A Quick Course in Ichthyology
by Jason Buchheim
Director, Odyssey Expeditions
|Question- What is a fish?|
|List all the defining features of fish.
|FISH: Any of a large
group of cold-blooded, finned aquatic vertebrates. Fish are generally scaled
and respire by passing water over gills.Modern fish are divided into three
I. AGNATHA, primitive jawless fish.Lampreys and Hagfish
II. CHONDRICHTHYES, the jawed fish with cartilaginous skeletons. Sharks, Rays, Rat-Fishes
III. OSTEICHTHYES, fish with bony skeletons.Lungfish, Trout, Bass, Salmon, Perch, Parrot Fish
|FISH: the members of a single species|
Fish of the class agnatha ("no jaw") are the most "primitive" of the fishes; they lack a jaw and a bony skeleton. The hagfish and the lamprey are the only living representatives of this once large class. As they lack true bones, these fish are very flexible, the hagfish can actually tie itself in a knot to rid itself of a noxious slime it can produce to deter predators. They have a smooth, scale less skin and are soft to the touch. In place of the jaws is an oral sucker in the center of which is the mouth cavity. Many of the agnathas are highly predatory, attaching to other fish by their sucker like mouths, and rasping through the skin into the viscera of their hosts. The juvenile lamprey feeds by sucking up mud containing micro-organisms and organic debris - as did the primitive agnatha. Agnathas are found in both fresh and salt water and some are anadromous [living in both fresh and salt water at different times in its life cycle]. The hagfish has no eyes, while the lamprey has well-developed eyes.
Sharks are animals that are superbly adapted to their environment. Almost all are carnivores or scavengers, although the species that live close to the sea floor feed mostly on invertebrates. Most possess a keen sense of smell, a large brain, good eyesight, and highly specialized mouth and teeth. Their bodies are usually heavier than water, and they do not have an air filled swim bladder for buoyancy like most bony fishes. All sharks have an asymmetric tail fin, with the upper lobe being larger than the lower one. This feature, together with flattened pectoral fins, and an oil-filled liver compensates for the lack of a swim bladder. There are 344 known species of sharks living in all parts of the oceans, from shallow to deep water and from the tropics to the polar regions. A few even venture into fresh water and have been found in rivers and lakes. Contrary to popular belief, most sharks are harmless to humans. Sharks are classified into eight orders:
1. Sawsharks (Pristophoriformes), one family, five sp.Live on the bottom in warm temperate or tropical seas. Easily recognized because of tube, blade like snouts. Bear live young.
2. Dogfish Sharks (Squaliformes), three families, 73 sp. Bottom dwelling deep water sharks, distributed worldwide. Bear live young and eat bony fishes, crustaceans, squid and other sharks. Harmless to humans.
3. Angel Sharks (Squatiniformes), one family, 13 sp. Flattened,
bottom dwelling sharks. Found on continental shelves and upper slopes of
cold temperate and tropical seas. Have very sharp, awl-like teeth that
are used to impale small fish and crustaceans.
|4. Bullhead Sharks (Heterodontiformes), one family, 8 sp. Live on rocky reefs where there are plenty of cracks and crevices. Found in Pacific and Indian Ocean. Eat invertebrates.|
6. Mackerel Sharks (Lamniformes), seven families, 16 sp. Small, highly diverse order. Found in tropical to cold temperate or even Arctic waters. Oceanic and coastal. Most very large, eat bony fish, other sharks, squid, and marine mammals. Includes the Mako and Great White and the plankton eating Megamouth and Basking Sharks.
7. Carpet Sharks (Otectolobiformes) seven families, 31 sp. Warm tropical to temperate waters. All members except whale shark live on bottom. Flattened. Most eat small fishes and invertebrates. Whale shark is plankton feeder. Some bear live young and others lay eggs.
8. Ground Sharks (Carcharhiniformes) 8 families, 193 sp. Largest order of sharks. Worldwide distribution, temperate and tropical waters. Most live near coast, although some found in deeper waters. Eat bony fishes, other sharks, squid, and small invertebrates. Includes the dangerous Tiger shark.
