skipjack Fish are cold-blooded vertebrates (animals with backbones) that live in the water. Most breath oxygen from the water with gills, are covered by scales, and have fins rather than arms and legs. Amphibians such as frogs, toads and salamanders can have backbones and can breath with gills in the early stages of their life. They can be distinguished from fish by the fact they have arms and legs and fish have fins.
There are around 18,800 known fish and lower chordates species. Of these there about 15,000 species of fish. Scientists believe there are about 20,000 species of fish in the oceans. New species of fish are being discovered at a rate of around 200 a year.
There are four classes of fish: 1) bony fish with ray fins (most fish); 2) cartilaginous fish such as sharks which have cartilage instead of bones; 3) jawless fish such as lampreys (see below); and 4) bony fish with lobe fins such as ancient coelacanths and lungfish.
The largest fish is the whale shark (See Sharks) The smallest fish is the quarter-inch-long south infantfish discovered the Great barrier Reef in 1979. The spiny-headed devilfish is the world’s shortest fish. An adult male is 0.24 inches long. The south infantfish is the world’s second shortest fish at.27 inches and the world’s lightest fish, weighing 1/500,000th of a pound. There are 700 species of poisonous fish.
Megalodon tooth and great white shark teeth The United Nations Convention on International Trade on Endangered Species (CITES), the United Nations wildlife trade body, covers fish and ocean creatures as well as land animals. Among the species that are listed are great white sharks, whale sharks and seahorses.
Websites and Resources: National Oceanic and Atmospheric Administration noaa.gov/ocean ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Ocean World oceanworld.tamu.edu ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Montery Bay Aquarium montereybayaquarium.org
Websites and Resources on Fish and Marine Life: MarineBio marinebio.org/oceans/creatures ; Census of Marine Life coml.org/image-gallery ; Marine Life Images marinelifeimages.com/photostore/index ; Marine Species Gallery scuba-equipment-usa.com/marine
Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Wikipedia article Wikipedia ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Coral Reef Pictures squidoo.com/coral-reef-pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.
Early History of Fish
Megalodon jaw The ancestors of fish were sea squirts and lamprey. The later evolved around 550 million years ago and are still around today. It has a primitive sort of backbone, two eyes, a single nostril and a tongue with sharp spines instead of a jaw. It latches itself onto the flanks of fish and literally eats them alive.
Early proto fish that appeared around 500 millions ago were the size of large minnows. They were heavily armored, often completely covered in bony plates. They had two eyes and a single nostril and no jaw like the lamprey and moved around by beating a tail that emerged from the armor. They had no fins and fed themselves by nuzzling along the ocean floor, sucking up mud and sand and filtering out edible parts. Slits on the sides of their throat were used to filter mud and acted as gills.
Over time proto fish grew in size and increased in variety. They sprouted dorsal fins which gave them stability but no paired lateral fins that made it possible to maneuver around with ease. For a hundred million years fish existed in this form.
Later History of Fish
cetacean vertebra bitten
by Megalodon Around 400 million years ago, proto-fish began to develop jaws from filtering slits and primitive gills. The bony plats which covered them developed into teeth. This freed them from nuzzling in the mud and transformed them to creatures that could bite in the open sea. Over time flaps of skin grew out of the sides of their bodies and these developed into lateral fins.
Over more time the first true fish shed their bony plates and developed an internal skeleton with a vertebrae column running through the body. Most had two pairs of lateral fins: the pectorals behind the throat and the pelvis fins near the anus. Over yet more time the bony plates on the bodies evolved into scales and the fish developed swim-bladders.
Also around 400 million years ago fish began to evolve into two separate families: 1) bony fish; and 2) cartilaginous fish like sharks and rays who lost much of their internal skeletons and instead supported their bodies with cartilage. A fairly complete fossil of a 30-centimeter-long bony fish that lived 418 million years was found in southern China in the mid 2000s. It has a jaw and fat, fleshy lobe fins as opposed to thin ray fins found on most modern fish.
The seas 400 million years ago were inhabited by all kinds of wild creatures, including sea scorpions that were twice the size of a man and had claws the size of a human head. A fossil of one such creature was found in 2007 in 390-million-year-old rocks in Germany. The creature, which had a 2.5-meter-long body, was sort of like a giant water bug—the largest known bug that ever lived— and is thought to be ancestor of modern scorpions, and perhaps spiders too.
