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CANADIAN WATERS |  |
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 Creature Feature!
Explore the special feature of the month - The Salmon Lifecycle.
Fish!
Fish are creatures that live in the water (freshwater, saltwater and brackish water). They have scales, a body shaped for swimming and fins to move them through the water. There are many types of many fish, living in all types of aquatic habitats. Examples of marine fish include; sharks, seahorses, flounder and tuna. Dolphins, whales and porpoises (Cetaceans) are very similar in shape to fish, but they are not fish. These much larger creatures are mammals. There are some very major differences between marine mammals and fish. In very simple terms here are some of the differences between fish and cetaceans:
What different body shapes do fish have?
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Many fish have a skeleton made out of bone (like mammals, reptiles etc...) and some more Primitive fish have a skeleton that is made of cartilage and some bone.
Fish have bones (or cartilage) that support their fins, their body structure and their tails. They have a backbone like many other animals and they have bones in their head which encase the brain and hold the gill arches inside the mouth (explained in section on Breathing). There is one unique aspect of fish skeletal structure that most other animals do not share. Because fish float in the water, they do not need their limbs (fins and tail) to support their body for walking. As a result, the skeletal structure of the fins and tails are not always attached to the rest of the skeleton. The bones and hard structure making up the fins and tail attach only to muscles and not the spinal column. This allows fish more flexible movement in the water.
Almost all fish have scales or scutes. Scales are transparent plate-like structures covering the body usually in an overlapping fashion (sort of like the shingles on a roof). There are four types of scales: Cycloid, Ctenoid, Placoid and Ganoid.
True bony fish have ctenoid or cycloid scales. Ctenoid scales are rough in texture because they have tiny teeth along their edges. Cycloid scales on the other hand have a smooth surface. Fish such as bass, perch and snappers have ctenoid scales (their skin is rough to the touch) and cycloid scales can be found on fish such as carp, salmon, herring and sardines.
Ganoid scales are found on more primitive bony fishes such as gars and sturgeon. These scales are diamond shaped. The rows of bony scutes on a sturgeons body are modified ganoid scales.
Sharks, skates and most rays have placoid scales which are rough and sharp. Placoid scales which are also referred to as dermal denticles, look like tiny teeth.
Some primitive fish do not have scales at all. Their bodies are smooth and slimy.
Did You Know?... The scales of some fish can tell scientists
how old fish are. By looking under a microscope scientists can see growth rings
on the scales which represent a season or period of active growth for the fish.
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All animals need to breathe in oxygen to survive. Mammals, including whales, dolphins and of course humans use lungs to take in air allowing oxygen to get into their blood for respiration. Fish and many marine invertebrates use GILLS instead of lungs for respiration. When water flows over the gills, the oxygen Diffuses through the gill tissues into the fishes' bloodstream.
Fish have a heart that pumps blood through a network of blood vessels to Oxygenate the body.
When a fish is swimming along, it simply opens its mouth to let water in. Once inside the oral cavity the water runs over the gills which are located in the cavity. A structure called an operculum on the sides of the fishes' head open to let water that has already oxygenated the gills, exit the body.
The following diagram shows the structure of the gills:
Gills have an interesting structure. The gill filaments (which absorb the oxygen) are very numerous and are attached to bony gill arches. On the opposite side of the gill arches are the gill rakers which face forward towards the mouth to filter clean water as it passes over the fills. During respiration, water enters the buccal cavity and flows over the gills. Oxygen is absorbed into the gill filaments and then into the bloodstream. Water that has oxygenated the gills leaves the fishes body through the operculum on the side of the head.
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Many fish have a structure called a "swim bladder" which controls Buoyancy. This means that the fish uses the swim bladder to move to different depths in the water column. The structure develops as a growth out of the intestine. Controlling buoyancy is accomplished by the volume of gas that is kept in the swim bladder. Some fish take in air to control gas volume (physostomous swim bladder) and others control the gases internally (physoclistous swim bladder) through chemical and physical processes.
Neutral buoyancy refers to being essentially weightless in the water. Having neutral buoyancy is important for fish because they need to stay at a specific level in the water column to carry out daily activities (feeding, escape from predators, reproduction etc...). If a fish could not stay neutrally buoyant, they would spend way too much energy on trying to keep their body suspended at the same level by swimming. Fish achieve neutral buoyancy in many ways for example, some fish have decreased bone and muscle mass which are heavy to support and others use a swim bladder to control what level they float at.