Sharks have numerous structural and physiological features that make them unique among the fishes. They have a simple cartilaginous skeleton with no ribs, and a cartilaginous jaw, backbone, and cranium.
Thick skin supports the flimsy skeleton. The skin is elastic and aids in movement; when the tail is arched, it pulls on the skin, which pulls back like a rubber band. The jaws are not connected to the skull and become unhinged, protruding forward from the skull allowing for a wider gape when feeding. The teeth are ossified with minerals known as 'apatite'. They form a conveyer belt with as many as eight teeth in a row. When a shark looses a tooth, another one just pops up. Sharks go through up to 2,400 teeth a year.
Sharks have placoid scales which are fixed, slightly ossified and layered. They are smooth to the touch in one direction and extremely course in another. Just rubbing a shark the wrong way can inflict serious wounds.
All sharks, rays, and skates are carnivores. They have normal sensory modalities, a small brain (most of which is dedicated to the olfactory lobes giving them an acute sense of smell) and well developed eyes with color vision and adaptation to low light levels.
Some sharks lay eggs (all skates and ratfish do), but most are ovoviviparous (all rays are). The young develop with their yolk sacks within the mother, but without a placenta or umbilical cord. Some sharks (the Great White) are oviphagous; the young eat the other developing young and embryos inside their mother and only the fiercest is born! A few sharks (hammerheads and reef sharks) are viviparous; like mammals, the young are nourished with a placenta within the mother. The gestation period is around 22 months and 2-80 pups are born per litter. Because most sharks are ovoviviparous or viviparous, they do not produce mass numbers of young like other fish do. They are slow to develop and for this reason shark population numbers have been decreasing rapidly due to the recent popularity of shark fin soup. Fishermen are taking many more sharks than the maximum sustainable yield will allow. Some sharks will soon be endangered species. Rays
|Rays in general are physiologically exactly
like sharks except the rays pectoral fins are fussed to their heads. Their
gills are ventrally located. They swim with their ventral fins, like wings.
Their eyes are dorsally [top] located and have spericules behind them.
The spericules are used to breathe in with.
Rays are modified as bottom feeders, feeding on invertebrates found in the sand. Sometimes you can watch a ray making quite a ruckus on the sand bottom in search of the invertebrates.
Manta rays are planktivores and cruise the open water filter feeding out small animals. Mantas are the largest of the rays.
|Electric rays swim with their caudal fin and
use their modified pectoral fins to electrically shock and stun their prey.
Sawfish look like sharks but have true fused pectoral fins and gills on the ventral surface.
Stingrays have a toxin filled spine at the base of their tail. Stingrays are not the mean creatures roaming the waters to hurt swimmers, as many people believe them to be. Stingrays are actually very approachable and can be hand fed and petted, just don't step on them!
The bony fish comprise the largest section of the vertebrates, with over 20,000 species worldwide. They are called bony fish because their skeletons are calcified, making them much harder than the cartilage bones of the chondrichthyes. The bony fishes have great maneuverability and speed, highly specialized mouths equipped with protrusible jaws, and a swim bladder to control buoyancy.
The bony fish have evolved to be of almost every imaginable shape and size, and exploit most marine and freshwater habitats on earth. Many of them have complex, recently evolved physiologies, organs, and behaviors for dealing with their environment in a sophisticated manner.
Eels -Anguilliformes 597 spp
Tarpon -Elopiformes 11spp
Salmon -salmoniformes 350 spp
|Deep Sea Fish -Stomiiformes 250 spp||Gobies -Gobiesociformes 114 spp||Trumpetfish -Syngnathiformes 257 spp|
Flyingfishes -Cyprinodontiformes 845 spp
Silversides -Atheriniformes 235 spp
Squirrelfishes -Beryciformes 164 spp
Scorpionfishes -Scopaeniformes 1160 spp
Flatfish -Pleuronectiformes 538 spp
Triggerfish -Tetraodontiformes 329 spp
Perch Like -Perciformes 7791 spp, largest order
Fish have come up with three modes of reproduction depending
on the method they care for their eggs.
|Modes of Reproduction|
Parental care: In fishes, parental care is very rare as most fish are broadcast spawners, but there are a few instances of parental care. Male gobies guard the eggs in a nest until they are born. The male yellowhead jawfish actually guards the eggs by holding them in his mouth! Weird Fish Sex!