Ocean Dinosaurs and Sea Creatures Move to Land
Tiktaalik Rosae is regarded as a transition fossil between sea creatures and land animals. Dated to 375 million years ago and discovered on Canada’s Ellsmere Island, it has fins like an ancestral fish but its pectoral fin contains arm bones like those of land-dwelling animals. With a bendable shoulder and elbow, plus a proto-wrist, this fin could support the body and propel it on land. Its ribs and limb bones resemble those of later four-legged amphibians and other terrestrial species.
The announcement of the discovery of Tiktaalik Rosae was made in an April 2006 article published in Nature by team of researchers lead by Neil Shubin of the University of Chicago. The scaly creature was 1.2 to 2.7 meter long and is seen as a link between scaley, armored fish like Eusthenopteron, which lived 385 million years, and Icthyostega, a land creature that lived around the same time and looked like a cross between a Komodo dragon and a frog.
Between 230 and 90 million years ago, fish-like predatory dinosaurs called ichthyosaurs cruised the seas, Although they were reptiles their body and fin shapes were very much like that of modern predatory fish and sea mammals such as bluefin tuna, great white sharks and dolphins. A detailed fossil imprint of ichthyosaur skin reveal their skin contained multiple layers of fiber bundles like those found on dolphins, sharks and tuna.
Bony fish make up nine out of 10 fish species. Of the four classes of fishes they evolved most recently and are regarded as the most advanced. They vary a great deal in size and shape but most tend to be small. All have light but strong internal skeletons, which supports their body and gives them precise control over their body movements. Most also have a gas-filled swim bladder that allows them to adjust their buoyancy within narrow limits. With a buoyancy bladder fish don’t have use their fins to maintain buoyancy like cartilaginous fish do.
The skeleton of a bony fish has three main parts: the skull, backbone and fin skeleton, The gills are paired and located behind the head. The gill flaps are covered by a bony flap and the lower gill chamber has bony supports, allowing the fish to gulp water and pump it through its gills. In this way the bony fish can respire while sitting stationary and doesn’t need to move to breath like sharks and cartilaginous fish. The bony supports in the gill chamber are used in gulping and seizing food.
Parts of a bony fish (Anoplogaster cornuta)
Most bony fish are covered in scales and they are covered by a thin layer of skin that secrets mucous that helps fight off parasites and disease-causing organisms. Even fish that don’t have scales still have the mucous secreting skin.
Bony fish tend to have keen senses of hearing and sight, which play an important roles in fish communication and schooling. The eyes are generally on the sides of the head, giving the fish a wide field of vision.
Bony fish are very precise and dexterous swimmers. A separate set of muscles controls each fin. The symmetrical tail is often what is used to propel the fish forward. Teeth may be found on the jaws, throat, roof of the mouth or tongue.
Sharks, rays and a group of deep-water fish called chimaeras and their relatives are cartilaginous fish, or elasmobranch, as opposed to bony fish, the classification most fish fall into. All cartilaginous fish possess skeletons made of cartilage rather than bone and specialized teeth that can be replaced throughout their lives. Some have cartilage strengthened by mineral deposits and bonelike dorsal spines.
Parts of a shark (cartilaginous fish)
Cartilaginous skeletons are much lighter and more flexible than bony skeletons. On the land they would be unable to support the weight of large animals but in water they are effective for animals up to 40 feet in length. Most have skin covered by thousands, even millions, of interlocking scales called dermal denticles. They have a similar composition to teeth and give skin a sandpaper-like texture, increase durability and reduce drag.
Cartilaginous fish have five to seven pairs of gills. When water enters the mouth the gill slits are closed. When waters passes through the open gills the mouth is closed. These fish also lack the air bladder that give bony fish their buoyancy and instead have an oil-rich liver that adds to their buoyancy. Even so many are negatively buoyant and need to swim to stay afloat.
All cartilaginous fish have a sensory system of pores called ampullae of Lorenzini, named after an Italian biologist who discovered them in 1678, that send out electrical signals that can be used to locate prey and avoid predators. Most also have an effective lateral line system running from their tail to their snout that helps them detect small vibrations.
swim bladder All cartilaginous fish are carnivorous but for some this means they feed primarily on zooplankton. Most feed on live prey but will feed on carrion if it is available. Few feed exclusively on carrion. Reproduction take place internally when the male passes sperm into the female’s cloaca with a modified pelvic fin. Some species release embryos in leathery egg cases. Other species give birth to live young that hatched from eggs that broke open inside the female. In yet others, embryos develop in placenta-like structures. In all cases the young do not go through a larval stage like many bony fish; instead they are born as miniature adults.
