman describes pulling up a 400-pound tiger shark flopping on his line, but completely finless. Overfishing and the shark's slow rate of reproduction  has brought some species to the brink of extinction. Such practices are repulsive and major international efforts are underway to prevent such occurrences.

Believe it or not, California law protects white sharks because they are a key predator for controlling populations of seals and pinnipeds. As a top level predator, they ultimately control populations of several other trophic levels . Without them, oceanic ecosystems would become unbalanced, having potentially catastrophic consequences for other organisms.

For that reason, and in the interest of advancing scientific knowledge, a number of researchers from various institutions are joining efforts to study shark biology and behavior and their interactions with humans. The Academy of Sciences , San Francisco State University , the Point Reyes Bird Observatory , and the California Department of Fish and Game  are cooperating to learn more about sharks. Additional efforts are underway by the Pelagic Shark Research Foundation  in Santa Cruz . Such studies are essential for understanding not only the role of sharks in oceanic ecosystems, but also for insuring a peaceful coexistence with humans.

15.14 The Blue Shark

Along the coast of southern California, and in most waters of the oceans, swims a shark who would qualify for all the benefits of a frequent-roamer mileage program, if such a program existed. The blue shark , or Prionace glauca , is a veteran swimmer of the world ocean, known for its extensive migrations in temperate and tropical waters. One blue shark , tagged off New York , was captured 16 months later off the coast of Brazil , a 3,740-mile journey.

Cousteau  calls the blue shark  "the most majestic of all sharks." The blue shark  is named for the brilliant blue color of its sides and back. Like most sharks, who exhibit countershading , the blue shark 's belly is bright white. Presumably, the blue shark 's blue topside provides camouflage  as it approaches its prey from below. Blending in with the brilliant blue waters off Catalina Island, this shark would be very difficult to see from below.

Perhaps the most distinctive feature about this sharks are its eyes. Its coal-black pupils rimmed with white have an impassive look to them. Blue shark eyes, like all shark eyes, are highly developed. In fact, shark eyes function much like cat eyes to given them excellent night vision. Behind the retina of the eye is a reflective structure called a tapetum lucidum . The granular, silvery crystals of the tapetum lucidum act to capture scattered light under low light intensities and improve the ability of the shark to see objects in dim light.

In addition to their reputation as long-distance swimmers, blue sharks  are also fast swimmers. Their sleek, slender body, long pointed snout, and long curved pectoral fins  provide powerful and swift locomotion when necessary. Blue sharks will even jump out of the water when hooked.

This speed allows them to devour large numbers of squid and small bony fishes of which they are fond. When feeding on squid, blue sharks  may race through the of squid with their mouth wide open or they may swim slowly sweeping their heads back and forth. They also can charge upwards in a vertical position to engulf their prey. Blue sharks are also well known for their love of whales. Whalers have long noted the ferocity with which blue sharks attack whale carcasses. In the midst of a full-on frenzy, blue sharks are even oblivious to the injurious pokes of a whale spade wielded by a wary seaman.

Growing to lengths up to 16 feet, the blue shark  is the most abundant shark along the east and west American coasts. They may travel alone or in groups, unlike most other species. This behavior of traveling in groups makes them especially susceptible to feeding frenzies and is one reason they are considered dangerous. Blue shark attacks on humans have been reported but they are not as common as the attacks of "man-eaters." Other than the four listed above, seventeen species, including the blue shark , are considered dangerous to man.

Blue sharks , like most elasmobranchs, give live birth to their young. However, blue sharks  are viviparous , meaning their young develop by receiving nourishment from a placenta. This is in contrast to ovoviviparous  species, who give live birth but whose young nourish on the yolk of their egg. Gestation periods in blue sharks last from 9 to 12 months with as few as 4 and as many as 135 pups being born per litter. During courting, the male blue sharks appear to bite the females. Female blue sharks are easily distinguished from males by the teeth scars on their backs. The skin of female blue sharks is twice as thick as male blue sharks and thicker than the male's teeth are long, an adaptation to their mating rituals. Following copulation, the female stores the sperm until the following spring, whereupon ovulation and fertilization occurs.

Sharks typically rely on several senses to discern and track prey in the water. Sharks can see, smell, hear, feel, touch, taste, detect vibrations and movements, and sense electric and magnetic fields. Their shark's keen sense of sound allows them to detect potential prey for a mile or more. Sharks use hearing, possibly through their inner ear and also the lateral line system , to detect low frequency vibrations (40 Hz and below) such as a struggling or splashing fish or mammal.

