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 today's lecture.
Three Kinds of Fishes
All of these 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, it 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.
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.
The second class of fishes need no introduction. The cartilaginous fishes in the Class Chondricthyes 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 chondricthyans 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 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 sharks 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 chondricthyes are the chimaeras, 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 chondricthyans do not have exposed gills or gill slits; rather they have an operculum that covers their gills, much like bony fish.
We focus our attention specifically on sharks in our next lecture, so don't be disappointed by their brief coverage here. These ancient lords of the sea deserve a separate page in the annals of this class and we will explore their good side and their bad side in considerable detail. In the meantime, let's learn a little about shark food.
The third class of fish, and the main subject of this lecture, is the Class Osteicthyes. 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 morays, or flattened, like flounders. These fish have fused their dorsal 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 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, our single lecture gives us only enough time to skim the surface. 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.
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.
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.
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, an 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.
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.
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.
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.
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.
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.
|Amount of parental energy devoted to care/nourishment of the young||NONE
Parents abandon fertilized eggs
Males guard fertilized eggs
Females give live birth after nourishment by placenta
|Number of eggs or young||MOST
up to 7,000,000
up to 500,000
|Size of egg||SMALL
|Development of young at hatching or birth||LEAST
|(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.