The answer to this is very simple. Colour can be very helpful in camouflaging fish from predators and prey. It is important that coral reef fish for example blend in with the bright colours of the reef. Another benefit of colour is to help fish of the same species recogize each other for reproduction. Some fish are colourful in order to attract a mate.
Some fish are not very colourful which helps them to blend in to more bland surroundings. Other fish do not need colour because the environment that they live in is entirely devoid of light (ie. the deep sea). Some deep sea fish however will produce colour with a special "light organ" in order to stun or distract their prey or predator (this is similar to the reaction animals have to automobile headlights).
For example: Anglerfish and frogfishes have a "fishing organ" that hangs off the front of the head in front of the mouth. The light produced by the organ stuns the prey allowing it to be quickly consumed.
Frogfishes
are bottom-dwelling fish that belong to the Family Antennariidae. There are
between 50 and 60 species of frogfishes worldwide. They are small, globular
fish that have short pectoral fins made for grasping. They have a trap door
mouth with which they snatch fish and crustaceans. The fishing lure on frogfishes
is located on the snout.
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Bioluminescence in the light organs of fish is produced by pigment containing bacteria that live inside the organ. It is a very unique Symbiotic relationship.
Many deep sea fish such as lantern fish have pigment containing photophores
(light emitting structures) placed all over the ventral side of the body. These
structures help to break up their silhouette so that they won't be seen by predators.
Similar photophores can be used all over the body for signalling in schooling
and reproduction.
Some pelagic fish such as mackeral are shaded dark on top of their bodies with a pale underside. These features allow the fish to blend in and it is therefore difficult to spot them when they are near the ocean surface. Alternatively animals below them have a hard time seeing them as their light underside blends in with the sunlight from above.
Here are some terms referring to the varying body shapes of fish:
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There are three main types of tails: Protocercal, Homocercal and Heterocercal. These types are separated by the structure of the two LOBES or forks of a fishes tail.
Heterocercal - lobes of Unequal length. There are two types of heterocercal tails:
Protocercal - in more primitive fishes (lamprey, stingrays). The fin rays radiate from the tip of the notochord, like in the coelacanth.
Homocercal - lobes of Equal or the same shape.
Mouth position can tell you a lot about what or how a fish eats its prey. Here are the three main mouth types with examples of the fish that have them.
There are many different types of fish with a wide variety of body colours, sizes and shapes. Fish fall into three main groups; Agnathans (jawless fish), Chondrichthyans (cartilaginous fish) and Osteichthyans (bony fish). The bony fish make up the largest group having over 25 000 species and several distinct subgroupings. Let us explore the three groups of living fish.
Click on the groups of fish listed below to learn more about them.
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There are approximately 45 species of jawless fish. Examples from this group make up two types of fish; lampreys and hagfish. This group has the following scientific classification:
Superclass: Agnatha
Order: Myxiniformes (hagfishes)
Order: Petromyzontiformes (lampreys)
Lampreys and hagfishes have a similar body form. There are however many major differences between the two groups.
| Feature | Lampreys | Hagfish |
| Eyes | Well developed | Rudimentary |
| Oral disc | Yes | No |
| Barbels | No | 3 pairs |
| Nostril location | Top of head | Front of head |
| External gill openings | 7 | from 1 to 14 |
| Internal gill openings | single tube connected to oral cavity | each enters into pharynx |
| Lateral line | developed | degenerate |
| Vertebrae (cartilaginous) | yes | no |
Lamprey and hagfish are not true fish. Ichthyologists however usually associate these agnathans with fish since both are aquatic vertebrates. Most agnathans prey on fish and they are very good at it.
Lamprey attach themselves to the sides of fish such as salmon, sucking on their blood and body fluids for nourishment. You could say that the lamprey is the "Vampire" of the sea. Most prey survive lamprey attacks, but they retain scars on their body.
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These unusual creatures have an oral disc that has tooth-like structures on it. There are also similar structures on the tongue. These hard plate-like teeth are used for attaching to prey by scraping a hole in their tissues.
Did You Know?... An interesting fact about the larvae of lamprey
is that they are not carnivorous like the adults. They are algae and Detritus
eaters.
There are approximately 40 species of lamprey worldwide. Most species are found in the northern hemisphere in Coastal Drainages.
Hagfish are not as aggressive as their relatives the lamprey. These creatures scavenge for dead and dying fish, which is not appreciated by fishermen who find hagfish feeding on the fish in their net catches.
