Phylum: Chordata (cord)
Subphylum: Vertebrata (animals with backbones)
Class: Chondricthyes and Osteicthyes
(Cartilaginous Fishes) (Bony Fishes)
I suppose there are those of us who love to dive because of an interest in corals, worms, sea stars, squids, lobsters or some other group of invertebrates. And undoubtedly there are specialists among us who have a keen interest in marine reptiles such as sea snakes or turtles. But there is not much doubt that most of us are drawn to diving, at least in part, because of our fascination with spectacularly colored fishes and marine mammals.
As the 10th article in this series, this piece is the first of two that deal with fishes and marine mammals, the “stars” of Neptune’s Kingdom. In this month’s issue we will take a look at the wonderful world of fishes, and in the concluding piece in this series we will examine the lives of marine mammals. All fishes and all mammals are vertebrates but there is little other scientific rationale to group them in a two-part series. However, being in the diving and educational business, we just want to be sure that both groups get their due as the “stars of Neptune’s show.”
Though they are included in different classes, all fishes and mammals are vertebrates described in the subphylum Vertebrata within the phylum Chordata.
The World of Fishes
In any fundamental discussion about fishes, one of the first questions that must be asked is “What is a fish?” The answer is not quite as simple as the question. Fishes are a very diverse group of animals that live in water, have backbones and use gills to respire. Most possess fins used for swimming and body control, have protective scales that cover their body and have a streamlined body that enables them to move efficiently through water. There are so many exceptions to this general description, however, that many specialists declare that fish are just about anything anyone decides to call a fish.
Some fishes are stunningly colorful; others drab. Some species are highly territorial; others only during their breeding season and territoriality might only involve one sex; and still others are not the slightest bit territorial. Some species are active by day; others by night. Some school; others are solitary. Some are herbivores (plant eaters); others are carnivores (meat eaters); and others are omnivores (eat plants and animals). Some are apex predators; others low on the food chain; and some scavenge. Some are bottom dwellers while others live in the water column, some inhabit reef communities; others sand or the open sea.
Fishes vary in size from a tiny goby that inhabits fresh-to-brackish-water lakes in parts of the Philippines to the behemoths we know as whale sharks. This specific goby is seldom longer than a half inch (1.25 cm) at adulthood, yet is so abundant it supports a fishery, while the whale shark, an animal known to attain proportions of close to 50 feet (15 m) long and 30,000 pounds (1,350 kg) is considered rare.
Fishes occur in a wide variety of body plans. The fastest swimmers, a group that includes tunas and many sharks, are torpedo-shaped or fusiform. Eels have an anguilliform (an- gweel-uh-form), or ribbon-like, shape which is rather elongate and snakelike. This design enables them to “s” their way in and out of tight quarters in a reef community. One of the most common body plans is said to be laterally compressed, meaning that the animals are rather thin from side to side. This design also enables these species to slip in and out of coral branches and tight quarters in rocky reefs when looking for food and when escaping predators.
Another major body plan known as dorsoventrally (door-so-ven-tral-lee) compressed can be seen in flatfishes such as halibuts, flounders and soles. These fishes are greatly flattened from top to bottom and are superbly equipped for their lives on the seafloor. Frogfishes, puffers and some porcupinefishes possess a globiform body plan, being lumpish or globe-shaped. Many of these species are relatively poor swimmers relying on camouflage and other strategies to capture food and escape predation. Several other species such as boxfishes, cowfishes and trunkfishes have box-shaped bodies. These fishes use their lone dorsal and anal fins to create forward propulsion while using their tail as a rudder. As you probably suspect, several other body plans can also be found in the diverse world of fishes.
Fishes comprise more than half of the world’s vertebrates, a fact which makes them the largest and most diverse group of animals with backbones. Ichthyologists, scientists who study fishes, estimate that worldwide there are roughly 28,000 living species inhabiting the waters of Earth. Of those, about 24,000 are marine species and the remaining 4,000 live in fresh water. For the past decade, new species have been discovered at a rate of 200 to 300 annually, and specialists expect this trend to continue.
Fishes are classified differently by different ichthyologists. In the most generally used system, the subphylum Vertebrata is divided into two superclasses: Gnathostomata (neh-THOS-teh-meh), which includes the fishes with hinged jaws and Agnatha (ag-NAY-theh), which includes the lamprey and other fishes without jaws. As divers, we pay little attention to lampreys and other fishes that lack hinged jaws.
Those species described in the superclass Gnathostomata are further divided into the class Chondrichthyes (kon-DRIK-thi-eez), the cartilaginous fishes, a group that includes all sharks, rays, skates and chimeras, and the class Osteichthyes (os-tea-ICK-thi-eez), the bony fishes such as angelfishes, damselfishes and sea basses. There are far more species of bony fishes (about 96 percent of the total) with just over 1,200 species of known cartilaginous species.