The primary fins possessed by most fish are: 1) the dorsal fin on the back; 2) the two pectoral fins on the sides behind the gills; 3) the two ventral fins on the bottom near the front; 4) the anal fin on the bottom near the back; and 5) the tail (caudal) fin. The fins are composed of a web of skin supported by horny rays. Boy fish lack hands of gripping forelimbs of any kind.
Most fish have scales which overlap like shingles on a roof. The scales grow with the fish and they produce rings like trees which reveal a great deal of data about the fish such as how much food they eat and when they lay their eggs. Over the scales is a layer of skin and a coating of slimy mucous.
Unlike humans, fish continue growing their entire lived. Old fish can be extremely large. The age of fish is estimated by counting lines of the scales or in otoliths, innner ear structure used in helping fish maintain balance. These lines are similar to the rings in a tree. In March 2007, a giant female shortraker rockfish caught in the Bering Strait that was over a meter long and weighed 27 kilograms was estimated to be around 100 years old based on lines in its scales and otoliths.
Most fish lay round, transparent eggs that contain oil globules to nourish the larvae. Their transparency allows them to hide better in the open water from predators. After they emerge from their eggs many creatures remain transparent so they can hide in plain site.
Breathing Fish, Their Air Bladder and Buoyancy
Fish gills Fish have hearts that pump blood through their bodies like terrestrial animals. Instead of lungs they use gills to inject oxygen into the blood. The gills are essentially filters that remove dissolved oxygen, which enters the blood stream through a delicate membrane of filaments. Straining devises called gill rakers prevent food and other object from striking the gills.
Most fish pump water across their gills using their head muscles. They breath by widening their jaws and sucking in water through their mouths and then squeezing their jaws shut to push the water through their gills.
Most fish also have an organ called an air bladder between the stomach and backbone. An appendage of the intestine, it is filled with gas and its purpose is not completely understood although it believed to be involved in maintaining balance and buoyancy and allows a fish to swim easily at any depth.
The gas in the air bladder is lighter than water. The fish can adjust buoyancy by regulating the volume and pressure of the gas in the bladder by moving gas between their bloodstream and the gas bladder. Gas enters the bladder through the gas gland which is supplied with blood by a network of vessels. The bladder of freshwater fish has a larger capacity than that of marine fish because freshwater is less dense than saltwater and does not support the fish’s body as well as saltwater.
Fish lack tongues. They rely on their jaws to the work done by both the jaws and tongue in mammals and reptiles. Many predatory fish rely in suction to pull prey into their mouths. Many fish eat their food whole. Others have sharp teeth and break their food into pieces that are swallowed whole. The vast majority of fish have a second set of jaws deep down their throat which break food as it goes down the gullet.
The back-of-the-throat choppers are called pharyngeal jaws. Adam Summers, a professor of bioengineering at the University of California at Irvine, wrote in Natural History magazine, they “can split, slice, tear or crush food as it goes down the gullet...and they come in an astonishing array of size, shapes and functions—all derived from gill arches, which hold in place the bright respiratory structures that lie behind the cheeks of most fishes. Pharyngeal jaws are equipped with their own set of teeth and move completely independently of the real jaws. Still, the problem persists on how to move prey back down from the mouth jaws to the throat. Suction generally works.”
The pharyngeal jaws on moray eels are like a second creature living inside the eel. See Moray Eels.
parts of a cod Fish are very streamlined. They have to be. Water is 800 times more dense than air and slight protrusions are more disruptive for a fish than for a bird. Eyes bulge barely above the surface and plates covering the gills fit close to the body. The coating of slim over the scales reduce drag and friction.
The fastest fish have bodies that are pointed in the front, grow quickly to maximum diameter and then tapper gently towards the tail. Along the top and bottom edges of the body on either side of the tail are tiny triangular blades that act as spoilers to prevent turbulence. The fastest swimmers like tuna have a deeply forked, half-moon tail.
Gray's Paradox is scientific problem first addressed in 1936 that explains why tuna, dolphin and other sea creatures swim as fast as they do when it seems they only have one seventh of the muscle to achieve this.
Fish move through the water by sweeping the tail fin laterally and moving their body back and forth with sideways muscular movements. The rear half of the fish is like an engine. Powerful muscles are attached to backbone that allow the tail to beat from side to side continuously throughout a fish’s life.
The other fins—the pectoral, pelvic and dorsal fins—are used primarily for stability and maneuvering and braking. Fish have fins at the fore and aft that swivel in almost any direction and give it great maneuverability in water. When the fish move at great speeds and stability is generated by the speed the fins tuck into slots, making the body is streamlined.
fish eye Many fish can quickly shoot forward from a resting position shooting water out of their gills. Flatfish are able to jump off the bottom of the ocean using the same method. Many open sea fish migrate between breeding ground and feeding grounds, often saving energy by riding the currents.