These fish do not have an oral disc, but they do have a ventral mouth with a tongue possessing 4 rows of "tooth-like" structures. The "teeth" rub against a plate made of cartilage forming a jaw that can tear flesh.
Hagfish produce a large amount of mucous over their body, which they use to cover their prey. This mucous is not very tasty to other fish wanting to steal the hagfishes catch. The hagfish itself is also very unpalatable to predators because of this mucous covering. Despite its benefits, the mucous can also clog the gill openings of the hagfish. To combat this problem the hagfish has developed an interesting method to get rid of unwanted mucous. The hagfish will twist itself into a knot to squeeze or slough off the mucous, so that water can pass through the gill openings for respiration.
There are between 30 and 35 species of hagfish worldwide, which are all marine. There are no freshwater species.
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This group contains two subclasses: Holocephali (ratfish and chimaeras) and Elasmobranchii (sharks, skates and rays).
These cartilaginous fish are an interesting group. There is not a person around who does not know what a shark is. These fascinating creatures are often feared by humans. Most species of shark however are harmless. The cartilaginous fishes are thought of as primitive compared to the bony fishes, but they have many unique specializations.
The Chondrichthyes are less numerous than the abundant Teleosts (bony fish). For the most part, they are predators of large invertebrates and fish. These predators are highly successful with well developed sensory systems as well as many other special characteristics. Let us examine the different characteristics of Elasmobranchs.
As we have discussed, neutral buoyancy is important for fish so that they can rest at whatever level in the water they desire. To not have neutral buoyancy would mean spending too much energy trying to get off the sea floor. Many Teleosts (bony fish) use a swimbladder to accomplish this. The swimbladder is a feature that elasmobranchs have not developed. The skates and rays stay on the bottom and other elasmobranchs have very large livers and/or hydrodynamic features that help them to keep off the bottom when swimming. Some hydrodynamic features that help with lift in the water are the protective coat of placoid scales on the body and the heterocercal tail that provides stability and thrust. All elasmobranchs have a cartilaginous skeleton which weighs less than bone and is therefore lighter to support in the water and less energy is spent.
Sharks and other cartilaginous chondrichthyans have gills like other fish. Water must get to the gills to oxygenate the blood and hence the tissues. These fish have spiracles which are located in front of the main gill opening on each side of the head. The spiracle is used to pull water into the gill chambers. Pelagic, fast swimming sharks do not rely on spiracles as much as bottom dwellers such as skates and rays do. Swimming fast pushes water along the gill openings, whereas bottom dwelling fish are often motionless looking for prey and hiding from predators and they need a method to get water into the gill chambers.
These fish have no swim bladder (see section on Swim Bladder above).
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The fish in this group have placoid scales (see above). Some species (most sharks) have the scales covering their entire body, while others like rays and skates only have a few rows of larger placoid scales called denticles on their back. Placoid scales are shed just as our skin cells are shed daily.
Did You Know?... that the spine used for stinging on a stingray
is a modified scale?
Many members of the Chondrichthyes are very efficient predators. Their diet may consist of any of the following: hard or soft shelled invertebrates, fish and marine mammals. Depending on their diet sharks, skates and rays have different tooth shapes. The teeth of this group are modified placoid scales of which they have many rows on their jaws. When the "teeth" are shed or lost, a tooth on the row behind flips into place.
Here are some examples of how tooth shape is adapted for the diet:
These unique fish have Hyostylic jaw suspension. This means that they can dislocate their jaw to make the maximum opening size possible for prey to go through. The jaw is also loosely attached to the cranial (head) bones. In skates and rays the loose jaw suspension allows the jaws to come out of the mouth a little to grab onto prey on or attached to the sea floor.
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The osmoregulatory system is well developed in elasmobranchs. They are able to store nitrogenous wates (from Excretion) in their body without be affected by them. They are able to keep concentrations of salts and other substances at the same concentration found in seawater or higher. Not all fish can do this. The rectal gland excretes sodium and choride Ions (main components of seawater) while the kidney excretes other ions that the body must get rid of.
Sharks are very agile. They move with a side to side motion and their fusiform (torpedo-shaped) body produces little drag. Their heterocercal tail provides them with extra lift in the water.
The embryos of elasmobranchs are kept in egg cases or in eggs with a shell in the body cavity of the female during development. A small number of large mature young are produced which is very different from many other fish who produce many small underdeveloped young. All young in this group receive nutrients from a yolk sac.
