Centropristis striata
Classification
Fish are a
paraphyletic group: that is, any clade containing all fish
also contains the tetrapods, which are not fish. For this reason, groups
such as the "Class Pisces" seen in older reference works are
no longer used in formal classifications.
Fish are classified into the following major groups:
-
Hyperoartia
-
Petromyzontidae (lampreys)
-
Pteraspidomorphi (early jawless fish)
-
Thelodonti
-
Anaspida
-
Cephalaspidomorphi (early jawless fish)
-
Galeaspida
-
Pituriaspida
-
Osteostraci
-
Gnathostomata (jawed vertebrates)
Some palaeontologists consider that
Conodonta are
chordates, and so regard them as primitive fish.
For a fuller treatment of classification, see the
vertebrate article.
Fish anatomy
-
Digestive system
The advent of jaws allowed fish eat a much wider variety
of food, including plants and other organisms. In fish, food
is ingested through the mouth and then broken down in the
esophagus. When it enters the stomach, the food is further
broken down and, in many fish, further processed in
fingerlike pouches called pyloric ceca. The pyloric ceca
secrete digestive enzymes and absorb nutrients from the
digested food. Organs such as the liver and pancreas add enzymes and various digestive chemicals as
the food moves through the digestive tract. The intestine
completes the process of digestion and nutrient absorption.
Respiratory system
Most fish exchange gases by using
gills that are located on either side of the pharynx. Gills
are made up of threadlike structures called filaments. Each
filament contains a network of capillaries that allow a
large surface area for the exchange of oxygen and carbon
dioxide. Fish exchange gases by pulling oxygen-rich water
through their mouths and pumping it over their gill
filaments. The blood in the capillaries flows in the
opposite direction to the water, causing counter current
exchange. They then push the oxygen-poor water out through
openings in the sides of the pharynx. Some fishes, like
sharks and lampreys, possess multiple gill openings.
However, most fishes have a single gill opening on each side
of the body. This opening is hidden beneath a protective
bony cover called an operculum. Some fishes, such as
lungfish, have developed an adaptation known as a labyrinth
that allows them to survive in oxygen-poor areas or places
where bodies of water constantly dry up. These species of
fish possess specialized organs that serve as lungs. A tube brings air containing oxygen to this organ
by way of the fish's mouth. Some kinds of lungfish are so
dependent on receiving oxygen from the air that they will
suffocate if not allowed to reach the surface of the water.
Circulatory system
Fish have a
closed circulatory system with a heart that pumps the blood
in a single loop throughout the body. The blood goes from
the heart to gills, from the gills to the rest of the body,
and then back to the heart. In most fishes, the heart
consists of four parts: the sinus venosus, the atrium, the
ventricle, and the bulbus arteriosus. Despite consisting of
four parts, the fish heart is still a two-chambered heart.
The sinus venosus is a thin-walled sac that collects blood
from the fish's veins before allowing it to flow to the atrium, which is
a large muscular chamber. The atrium serves as a one-way
compartment for blood to flow into the ventricle. The
ventricle is a thick-walled, muscular chamber and it does
the actual pumping for the heart. It pumps blood to a large
tube called the bulbus arteriosus. At the front end, the
bulbus arteriosus connects to a large blood vessel called
the aorta, through which blood flows to the fish's gills.
Excretory system
As with many aquatic animals, most fishes release their
nitrogenous wastes as ammonia. Some of the wastes diffuse
through the gills into the surrounding water. Others are
removed by the kidneys, excretory organs that filter wastes
from the blood. Kidneys help fishes control the amount of
ammonia in their bodies. Saltwater fish tend to lose water
because of osmosis. In saltwater fish, the kidneys
concentrate wastes and return as much water as possible back
to the body. The reverse happens in freshwater fish, they tend to gain water continuously.
The kidneys of freshwater fish are specially adapted to pump
out large amounts of dilute urine. Some fish have specially
adapted kidneys that change their function, allowing them to
move from freshwater to saltwater.
