Unit Five. Evolution of Animal Life


20. History of the Vertebrates


20.4. Fishes Dominate the Sea


A series of key evolutionary advances allowed vertebrates to first conquer the sea and then the land. Figure 20.9 shows a phylogeny of the vertebrates. Branch points in the family tree indicate key adaptations that lead to great diversity. About half of all vertebrates are fishes. The most diverse and successful vertebrate group, they provided the evolutionary base for the invasion of land by amphibians.



Figure 20.9. Vertebrate family tree.

Primitive amphibians arose from lobe-finned fishes. Primitive reptiles arose from amphibians and gave rise in turn to mammals and to dinosaurs, which are the ancestors of today's birds.


Characteristics of Fishes

From whale sharks that are 12 meters long to tiny cichlids no larger than your fingernail, fishes vary considerably in size, shape, color, and appearance. However varied, all fishes have four important characteristics in common:

1. Gills. Fish are water-dwelling creatures, and they must extract dissolved oxygen gas from the water around them. They do this by directing a flow of water through their mouths and across their gills. Gills are composed of fine filaments of tissue rich in blood vessels. They are located at the back of the mouth. When water passes over the gills as the fish swallows water, oxygen gas diffuses from the water into the fish’s blood.

2. Vertebral column. All fishes have an internal skeleton with a spine surrounding the dorsal nerve cord, although it may not necessarily be made of bone. The brain is fully encased within a protective box, called the skull or cranium, which is made of bone or cartilage.

3. Single-loop blood circulation. Blood is pumped from the heart to the gills. From the gills, the oxygenated blood passes to the rest of the body and then returns to the heart. The heart is a muscular tube-pump made up of four chambers that contract in sequence.

4. Nutritional deficiencies. Fishes are unable to synthesize the aromatic amino acids and must consume them in their diet. This inability has been inherited by all their vertebrate descendants.


The First Fishes

The first backboned animals were jawless fishes that appeared in the sea about 500 million years ago. These fishes were members of a group called ostraco- derms, meaning “shellskinned.” Only their head-shields were made of bone; their elaborate Chordate ancestor internal skeletons were constructed of cartilage. Wriggling through the water, jawless and toothless, these fishes sucked up small food particles from the ocean floor. Most less than a foot long, the earliest groups respired with gills but had no fins—just a primitive tail to push them through the water. These fishes were a great evolutionary success, dominating the world’s oceans for about 100 million years. By the end of this period, some groups of ostracoderms had developed primitive fins to help them swim and massive shields of bone for protection. They were eventually replaced by new kinds of fishes that were hunters. One group of jawless fishes, the agnathans, survive today as hagfish and parasitic lampreys, shown in figure 20.10.





Figure 20.10. Specialized mouth of a lamprey.

Lampreys use their suckerlike mouths to attach themselves to the fish on which they prey. When they have done so, they bore a hole in the fish with their teeth and feed on its blood.


The Evolution of Jaws

The evolution of fishes has been dominated by adaptations to two challenges of surviving as a predator in water:

1. What is the best way to grab hold of potential prey?

2. What is the best way to pursue prey through water?

The fishes that replaced the jawless ones 360 million years ago were powerful predators with much better solutions to both evolutionary challenges. A fundamentally important evolutionary advance was achieved about 410 million years ago—the development of jaws. As illustrated in figure 20.11, jaws seem to have evolved from the frontmost of a series of arch supports (colored red and blue) made of cartilage that were used to reinforce the tissue between gill slits, holding the slits open. As you step through the figure from left to right, you can see how the gill arches evolved, reforming into the jaws.



Figure 20.11. A key adaptation among fishes: evolution of the jaw.

Jaws evolved from the anterior gill arches of ancient, jawless fishes.


Extinct armored fishes called placoderms and spiny fishes called acanthodians both had jaws and paired fins. Spiny fishes were predators and far better swimmers than ostracoderms, with as many as seven paired fins to aid their swimming. The larger placoderm fishes had massive heads armored with heavy bony plates. Many placoderms grew to enormous sizes, some over 9 meters long!

Both spiny fishes and placoderms are extinct now, replaced in turn by fishes that evolved even better ways of moving through the water, the sharks and the bony fishes. The earliest sharks and bony fishes appear in the fossil record soon after spiny fishes and placoderms do. However, after sharing the seas for 150 million years, the long competition finally ended with the complete disappearance of the less maneuverable early jawed fishes. For the last 250 million years, all jawed fishes swimming in the world’s oceans and rivers have been either sharks (and their relatives, the rays) or bony fishes.



The problem of improving speed and maneuverability in swimming was solved in sharks by the replacement of the heavy bony skeleton of the early fishes with a far lighter one made of strong, flexible cartilage. Members of this group, the class Chondrichthyes, consist of sharks, skates, and rays. Sharks are very powerful swimmers, with a back fin, a tail fin, and two sets of paired side fins for controlled thrusting through the water (figure 20.12). Skates and rays are flattened sharks that are bottom-dwellers; they evolved some 200 million years after the sharks first appeared. Today there are about 750 species of sharks, skates, and rays.