Did You Know?... The first embryo to hatch inside the mother
in some species of sharks eats the other offspring inside the body cavity. These
young are later fed by non fertilized eggs that the mother releases. These young
are born large because of all the nutrition that they have received.
Cartilaginous fishes have well developed sensory systems. These senses help them to carry out daily activities such as hunting prey. They are able to pick up the odor of injured prey very well as well as detecting weak sound vibrations from other animals that may be injured, swimming fast or in a school of fish. These fish have a well developed visual sense as well as being able to detect the electrical fields made during muscular movement. There are some interesting parts of a sharks body that allow many of these senses to be so good.
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These are the bottom dwelling ratfishes which have a similar shape to other bottom dwelling fish. Ratfishes live in deep water and there are approximately 30 species worldwide. Their diet consists of bottom dwelling crustaceans and molluscs. Like the skates and rays these fish have flat teeth developed for crushing the hard shells of their prey. The eggs of these fish have a thick outer covering.
Did You Know?... Chimaeras have a very interesting feature.
It is the spine in front of the dorsal fin which has a venom gland associated
with it. This is used for defense against predators.
There are too many fish in the Class Osteichthyes to try to name and describe them all. Instead we will examine bony fish characteristics in general as well as learning about some of the better known groups in this class.
There are freshwater, marine and Anadromous types of sturgeon. They are the largest freshwater and anadromous fish growing up to 8.5 metres in length. Their body is covered by five rows of bony scutes which are modified ganoid scales (see Scales above). They have a ventral mouth that has four barbels in front of it. They are bottom feeders and the adults lack teeth. Their diet consists of benthic invertebrates and fish. In Canada there are sevral species of sturgeon.
The Order Clupeiformes is comprised of 357 species, 72 of which are freshwater. These fish (herring, anchovies) are important in fisheries where 16 million tons are caught each year.
Herring, Clupea sp. and their kin are plankton feeders. They have a keeled (ridged) belly, silver scales which are shed easily and a compressed body which reduces their visibility to predators.
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Salmonids (Family Salmonidae) are freshwater, anadromous and landlocked fish that are important in commercial fisheries and sport fishing in North America and Eurasia. These fish have Parr Marks (see diagram below) as young and all species have an adipose fin. This group has 66 species. The Chinook Salmon, Oncorhynchus tshawytscha can grow to 1.5 metres in length. The life history of this group is very complex. Here is a diagram of the life history of Sockeye Salmon. There are seven stages in the life cycle of Sockeye Salmon.
Explore
the special feature of the month - The Salmon Lifecycle.
The Order Gadiformes has 482 species of which all are marine except for one. They are bottom-dwelling fish. In the past these fish have had some economic importance, but the populations have declined. They are characterized by long dorsal and anal fins. In the Cod family a chin barbel is usually present. The Atlantic Cod is known as Gadus morhua.
The Scombridae are large pelagic fish with well developed jaws. Several fish in this group are important in commercial fisheries. Tuna and mackeral are examples from this group. These fish have torpedo-shaped bodies for moving fast through the water. There are approximately 50 species of tuna and mackeral. They are the top carnivores in the epipelagic zone of tropical and subtropical zones. Their prey consist of small schooling fish and squid.
Did You Know?... Fish are cold blooded, but Tuna and a few
other fish can heat their bodies using special process in their muscles.
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These bodies of these fish are flattened and both eyes appear on one side of the head. These fish are mostly marine with approximately 570 species. Since they are bottom-dwelling fish they must sometimes combat cold temperatures. Special "antifreeze" compounds in their body allow them to withstand up to minus one degree Celsius. Cod also have these proteins.
There are 300 or more species of sculpins, which are bottom-dwelling fish. Most have a broad flattened head, usually with spines and large dorsal eyes, a smooth body and large pectoral fins. Most of these fish live in turbulent water hiding under rocks and using their body shape to stay on the bottom. When water flows over their large pectoral fins they are pressed against the bottom.
Links!Canada's Aquatic Environments - Fish
http://www.aquatic.uoguelph.ca/fish/index.htm
MI-Net - Fish Facts
http://www.mi.mun.ca/mi-net/faq's/fishfaq.htm
Artwork:
Clipart by the Corel Corporation
Original Art by Jennifer Lalonde
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Created: 2001-06-08 Updated: 2005-01-27 Reviewed: 2003-09-03 |
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