At somewhat closer distances, on the scales of perhaps a quarter of a mile or more, sharks rely on their sense of smell to locate prey. Sharks have large olfactory organs  on their snout into which water flows. Sharks are very selective in their sense of smell, able to differentiate amino acids , amines, and small fatty acids, stimulating smells, from sugars and simple carbohydrates, which don't appear to attract sharks. Sharks track down the source of a preferred smell by turning towards the direction of the smell as the swim, tracing a sinuous path back and forth like a hunting dog to find its prey.

At distances up to 300 feet, sharks rely on their lateral line system  to detect vibrations, changes in pressure, and movements in the water. Because all living organisms produce electrical fields, sharks can locate their prey at close range with surprising accuracy; they can even locate and catch flounder and stingrays  buried in the sand.

At distances of tens of feet, a shark's far-sighted vision  allow it to home in on its prey. Sharks are most sensitive to light, movement, and contrast. It is also likely that they can determine shape, to some extent, as this is the commonly offered reason for surfers being attacked, i.e. they look like seals.

At very close distances, on the scales of inches, sharks use another type of electrical sense organs called the ampullae . These organs give the shark another level of electrical sensing. At point-blank range, sharks have a sense of taste that is highly refined. Many sharks bump their prey prior to biting it, apparently as a means to identify the prey. Many sharks will bite their prey and discontinue their attack if the victim is not the preferred food.

Blue sharks are well-noted for their keen sense of perception. They appear to have a highly-developed lateral line system  and studies are underway to better determine the nature of this sense. Only by examining the true nature of these animals will we come to appreciate the extent to which they have fine-tuned their sensory systems for life in the sea.

15.15 The Hammerhead

Undoubtedly one of the most distinctive sharks is the hammerhead  shark. With its spade-shaped head punctuated by singular eyes and nostrils at each end, the hammerhead  is a shark uniquely its own.

Hammerheads are found around the world in shallow coastal waters  and occur locally in the Gulf of California . Several species exist; the largest, the great hammerhead , grows to lengths of more than 18 feet.

One of the most interesting behaviors about hammerheads  is their penchant to form groups. Schools of scalloped hammerheads  (Sphyrna lewini ) with more than 100 individuals have been observed in the Gulf of California . Why they form groups is not clear. Groups appear to form during the day in association with seamounts . All the individuals swim in the same direction, apparently following the designated leaders. Some theorize that grouping is related to breeding, but this appears to be only part of the answer since no copulation has ever been observed. Grouping for defense has been ruled out because hammerheads  have no natural enemies. Research on these sharks, conducted in the Gulf of California over a period of several years, are only now beginning to yield some clues as to why hammerheads  form groups.

Here's the story so far . Unlike typical schools of fishes where individuals of nearly equal size swim in close formation at the same speed, schools of hammerheads  contain individuals of many sizes who swim in haphazard and uncoordinated patterns. Large female hammerheads  dominate the center of the school while smaller ones circle along the edges. This central position appears to be a power position; younger females constantly "battle" for the center by striking their rivals with the undersides of their jaws. Dominant females also bully their rivals by performing what is known as a corkscrew display. In this behavior, the female performs a twisting loop, rotating her body as she accelerates into a tight somersault. At the height of her loop, a white flash of light reflects off her body, which appears to intimidate the other females and cause them to retreat to the sidelines, shaking their heads.

This center position is important to the females because this is where the most desirable male hammerheads  can be found. Sexually mature males will dash into the cluster of females and twist his body, revealing to the "queen" his handsome pair of claspers, the male reproductive organs If the central female takes a liking to the male, the pair will leave and swim to the bottom of the seamount where they will mate. Thus, it is clear that one function of schools is to identify the most fit mates, a process that would be difficult if hammerheads  were solitary.

Scientists studying hammerheads  in the Gulf of California  have also observed that hammerheads  leave their schools and seamounts  at night when they go to feed. Every evening, hammerheads  complete a ten to fifteen miles journey into deeper waters, always returning at dawn. By attaching transmitters to the animal's body, scientists discovered that hammerheads  travel to abundant feeding grounds, sometimes near another seamount. What is so extraordinary about their travels is their ability to find their way back and forth between seamounts. Hammerheads travel in a yo-yo pattern at mid-depths, following each other like cars on a highway. When they reached the outermost point in their journeys, the stayed in one place and made jerky, random movements, as if they were feeding. In one case, scientists were able to confirm that one individual had traveled to a distant seamount where abundant squid were located.

From these observations has come the extraordinary hypothesis that hammerhead  sharks use a magnetic sense  to navigate within the oceans. Geophysical data collected at the sites using magnetometers reveals distinctive patterns and magnetic anomalies that could serve as navigational points, just like landmarks serve as convenient reference points for hikers or sailors. Magnetic navigation has been proposed for other species of fish, including salmon, but no direct observations of magnetic sense  organs in hammerheads  has yet been found.