Sensory and nervous system
Fish have well-developed nervous systems that organize
around a central brain, that is divided into different
parts. The most anterior, or front, end of the brain are the
olfactory bulbs, which are involved in the fish's sense of
smell. Unlike most vertebrates, the cerebrum of the fish
primarily processes the sense of smell rather than being
responsible for all voluntary actions. The optic lobes
process information from the eyes. The cerebellum
coordinates body movements while the medulla oblongata
controls the functions of internal organs. Most fishes
possess highly developed sense organs. Nearly all daylight
fish have well-developed eyes that have color vision that is
at least good as a human's. Many fish also have specialized
cells known as chemoreceptors that are responsible for
extraordinary senses of taste and smell. Although they have
ears in their heads, many fish may not hear sounds very
well. However, most fishes have sensitive receptors that
form the lateral line system. The lateral line system allows for
many fish to detect gentle currents and vibrations, as well
as to sense the motion of other nearby fish and prey. In
2003, it was also found by Scottish scientists at Edinburgh
University performing research on rainbow trout that fish
experience pain.
Some fishes, such as catfish and sharks, have organs that
detect low levels electric current. Other fishes, like the
electric eel, can produce their own electricity.
Muscular system
Fish locomotion
Most fish move by contracting paired sets of muscles on
either side of the backbone alternately. These contractions
form S-shaped curves that move down the body of the fish. As
each curve reaches the back fin, backward force is created.
This backward force, in conjunction with the fins, moves the
fish forward. The fish's fins are used like an airplane's
stabilizers. Fins also increase the surface area of the
tail, allowing for an extra boost in speed. The streamlined
body of the fish decreases the amount of friction as they
move through water. Since body tissue is more dense than
water, fish must compensate for the difference or they will
sink. Many bony fishes have an internal organ called a
swim bladder that adjust their buoyancy through
manipulation of gases.
Reproductive system
The eggs of fish are fertilized either externally or
internally, depending on species. The female usually lays
the eggs, and the embryos in the eggs develop and hatch
outside her body. These kind of fish are called oviparous
fish. Oviparous fish develop by obtaining food from the yolk
in the egg. Salmon, for example, are oviparous.
Ovoviviparous fish keep the eggs inside of the mother's
body after internal fertilization. Each embryo develops in
its own egg. The young are "born alive" like most mammals.
Some species of fish, such as various sharks, are
viviparous. Viviparous fish allow their embryos to stay
in the mother's body like ovoviviparous fish. However, the
embryos of viviparous fish obtain needed substances from the
mother's body, not through material in the egg. The young of
viviparous species are also "born alive".
Immune system
Types of immune organs vary between different types of
fish.[1]
In the jawless fish (lampreys and hagfishes), true lymphoid
organs are absent. Instead, these fish rely on regions of
lymphoid tissue within other organs to produce their immune
cells. For example, erythrocytes, macrophages and plasma
cells are produced in the anterior kidney (or pronephros)
and some areas of the gut (where granulocytes mature)
resemble primitive bone marrow in hagfish. Cartilaginous
fish (sharks and rays) have a more advanced immune system
than the jawless fish. They have three specialized organs
that are unique to chondrichthyes; the epigonal organs
(lymphoid tissue similar to bone marrow of mammals) that
surround the gonads, the Leydig’s organ within the walls of
their esophagus, and a spiral valve in their intestine. All
these organs house typical immune cells (granulocytes,
lymphocytes and plasma cells). They also possess an
identifiable thymus and a well-developed spleen (their most
important immune organ) where various lymphocytes, plasma
cells and macrophages develop and are stored. Chondrostean
fish (sturgeons, paddlefish and birchirs) possess a major
site for the production of granulocytes within a mass that
is associated with the meninges (membranes surrounding the
central nervous system) and their heart is frequently
covered with tissue that contains lymphocytes, reticular
cells and a small number of macrophages. The chondrostean
kidney is an important hemopoietic organ; where
erythrocytes, granulocytes, lymphocytes and macrophages
develop. Like chondrostean fish, the major immune tissues of
bony fish (or teleostei) include the kidney (especially the
anterior kidney), where many different immune cells are
housed[2]. In addition, teleost fish possess a thymus,
spleen and scattered immune areas within mucosal tissues
(e.g. in the skin, gills, gut and gonads). Much like the
mammalian immune system, teleost erythrocytes, neutrophils
and granulocytes are believed to reside in the spleen
whereas lymphocytes are the major cell type found in the
thymus[3][4]. Recently, a lymphatic system similar to that
described in mammals was described in one species of teleost
fish, the zebrafish. Although not confirmed as yet, this
system presumably will be where naive (unstimulated) T cells
will accumulate while waiting to encounter an antigen.