When a ship moves through the water drag is created by vortices that spin off the hull. Fish flex their bodies in such a way that the vortices roll off and no longer produce drag. Through the lateral motion of the body, the vortices roll down the sides and off the tail and in doing this create propulsion forces rather than drag.
Fish Vision and Senses
Fish have eyes with parts that are similar to those of terrestrial animals and in fact fish are the evolutionary source of eyes on terrestrial animals. Fish eyes don't have eyelids because they don't need to keep their eyes moist. Fish tend to be nearsighted and most species have rods and cones, which means they can detect color.
A fish eyes has a totally spherical lens that corrects for refraction, or bending of light, in water and corrects fickle light conditions in the sea. Cones provide acute vision in bright light conditions while close-packed rods pull in light in dim conditions.
Diurnal reef fish can often more see colors than humans and parts of the spectrum that humans can't discern. Their eyes are generally small and usually have a thick layer of melanin that prevents light from bouncing around inside the eye, disrupting their visual images.
Nocturnal fishes often only see in black and white. Having evolved from species that once lived in deeper water, they often have big eyes with big pupils that can perceive a lot with only a little light. It is not clear how good reef fish vision. Some species have been observed feeding on diver's bubbles, apparently confusing them for small silvery fish.
Predator fish such as barracuda and groupers are most active at dawn and dusk and their eyes contains features found in both diurnal and nocturnal fish. Some sea creatures may react to contrasts rather than colors themselves. Shrimp that are bright red and white might actually look black and white to a reef fish, who respond to the contrast not the color.
Most fish have a very developed sense of smell. Their nostrils open into cups that can detect very minute amounts of chemicals in the water
All fish have rods of nerve cells, called lateral lines , that run down their flanks and branch over the head. They have a slightly different texture from the rest of the body and consist of vibration-sensitive hairs and pores connected by a canal running just below the surface. Tiny hair cells in the lines detect difference in pressure and movements in the water and changes in dozens of meters away.
Fish Hearing and Noises
Fish don't have outer ears. They can sense vibrations through the water with an inner ear comprised of three canals and a large sac, all of which have sensitive linings and small particles that move and vibrate. Sound moves better through water than air. Sound waves pass through the skull and reach the inner ear without the aid of a passageways like those provided by the outer ears that terrestrial animals have.
Many fish have bony connections between their ears and swim bladders and use their swim bladders as amplifiers of sound. Some have even developed special muscles that vibrate the swim bladder to produce a loud drumming noise used to call other fish.
Fish don't have voice boxes. They produce a variety of sound with different parts of their bodies. Sea robins, drum fish and other fish make booming, grunting and drumming noises with their air bladders. Other fish grind their teeth or scape their fins against their bodies.
Croakers make enough noise to keep fishermen away at night. Their two- and three-beat drumming sounds are produced by muscles that drum against the air bladder.
Reef fish make quite a bit of noise underwater if you are tuned into the right frequency. Underwater noise includes groupers making a creaking door noise when they spot prey, cichlids emitting grunts, hamlet fish letting out loud squeals, the popping sound made by pistol shrimps, the crunching sound of feeding parrotfish and the grunts of damsel fish.
Do Fish Experience Pain?
mola mola A question that some have addressed—including Kurt Cobain in a Nirvana song—is do fish feel pain when they are caught with a hook? For a long time it was argued that they either didn’t feel anything or the pain they felt was minimal. In a study by University of Edinburgh researchers published in 2003, mouth area of rainbow trout were injected with bee venom, acid and other substances and scientists found that fish possess neurons for detecting pain that are almost identical to those of humans. The study found that fish also responded physically to the pain by wriggling from side to side, rubbing their mouths against their tanks, experiencing accelerated heart rates and abstaining from eating. Critics of the study argue that avoidance reactions does not necessarily mean the fish experienced pain and point out that fish lack the part of the brain that humans have that registers pain and suffering. In 2009 scientists at Purdue University injected some goldfish with morphine and some were given a saline solution. Both groups were subjected to “unpleasant temperature.” The scientists concluded the fish could feel pain.