The major difference between bony fishes and cartilaginous fishes is that the skeleton of bony fishes is, as the group name suggests, composed of bone, while the skeletons of all sharks, rays, skates and chimeras are made of cartilage. This means that fishes such as whale sharks — the largest fish in the world — and basking sharks, great white sharks, manta rays and all the rest of their cartilaginous cousins do not have a single bone in their entire body.
Specialists believe that cartilaginous fishes first appeared in the Earth’s waters roughly 450 million years ago. This is not to suggest that any species that were alive then are alive today in an unaltered form. Modern sharks, for example, are estimated to have roamed the seas for “only” the last 120-150 million years, or so. Even though they do not have bones to fossilize, sharks are “tooth-making machines,” with individual animals growing thousands of teeth during their lifetime. As a result, shark teeth are among the most commonly found fossils and there is plenty of evidence for paleontologists to examine.
For many years bony fishes were thought to have appeared nearly 300 million years later than cartilaginous fishes, but recent evidence from fossil discoveries suggests that bony fishes appeared at least 350 million years ago.
Of course, bony fishes and cartilaginous fishes differ in other ways as well. Most bony fishes possess a swim bladder, an internal organ that helps them ascend, descend and maintain neutral buoyancy by adding or subtracting gases from the blood stream to expand or contract the swim bladder. Cartilaginous fishes lack a swim bladder, and these animals will sink if they stop swimming. Their lighter weight skeleton made of cartilage is an adaptation that helps sharks, rays, skates and chimeras overcome their lack of a swim bladder. Interestingly, many sharks and rays live in the water column in the open sea and must swim all day, every day of their lives in order to pass oxygen-rich water over their gills so they can respire. Most bony fishes, by contrast, move a bone-like flap called an operculum back and forth in order to pump oxygenated water over their gills.
Bony fishes and cartilaginous fishes also differ in that the skin of most bony fishes is covered with scales. The skin of shark, rays, skates and chimeras is covered with relatively small, tooth-like structures called dermal denticles. Bony fishes reproduce via external fertilization while fertilization is internal in all cartilaginous fishes.
Fishes inhabit almost every known underwater habitat from the shallow reefs of tropical and temperate seas that sport divers explore to hot desert springs, near-freezing Arctic waters, mud in dried up tropical ponds and the abyssal depths of the ocean. They rely on a combination of the senses of sight, feel, hearing, smell and taste to interpret their surroundings. Eyesight is extremely important to some fishes, especially those species that inhabit relatively clear, well-lit, shallow water. Fish eyes are well-developed and each can be rotated independently to enable a fish to see a very wide area, hence the term “fisheye lens” is used to describe photographic lenses that have an extremely wide angle of view. However, sight is surprisingly unimportant to many other species, especially those that live in deep, dark waters.
All fishes possess a system of nerves and organs known as the lateral line system that runs down the outside of their bodies. The lateral line system, which is especially well-developed in bony fishes, provides the ability to feel pressure waves created by the motion of other animals and objects in the water. The lateral line system is extremely sensitive, commonly enabling fishes to feel predators and prey before they can see them. This sensory system also empowers schooling species to swim close to one another while making collision-free, near instantaneous turns.
One way in which sharks, rays, skates and chimeras compensate for a less well-developed lateral line is by using organs called ampullae (am-POOL-ee) of Lorenzini (lore-en-zee-knee) that are concentrated in their snout and head. These gel-filled pits are extremely sensitive to pressure and electrical fields. In fact, sharks have the keen ability to detect electrical fields that are as much as 10,000 times more faint than the electrical fields that can be detected by any other known group of animals. Biological functions such as a beating heart and any contracting muscle create electrical fields that surround all living creatures. The ability to detect these fields translates into the ability to detect prey and helps make sharks such capable predators.
Sound also plays an important role in the lives of many fishes. In fact, species such as triggerfishes, squirrelfishes, groupers and others make grunting sounds that are considered an important part of their intraspecies communication, including the sound of alarm when danger threatens. Smell, too, plays a vital role for many species ranging from sharks to moray eels (yes, eels are fishes). It is well-known that octopi emit ink to anesthetize the sense of smell in eels as much as to create a “smokescreen.” While fishes are extremely sensitive to touch, studies have demonstrated that they are not very sensitive to taste.
“Fish” is the correct term to use when referring to animals of the same species. The word “fishes” is used to refer to more than one species. About 80 percent of fishes live in schools as juveniles, while about 20 percent school as adults. Schooling provides safety via the concept of “safety in numbers.” Schooling also provides proximity to potential mates, the ability to overcome the defenses of territorial fishes, and increases swimming efficiency by reducing “drag” in much the same fashion that cyclists drafting one another in a pack reduces drag for the trailing bicyclists.