[5]
Evolution
The early fossil record on fish is not very clear. It
appears it was not a successful enough animal early in its
evolution to leave many fossils. However, this would
eventually change over time as it became a dominant form of
sea life and eventually branching to include land
vertebrates such as amphibians,
reptiles, and
mammals.
The formation of the hinged jaw appears to be what
resulted in the later proliferation of fish because un-jawed
fish left very few ancestors.
Lampreys may be a rough representative of pre-jawed fish.
The first jaws are found in Placodermi fossils. It is unclear if the advantage of a
hinged jaw is greater biting force, respiratory-related, or
a combination.
Some speculate that fish may have evolved from a creature
similar to a coral-like
Sea squirt, whose larvae resemble primitive fish in some
key ways. The first ancestors of fish may have kept the
larval form into adulthood (as some sea squirts do today,
see
Neoteny), although the reversal of this case is also
possible. Candidates for early fish include
Agnatha such as Haikouichthys, Myllokunmingia, and
Pikaia.
Fish disease
-
Fish are susceptible to disease as any other organism.
Fish diseases can be refered to as etiology:
- Bacterial Disorders
- Fungal Disorders
- Parasitic Disorders
- Viral Disorders
- Metabolic Disorders
- Water conditions
- Malnutrition
or the organ system most affected
- Neurological Disorders
- Body Cavity
- Eye Disorders
- Fecal Disorders
- Fin Disorders
- Gallbladder
- Gill Disorders
- Intestinal Disorders
- Kidney Disorders
- Liver Disorders
- Locomotor Disorders
- Skin Disorders & Changes In Color
- Swim Bladder
See also
-
The Wikipedia Fish Category page which provides
links to all aspects of the subject from icthyology to
aquariums to sharks.
-
Fish diseases Fish disease category
-
Ichthyology (the study of fish)
-
List of fish families
-
List of fish common names
-
Fish anatomy
-
List of freshwater aquarium fish species
-
Marine aquarium fish species
-
Antimycin A piscicide
-
Deep sea fish
-
Walking fish
Note on usage: "fish" vs.
"fishes"
"Fishes" is the proper
English plural form of "fish" that biologists use when
speaking about two or more fish species, as in "There are
over 25,000 fishes in the world" (meaning that there are
over 25,000 fish species in the world). When speaking of two
or more individual fish organisms, then the word "fish" is
used, as in "There are several million fish of the species
Gadus morhua" (meaning that G. morhua comprises several
million individuals). To see both in action, consider the
statement "There are twelve fish in this aquarium,
representing five fishes" (meaning that the aquarium
contains twelve individuals, some of the same species and
some of different species, for a total of five species). The
usage of the two words is similar to that of the words
"people" and "peoples". The collective noun for fish is
shoal (or school).
References
- ^
A.G. Zapata, A. Chiba and A. Vara. Cells and tissues
of the immune system of fish. In: The Fish Immune
System: Organism, Pathogen and Environment. Fish
Immunology Series. (eds. G. Iwama and T.Nakanishi,), New
York, Academic Press, 1996, pages 1-55.
- ^
D.P. Anderson. Fish Immunology. (S.F. Snieszko
and H.R. Axelrod, eds), Hong Kong: TFH Publications,
Inc. Ltd., 1977.
- ^ S.
Chilmonczyk. The thymus in fish: development and
possible function in the immune response. Annual
Review of Fish Diseases, Volume 2, 1992, pages 181-200.
- ^
J.D. Hansen and A.G. Zapata. Lymphocyte development
in fish and amphibians. Immunological Reviews,
Volume 166, 1998, pages 199-220.
- ^
Kucher et al.,. Development of the zebrafish
lymphatic system requires VegFc signalling. Current
Biology, Volume 16, 2006, pages 1244-1248.
External links
Aquarium Fish Resources
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