On whether fish experience pain or not, Victoria Braithwaite, a behavioral biologist at Edinburgh University, wrote in the Los Angeles Times, “Every year, sportsmen around the world drag millions of fish to shore on barbed hooks. It's something people have always done, and with little enough conscience. Fish are ... well, fish. They're not dogs, who yelp when you accidentally step on their feet. Fish don't cry out or look sad or respond in a particularly recognizable way. So we feel free to treat them in a way that we would not treat mammals or even birds. [Source: Victoria Braithwaite, Los Angeles Times, October 8, 2006]
But is there really any biological justification for exempting fish from the standards nowadays accorded to so-called higher animals? Do we really know whether fish feel pain or whether they suffer -- or whether, in fact, our gut sense that they are dumb, unfeeling animals is accurate? Determining whether any type of animal really suffers is difficult. A good starting place might be to consider how people feel pain. When a sharp object pierces the human body, specialized nerve endings called nociceptors alert us to the damage. Incredibly, no one ever seems to have asked before whether fish have nociceptors around their mouths. My colleagues and I in Edinburgh, Scotland, recently looked in trout and found that they do. If you look at thin sections of the trigeminal nerve, the main nerve for the face for all vertebrates, fish have the same two types of nociceptors that we do -- A-delta and C fibers. So they do have the necessary sensory wiring to detect pain.
black rockfish, a bony fish And the wiring works. We stimulated the nociceptors by injecting diluted vinegar or bee venom just under the skin of the trout. If you've ever felt the nip of vinegar on an open cut or the sting of a bee, you will recognize these feelings as painful. Well, fish find these naturally irritating chemicals unpleasant too. Their gills beat faster, and they rub the affected area on the walls of their tank, lose interest in food and have problems making decisions.
When I have a headache, I reach for the aspirin. What happens if we give the fish painkillers after injecting the noxious substances? Remarkably, they begin to behave normally again. So their adverse behavior is induced by the experience of pain.
Do Fish Feel Pain? How Intelligent Are They?
On whether fish feel pain Braithwaite wrote in the Los Angeles Times, “But just because fish are affected by pain, does that mean they actually feel it? To answer that, we need to probe deeper into their brains (and our own) to understand what it means to feel pain. To determine what fish go through mentally when they experience painful stimuli, we also need to determine whether they have a capacity to feel emotion and to suffer. This is a much harder problem. It goes to the very heart of one of the biggest unresolved issues in biology: Do nonhuman animals have emotions and feelings? Are nonhuman animals conscious?
Scientists and philosophers have long debated consciousness and what it is and whether it is exclusively human. There are multiple definitions and, frankly, we haven't really come to grips with what it means to be conscious ourselves. Are we conscious because we are capable of attributing mental states to others, or perhaps because we have a qualitative awareness of feelings, whether positive or negative? And if we can't define our own consciousness, can we expect to detect it in fish? Perhaps not, but we can look for behaviors and abilities that we believe contribute to human consciousness -- for example, complex cognitive abilities and specialized brain regions that process emotion and memory.
blue shark It turns out that the stereotype of fish as slow, dim-witted creatures is wrong; many fish are remarkably clever. For example, they can learn geometrical relationships and landmarks -- and then use these to generate a mental map to plan escape routes if a predator shows up. And their brains are not as different from ours as we once thought. Although less anatomically complex than our own brain, the function of two of their forebrain areas is very similar to the mammalian amygdala and hippocampus -- areas associated with emotion, learning and memory. If these regions are damaged in fish, their learning and emotional capacities are impaired; they can no longer find their way through mazes, and they lose their sense of fear.
None of this tells us that fish are “conscious,” but it does demonstrate them to be cognitively competent: They are more than simple automata. So do we have to change the way we treat fish? Some still argue that fish brains are so less well developed than those of birds and mammals that it isn't possible for fish to suffer. In my view, that case is not proven.
Moreover, we actually have as much evidence that fish can suffer as we do that chickens can. I think, therefore, that we should adopt a precautionary ethical approach and assume that in the absence of evidence to the contrary, fish suffer. Of course, this doesn't mean that we necessarily must change our behavior. One could reasonably adopt a utilitarian cost-benefit approach and argue that the benefits of sportfishing, both financial and recreational, may outweigh the ethical costs of the likely suffering of fish.
But I do find it curious that it has taken us so long even to bother to ask whether fish feel pain. Perhaps no one really wanted to know. Perhaps it opens a can of worms -- so to speak -- and begs the question of where do we draw the line. Crustacean welfare? Slug welfare? And if not fish, why birds? Is there a biological basis for drawing a line?
Image Source: National Oceanic and Atmospheric Administration (NOAA) noaa.gov/ocean ; Wikimedia Commons
Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
© 2009 Jeffrey Hays
Last updated March 2011