In some species, different physical characteristics such as markings, coloration or size make it easy to distinguish adults from juveniles and to tell males from females. However, things are not always so simple. Some fishes, most notably members of the family of seabasses, are male and female at the same time, while others such as parrotfishes and wrasses commonly change sex during their lifetime.
Although freshwater lakes and rivers comprise less than one one-thousandth of the volume of water on Earth, 40 percent of fish species live in fresh water. Most of the rest are found in the salty oceans, while only 2-3 percent are found in somewhat salty or brackish waters.
The greatest diversity in fishes is found in and around coral reefs. Although coral reefs account for less than 1 percent of the world’s oceans, about 50 percent of all known species of fishes are associated with coral reefs. The reef species are estimated to be close to 7,000 while about 14,600 species of fishes inhabit the world’s oceans.
Diversity in some locations is equally astonishing. For example, about 2,500 species of fishes occur in the Philippines alone, while close to 2,000 can be found in the waters of Papua New Guinea and as many as 1,500 are associated with the waters of Australia’s Great Barrier Reef. More than 500 species can be observed in the Bahamas and as many as 600 in the waters of the Florida Keys.
In fresh waters, the greatest diversity of fish species occurs in the warm tropics of South America, Africa and southeast Asia.
While many species occur in the same geographical region, a closer examination reveals that each species is able to find a niche within various ecosystems that enable them to survive. For example, within the same reef community one species might feed only on a particular alga during the day while a neighboring species feeds on the same resource at night. Another species might feed on different algae that occur in the surrounding sand, while still others prey on other fishes, mollusks, echinoderms or other fishes, etc. Of course, some of these species might be associated with the sea floor while others are midwater animals.
The majority of marine fishes are found associated with the seafloor or with other natural or artificial features such as reefs or docks. Only 13 percent of fish species live primarily in the open ocean. Many of these are highly migratory. Some tunas can travel as fast as 50 mph (80.5 kph) in short lasting bursts, and often travel well over 100 miles a day in annual migrations that cover many thousands of miles. Other species live in home ranges that are as small as a few square yards.
After digesting the material that has been presented in this overview, it is easy to see that fishes are not randomly distributed in the world’s waters. The continental shelves — shallow, sunlit areas of the ocean that are typically less than 650 feet (197 m) deep and contain nutrients from the continents — contain most of the ocean’s fishes. The continental shelves promote large populations of tiny invertebrate animals so it logically follows that the upper 650 feet (197 m) of the ocean holds 78 percent of marine fish species. Considerably fewer species inhabit the abyssal depths.
And what does all this mean to you? I suppose a logical answer to this hypothetical question is that this information can mean as much or as little as you like. Should you become an ichthyologist, the world of fishes can easily occupy an entire lifetime. If you pursue other interests professionally and dive primarily for recreation, fishes can provide a marvelous diversion and clearly, the more you learn about fishes, the greater insight you will have into the workings of the aquatic world that we all enjoy exploring.
Do fish sleep?
If the definition of sleep is a period of rest in which the eyes are closed and there is little or no thought or movement, then the answer is “not exactly.” Many species that live in midwater never stop moving. By contrast, many fishes live nearly motionless lives, and many others function on a regular diurnal/nocturnal cycle, some being active by day, others by night. In addition, most fishes can’t close their eyes because they lack eyelids. Sharks do have eyelid-like membranes and are a notable exception. The point here is that it is not accurate to state that all fishes sleep like we do. However, most fishes do rest at times, and many enter a state of torpor (tore-per), a kind of semiconscious fog in which they are somewhat alert for danger, but not fully alert as they are resting.
It is impossible to overstate the importance of fishes to human populations around the world. Throughout history, humans have used fish protein as a food source, with wild fishes providing the bulk of fish protein. Fishes have also been farmed in large quantities for more than 2,000 years in China. Recent advances in fish farming have alleviated hunger in some parts of the world. In industrialized countries, farm-raised fish provide relief for overfished wild stocks.
Today, fish protein represents about 25 percent of the total animal protein consumed by the world’s population, second only to beef.
Fishes also serve as sources of recreational pleasure for many people including divers. The catches from sports fisheries are far larger than commercial catches from most bodies of fresh water and in marine waters close to large population centers. More than 20 million home aquariums are kept in the United States alone.
However, not all is rosy in the world of fishes and humans. The United Nations has declared that every one of the world’s 20 leading fisheries is in a state of collapse because the various species are incapable of replenishing their own stocks at a rate that commercial fisheries are catching them.