One way that sharks—and hammerheads  in particular—might sense magnetic fields is through use of their ampullae , the electrical sensing organs in the snout of sharks. In hammerheads , the distance between ampullae is exaggerated due to the shape of its head, and this feature could allow hammerheads  to detect magnetic field lines. Even the yo-yo behavior of their swimming would be consistent with magnetic navigation as hammerheads  would be better able to distinguish local magnetic features.

Whether or not hammerheads  can actually detect magnetic fields is the subject of a study to be conducted at Bodega Bay  Research Station in northern California. By burying electric cables in a maze-like pen , scientists will be better able to determine whether hammerheads  are actually capable of following a magnetic field. In any event, the hammerhead 's relationship with seamounts  will continue to intrigue us for some time, but the implications for other species of migrating fish could be profound.

15.16 The Great White Shark

The great white shark  needs no introduction. Its reputation as a killer, an "eating" machine, the lord of the sea, etc. has been heralded for centuries. Its scientific name, in fact, Carcharodon carcharias , means "ragged tooth," an all too descript pseudonym for a shark that kills ruthlessly. Great whites have been reported in practically all oceans, but they seem to prefer cool, temperate and coastal waters . Regardless, they have also been reported at depths greater than 3000 feet and seen in the surf line and in shallow bays. Apparently, the white shark goes where it wants to because it can!

As with all sharks, females tend to be larger than males. The average length of a females is reported at 15 feet, but specimens longer than 25 feet have been caught. The largest white shark ever taken was harpooned in the Azores , a 29.25-foot-long giant with a pectoral span of nearly 14 feet and teeth as long as 3 inches. Weights of these large sharks range from more than 2,000 pounds up to 7,000 pounds.

The great white shark  gets its name from its pure white belly, which is often the first (or last) image presented to a fisherman or victim. However, the back and sides of the great white are a dark grayish black and some have suggested that "black" shark would be a more apt name.

The shark's reputation as a killer of humans is not without substantiation. As far back as the 16th century, naturalists reported finding whole men in armor in the stomachs of great whites. While these reports may be approached with skepticism, there is no doubt that great whites can swallow huge prey. In July 1976, a Los Angeles  fisherman caught a 16-foot white shark that "contained the bodies of two whole sea lions, one weighing 175 pounds and the other 125 pounds." In 1954, the body of a 13-year-old boy was apparently found in the stomach of a great white caught off Nagasaki .

The preferred prey of adult great white sharks off the coast of California are seals and sea lions, although any marine mammal will probably do. The blubber  of whales and porpoises is especially satisfying to a great white shark . Juvenile sharks will feed on fishes, such as menhaden  or tuna, and even other sharks, such as houndsharks, requiem sharks, hammerheads , and the spiny dogfish. Recent studies on the dietary cuisine of great whites indicate that they prefer meals with a high fat content (fat-free is not in the great white's vocabulary). They typically will reject low-fat prey, such as birds or sea otters , an observation that suggest one reason why great whites call off their attacks on humans (see below).

This pattern of food preference correlates well with the reproductive habits of great whites along the coast of California. Great white sharks are viviparous , like blue sharks , and give live birth to their young, which may weigh from 36 to 60 pounds! In California, most young sharks are born in southern California between San Diego  and Catalina Island . As the sharks mature, they move further up the coast towards the Farallons, as their youthful diet of fish gives way to their adult preference for seals and sea lions.

While identification and reporting of great white attacks have become more accurate in recent years, there seems little doubt that the occurrence of great whites is on the rise. Along the coast of California, sightings and attacks have grown in number, from one or two per year in the 1950s to nearly five per year in the 70s and 80s. In the period from 1973-1983, surfers were attacked thirteen times in waters near San Francisco .

So notorious have attacks become off San Francisco  that a zone called the “red triangle ” has been designated. The red triangle extends from Tomales Point  in the north, south to Monterey Bay , and west to the Farallon Islands  off the coast. One reason for the high concentration of great whites attributed to this area appears to be the abundant populations of seals and sea lions. Since implementation of the Marine Mammal Act , populations of marine mammals of all species have grown in number; so too, have populations of great white sharks.

The feeding behavior of great whites has been a topic of intense research in the Farallon Islands  and elsewhere. Because great whites are elusive and highly mobile, natural observations of its behavior are difficult to obtain. Still, a picture of their attack patterns and feeding preferences is emerging that suggests predictable patterns in time and space.