Figure 20.12. Chondrichthyes.

The Galapagos shark is a member of the class Chondrichthyes, which are mainly predators or scavengers and spend most of their time in graceful motion. As they move, they create a flow of water past their gills, from which they extract oxygen.


Some of the largest sharks filter their food from the water like jawless fishes, but most are predators, their mouths armed with rows of hard, sharp teeth. Sharks are well-adapted to their predatory life due to their sophisticated sensory systems. From a distance, sharks can detect prey using their highly developed sense of smell. Also, a sensory system called the lateral line system allows sharks to sense disturbances in the water. At close range, special electroreceptors located primarily on the shark’s head allow a shark to detect electric fields that surround all animals. Reproduction among the Chondrichthyes is the most advanced of any fish. Shark eggs are fertilized internally. During mating, the male grasps the female with modified fins called claspers. Sperm pass from the male into the female through grooves in the claspers. About 40% of sharks, skates, and rays lay fertilized eggs. In some of these species, the eggs are laid in a hard capsule, sometimes called a “mermaid’s purse.” The eggs of other species develop within the female’s body, and the pups are born alive. In still other species, embryos develop within the mother and are nourished by maternal secretions or by a placenta-like structure (mammalian placentas are discussed in section 20.8).


Bony Fishes

The problem of improving speed and maneuverability in swimming was solved in bony fishes (figure 20.13) in a very different way. Instead of gaining speed through lightness, as sharks did, bony fishes adopted a heavy internal skeleton made completely of bone. Such an internal skeleton is very strong, providing a base against which very strong muscles can pull. Bony fishes are still buoyant though because they possess a swim bladder. The swim bladder is a gas-filled sac that allows fish to regulate their buoyant density and so remain effortlessly suspended at any depth in the water. You can explore how a swim bladder works by examining the enlarged drawing in figure 20.14. The amount of air in the swim bladder is adjusted by extracting gases from the blood passing through blood vessels near the swim bladder, or by releasing gases back into the blood. Using a swim bladder, a bony fish can rise up and down in the water the same way a submarine does. The swim bladder solution to the challenge of swimming has proven to be a great success in bony fish. Sharks, by contrast, increase buoyancy with oil in their liver, but still must keep swimming (or moving through the water) or they will sink, because their bodies are denser than water.





Figure 20.13. Bony fishes.

Bony fishes are extremely diverse, containing more species than all other kinds of vertebrates combined. This Korean angelfish, Pomacanthus semicircularis, in Fiji, is one of the many striking fishes that live around coral reefs in tropical seas.



Figure 20.14 Diagram of a swim bladder.

The bony fishes use this structure, which evolved as a dorsal outpocketing of the pharynx, to control their buoyancy in water. The swim bladder can be filled with or drained of gas to allow the fish to control buoyancy. Gases are taken from the blood, and the gas gland secretes the gases into the swim bladder; gas is released from the bladder by a muscular valve.


Bony fishes (class Osteichthyes) consist of the lobe-finned fishes (subclass Sarcopterygii) and the ray-finned fishes (subclass Actinopterygii), which include the vast majority of today’s fishes. In ray-finned fishes, the fins contain only bony rays for support and no muscles; the fins are moved by muscles within the body. In lobe-finned fishes, fins consist of a fleshy muscular lobe that contains a core of bones that form joints with each other; bony rays are only at the tips of each lobed fin (see figure 20.15 1). Muscles within each lobe can move the fin rays independently of each other. Lobe-finned fishes evolved 390 million years ago, shortly after bony fishes appeared. Eight species survive today, two species of coelacanth and six species of lungfish. Although rare today, lobe-finned fishes played an important evolutionary role, as they gave rise to the first tetrapods (four-legged animals), the amphibians.

The major groups of fishes, both living and extinct, are summarized in table 20.2. Bony fishes are the most successful of all fishes, indeed of all vertebrates. Of the approximately 30,800 living species of fishes in the world today, about 30,000 species are bony fishes with swim bladders. That’s more species than all other kinds of vertebrates combined! In fact, if you could stand in one place and have one representative of every vertebrate animal species alive today pass by you, one after the other, half of them would be bony fishes.





The remarkable success of the bony fishes has resulted from a series of significant adaptations. In addition to the swim bladder, they have a highly developed lateral line system, a sensory system that enables them to detect changes in water pressure and thus the movement of predators and prey in the water. The lateral line system is discussed in more detail in chapter 29. Also, most bony fishes have a hard plate called the operculum that covers the gills on each side of the head. Flexing the operculum permits bony fishes to pump water over their gills. Using the operculum as very efficient bellows, bony fishes can pass water over the gills while stationary in the water. That is what a goldfish in a fish tank is doing when it seems to be gulping.


Key Learning Outcome 20.4. Fishes are characterized by gills, a simple, single-loop circulatory system, and a vertebral column. Sharks are fast swimmers, whereas the very successful bony fishes have unique characteristics such as swim bladders and lateral line systems.