Some fish are very kinky creatures by human standards,
displaying behavior that would probably get a human incarcerated for a
A classic example of protogyny is found in the wrasses and parrotfishes. The males in these species form harems, with one large male sequestering and defending a group of smaller females. The male enjoys spectacular reproductive success, as it has many females to mate with. The females also enjoy a limited reproductive success, producing as many eggs as they can, all fertilized by the one male. The male has the advantage over the females; it has many females producing eggs for him to fertilize, whereas the females only have themselves. It is great to be the king!
The weird sex stuff comes in when we analyze what the reproductive success of a smaller male may be. As only the largest male, the 'SuperMale' gets to mate with the females, a smaller male would enjoy zero reproductive success. There is no advantage to being a small male, and this is where the hermaphrodism comes in. If all the smaller fish were females, they could all enjoy a limited reproductive success while they are growing. If the male dies, the one that has grown to be the largest female will change sexes and become the male, in turn enjoying a much greater reproductive success than if she did not switch. So there are no small males and everything is all said and done, but wait! Evolution has a keen ability in finding weaknesses in any system, and it has done so with the parrotfish. In nature, we do find smaller male parrotfish, why should this be so? It has to do with the kind of thing that if a parrotfish was a human, could get the parrotfish into a great deal of trouble. The 'supermale' has to run around all of the time keeping track of and protecting all of his females as well capturing and eating food himself, so he does not necessarily have time to pay close attention to the details. When parrotfish mate, they form a spawning aggregation where the supermale will release his sperm into the water and the many females release their eggs. The sperm and egg find each other in the water column and fertilization takes place, and this is where the weakness of the system lays. Along comes the smaller male, who has evolved to look just like a female. Most of the time the smaller male will make itself completely inconspicuous by behaving just like the females, but during the spawning aggregations, he will be releasing sperm instead of eggs. The supermale will probably not even know that he has been conned. Everything gets really mixed up as males are changing into females changing into males. FISH- Schooling Behavior
Everyone has heard of a school of fish, an aggregation of fish hanging out together; but why, they are obviously not learning reading, writing, and arithmetic. Schools of fish may be either polarized (with all the fish facing the same direction) or non polarized (all going every which way)
There are some factors that can make it advantageous to hang out with other fish.
Antipredator: by hanging out with other fish, each individual fish may gain an advantage in not being eaten by other fish.
B. Dilution affect. If a fish hangs out with a lot of other fish and a predator does come around, the predator must usually select one prey item. With so many choices, the chances are that it will not be you. This is known as the 'selfish herd'.
C. Predator detection. A bunch of fish has many times the eyes and other senses than a solitary fish; so a school of fish may be better at detecting predators. But a school may also attract predators due to its large size.
Enhanced Foraging: A school of fish may have better abilities to acquire food. With many more eyes to detect food, many more meals may be found; but there would also be many more mouths to feed. By working as a team, the school may be able to take larger food items than any one individual could manage to capture.
Migration: The migration abilities of fish in schools may possibly be enhanced due to better navigation, etc. Hydrodynamic efficiency: Due to the complex hydrodynamic properties of water (properties the fish probably discovered only by accident), a fish may gain a swimming advantage by being in a school. The slipstream from the fish ahead of it may make it easier to pass through the water. Good for all the fish except for the ones in front.
FISH- how fish swim
The density of water makes it very difficult to move in, but fish can move very smoothly and quickly.
A swimming fish is relying on its skeleton for framework, its muscles for power, and its fins for thrust and direction.
The skeleton of a fish is the most complex in all vertebrates. The skull acts as a fulcrum, the relatively stable part of the fish. The vertebral column acts as levers that operate for the movement of the fish.
The muscles provide the power for swimming and constitute
up to 80% of the fish itself. The muscles are arranged in multiple directions
(myomeres) that allow the fish to move in any direction. A sinusoidal wave
passes down from the head to the tail. The fins provide a platform to exert
the thrust from the muscles onto the water.
|Diagram of forces when a fish swims.