One of the more famous great white attacks occurred in Australia  in 1963. An Australian skin diver, Rodney Fox , was participating in a spearfishing  tournament when he felt jaws close on his chest and back, hurtling him through the water with the impact of the strike. Fox drove his fist at the shark's eyes but his arm slipped into the shark's mouth tearing his hand and arm to the bone. The shark regrouped to attack again only this time it went for the fish bag strapped to Fox's waist. As the shark pulled him downwards, Fox desperately struggled to free the bag. Just at the end of his air, the bag snapped and Fox raced to the surface, where he was picked up by a nearby boat, who noticed an unusual amount of blood in the water.

Fox's  condition was horrifying: "his rib cage, lungs and the upper part of his stomach were exposed, the flesh had been stripped from his arm, his lung was punctured and his ribs were crushed." Miraculously, he lived, albeit with the scars of 462 stitches in his body.

A few years earlier, a friend of Fox's, Brian Rodgers , had been attacked by a great white. Only after firing his spear into the shark's head did it call off its attack. Rodgers managed to struggle to shore, where he was rushed to a hospital. After 3 hours in the operating room, he managed to survive.

In 1964, another Australian diver, Henri Bource , lost a leg to a great white. Four years later, he was attacked again, only this time all the shark got was the artificial leg.

All of these attacks reveal a single pattern and a decidedly unusual response. All three of these divers and many others attacked since that time were subjected to the "bite-and-spit" kind of attack. Observations on white shark attacks on seals and sea lion populations reveal a couple different kinds of attacks, depending on the type of prey.

In attacks on seals, the shark apparently grabs the prey and holds it until it bleeds to death, a killing mode called exsanguination , or blood deprivation. Once the seal quits bleeding, the shark begins to eat.

Great white shark attacks on sea lions begin somewhat differently. The strike begins with an explosive splash‹the shark appears to ram its victim‹whereupon the sea lion often struggles free. However, the shark is relentless and soon grabs its victim again until it stops bleeding. Once it has stopped bleeding, the shark finishes off its prey.

Sharks are opportunistic feeders and their feeding patterns on seals and sea lions are quite interesting when contrasted with the attacks on divers or other "non-food" items, such as pelicans and sea otters . A commercial abalone  diver recalls being attacked while swimming at a depth of 15-20 feet. The shark grabbed him by the leg and carried him downwards. All the while the diver was bleeding profusely. Only by pounding on the shark's head with a metal rod was he able to convince the shark to set him free. This attack is very similar to the kind of attacks great whites mount on seals.

It appears that white sharks release people because they find them unpalatable, not suited to their palate. The Farallon shark scientists observed a great white attack on a brown pelican , in which the animals was attacked and disabled, but never pursued, even though the bird was incapable of going anywhere. The pelican died two minutes later. Similar attacks have been postulated for sea otters , whose dead bodies was ashore intact, but with great white teeth in their wounds. A sea otter has never been found in the stomach of a great white.

As mentioned above, great white sharks appear to prefer the fat and blubber  of marine mammals. Scientists have hypothesizes that this diet of fat enables the great whites to maintain high rates of growth, about 5% per year, which is twice the growth rate of other sharks. A diet of blubber and high fat is consistent with high growth rates and makes sense for a predator who prefers cooler waters.

One other interesting observation of great white sharks bears mention. Farallon Island scientists often observed "confrontations" between two great white sharks to decide who would eat a freshly killed prey. In what these scientists call a "ritualized combat ," two great whites would approach each other head on, then sharply slap their tails in the water, splashing water towards their opponent. Water splashing was quite vigorous on occasion and some sharks would lift their bodies two-thirds of the way out of the water to make a larger splash. The sharks would circle and repeat their tail slapping until one shark called it quits. The victor fed on the remains of the prey.

This "before-dinner dance" has been interpreted as a kind of communication between great whites. A stronger shark sees a rival shark as a threat to its feeding, and warns it away. In this way, great white sharks may avoid killing each other.

Finally, there appears to be some evidence that great white populations are in danger in California. Intensive sport fishing appears to have reduced the numbers of great whites and their population is though to be small. Because they reproduce approximately once every two years, and because their litter sizes are small (7-9 pups per litter), it is difficult for great whites to make a comeback. For that reason, the state of California has passed legislation protecting the great white shark .

While the threats of these great animals on humans still remains, the knowledge to be gained through scientific research on these animals far outweighs their danger. Through research, we may find a way to peacefully coexist with these magnificent creatures. Certainly their success in surviving catastrophic changes in our planet over the past 300 million years deserves some attention. They may have much to teach us that we have yet to learn.