Thrust- force in animal's direction
Lift- force opposite in right angles to the thrust
Drag- force opposite the direction of movement
** All lift forces cancel out over one complete tail stroke.
|Drag is minimized by the streamlined shape of the fish
and a special slime fishes excrete from their skin that minimizes frictional
drag and maintains laminar (smooth) flow of water past the fish.
|Two swimming types in fishes|
|Defined by their method of living, and reflected in their
|Fins- fins give a fish control over its movements by directing thrust, supplying lift and even acting as brakes. A fish must control its pitch, yaw, and roll.|
|VARIATIONS IN BODY FORM|
|Fish shape has a great bearing on ability
to move through the water.
|Fish Thermal Strategies|
|In general, fishes are cold blooded. They derive their
body heat from their environment and conform to its temperature. As water
has a high heat capacity, it is able to easily suck any excess heat out
of a fish and into the environment.
They maintain a higher body temperature through the use of a specialized counter-current heat exchanger called a reta mirabile. These are dense capillary beds within the swimming muscle that run next to the veins leaving the muscles. Blood passes through the veins and arteries in a counter current (opposite) direction. The heat produced from the muscle contraction flows from the exiting veins into the incoming arteries and is recycled.
Why should they bother having an elevated body temperature? To increase the speed of the fish. The higher the body temperature, the greater the muscular power. Thirty degrees Celsius is the optimum temperature for muscular speed. With increased speed, the tuna can capture the slower, cold blooded fish it prey upon. Tuna have been clocked at record speed of 50-70 mph!
Bony fish have swim bladders to help them maintain buoyancy in the water. The swim bladder is a sac inside the abdomen that contains gas. This sac may be open or closed to the gut. If you have ever caught a fish and wondered why its eyes are bulging out of its head, it is because the air in the swim bladder has expanded and is pushing against the back of the eye. Oxygen is the largest percentage of gas in the bladder; nitrogen and carbon dioxide also fill in passively.
Physoclistous- swim bladder is closed to the gut. The gas gets in through a special gas gland in the front of the swim bladder. Gas leaves the bladder through an oval body in the back of the swim bladder. The system works in a pretty miraculous way. Oval body, filled by venous blood -gasses leave here
Gas gland, fed by arterial blood -gasses enter here
inside the spots= giant secretory cells- secrete lactate -in capillary clusters rete mirabile
Increased lactate levels from the giant secretory cells lower the surrounding pH, causing the blood hemoglobin to dump off its oxygen. The oxygen diffuses back into the incoming capillary, increasing the partial pressure of oxygen in the incoming capillary. This continues until the partial pressure of the oxygen in the capillary is higher than that of the swim bladder (which has a high concentration of oxygen). This complex system is necessary because the concentration of oxygen is higher in the swim bladder than it is in the blood, so simple diffusion would tend to pull the oxygen out of the bladder instead of pushing it in. If the fish wants more buoyancy, it must tell its secretory cells to release more lactate. Since oxygen diffuses easily with oxygen-poor venous blood, the gas can be forced out.
*Fish that migrate vertically tend to have high oxygen levels in their bladders because it fills in faster and leaves faster.
*Fish that maintain a stable depth tend to have more nitrogen because it is inert, enters slowly, and exits slowly.
Fishes- How Fish Breathe
How in the heck can a fish, which is underwater, breath if there is no air? When we go under water, we have to bring air with us to survive. Whales and dolphins have lungs that store air from the surface. Fish don't have lungs, and they rarely ever venture into the air, so how do they survive. We all know it has something to do with gills, but what exactly.
The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about 5 ml. of oxygen per liter of water. This is much less than the 210 ml. of oxygen per liter of air that we breath, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs. Here it comes again, a counter current exchange system, similar to the one we found in the fish's swim bladder and in the tuna's muscles.
The circulation of blood in fish is simple. The heart only has two chambers, in contrast to our heart which has four. This is because the fish heart only pumps blood in one direction. The blood enters the heart through a vein and exits through a vein on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart. A very simple closed-circle circulatory system.
The gills: the gills are composed of a gill arch (which gives the gill rigid support), gill filaments (always paired), and secondary lamellae, (where gas exchange takes place).
|Blood Flow, Counter Current Exchanger|
--Ram Ventilation: Swim through the water and open your mouth. Very simple, but the fish must swim continuously in order to breathe, not so simple.
Fishes- How Do Fish Sense
Successful survival in any environment depends upon an organism's ability to acquire information from its environment through its senses. Fish have many of the same senses that we have, they can see, smell, touch, feel, and taste, and they have developed some senses that we don't have, such as electroreception. Fish can sense light, chemicals, vibrations and electricity.
Light: photoreception [Vision]. Fish have a very keen sense of vision, which helps them to find food, shelter, mates, and avoid predators. Fish vision is on par with our own vision; many can see in color, and some can see in extremely dim light.
Fish eyes are different from our own. Their lenses are perfectly spherical, which enables them to see underwater because it has a higher refractive index to help them focus. They focus by moving the lens in and out instead of stretching it like we do. They cannot dilate or contract their pupils because the lens bulges through the iris. As the depth at which fish are found increases, the resident fish's eye sizes increase in order to gather the dimmer light. This process continues until the end of the photic zone, where eye size drops off as their is no light to see with. Nocturnal fish tend to have larger eyes then diurnal fish. Just look at a squirrelfish, and you will see this to be so. Some fish have a special eye structure known as the Tapetum lucidum, which amplifies the incoming light. It is a layer of guanine crystals which glow at night. Photons which pass the retina get bounced back to be detected again. If the photons are still not absorbed, they are reflected back out of the eye. On a night dive, you may see these reflections as you shine your light around!
Chemicals: chemoreception [Smell and Taste]. Chemoreception is very well developed in the fishes, especially the sharks and eels which rely upon this to detect their prey. Fish have two nostrils on each side of their head, and there is no connection between the nostrils and the throat. The olfactory rosette is the organ that detects the chemicals. The size of the rosette is proportional to the fish's ability to smell. Some fish (such as sharks, rays, eels, and salmon) can detect chemical levels as low as 1 part per billion.
Fish also have the ability to taste. They have taste buds on their lips, tongue, and all over their mouths. Some fish, such as the goatfish or catfish, have barbels, which are whiskers that have taste structures. Goatfish can be seen digging through the sand with their barbels looking for invertebrate worms to eat and can taste them before they even reach their mouths.
Vibrations: mechanoreception [Hearing and touch]. Have you ever seen a fish's ear. Probably not, but they do have them, located within their bodies as well as a lateral line system that actually lets them feel their surroundings.
Fish do not have external ears, but sound vibrations readily transmit from the water through the fish's body to its internal ears. The ears are divided into two sections, an upper section (pars superior) and a lower section (utriculus) The pars superior is divided into three semicircular canals and give the fish its sense of balance. It is fluid-filled with sensory hairs. The sensory hairs detect the rotational acceleration of the fluid. The canals are arranged so that one gives yaw, another pitch, and the last- roll. The utriculus gives the fish its ability to hear. It has two large otoliths which vibrate with the sound and stimulate surrounding hair cells.
Fish posses another sense of mechanoreception that is kind of like a cross between hearing and touch. The organ responsible for this is the neuromast, a cluster of hair cells which have their hairs linked in a glob of jelly known as 'cupala'. All fish posses free neuromasts, which come in contact directly with the water. Most fish have a series of neuromasts not in direct contact with the water. These are arranged linearly and form the fishes lateral lines. A free neuromast gives the fish directional input.
A lateral line receives signals stimulated in a sequence, and gives the fish much more information (feeling the other fish around it for polarized schooling, and short-range prey detection 'the sense of distant touch').
Electricity: electroreception. Sharks and rays
posses special organs for detecting electrical potential [voltage]. A set
of pits comprise the electroreceptive system called the ampullae of Lorenzini.
These are canals in the skin filled with a gelatin-like material that also
contain sensory cells. Movements or disturbances near the shark change
the voltage drop along the canals, which allows the shark to sense other
organisms nearby. These sensors are so sensitive that if there were not
any other distortions a shark could detect the heartbeat of a fish 500
miles away! They can detect muscular contractions of struggling prey and
even the earth's magnetic field (which sharks use for navigation).
Marine Biology resources by Odyssey Expeditions Tropical Marine Biology Voyages