2. Finfish Physiology & Anatomy
It is important to understand how fish function, so that you can provide the correct or
optimum environment and alleviate stress. How fish work is referred to as their
‘physiology’ and their body structure is their anatomy. There are large differences in
the physiology and anatomy between species which leads to a range of differences
in their biological needs.
The study of Fish physiology and anatomy is a very complex topic – this session
provides an introduction only, and there are many exceptions to generalisations.
This section provides an introduction to this topic.
3. Basic Anatomy of Finfish
Finfish are the most diverse group of vertebrates in the world and there are many
variations of form (ie body shape, colours, fin arrangement etc). These body forms
are important for taxonomy and identifying species; and they are largely due to
evolutionary adaptations to their particular environment over many generations. Our
primary interest is in the Teleosts (bony fish).
Other groups include the lampreys and sharks, which are not of significant
aquacultural interest at present.
The following slide provides a generalised diagram of the external anatomy of fish.
4.
5. Body Shape
The body shape of fish varies considerably between species, and is often a reflection
of their habitat, for example:
● Fast swimming fish are streamlined (eg Tuna)
● Mid-water species in slow currents or still may be flattened from side to side (eg
Angelfish)
● Bottom dwelling species are flattened from top to bottom (eg catfish, flounder).
These different body shapes reflect the best adaption for living in that environment
6. Body Shape
Body shape may also be effected by deformities that
can be caused by many different factors. These
include:
● Physical damage/rough handling or aggression
with other fish, predators etc
● Genetic abnormalities
● Nutritional deficiency (mainly Vitamin C & B)
during early development
Deformities that cause damage and disfigurement to
fish may reduce marketability of the fish i.e. for
ornamental fish or live fish tarde for restaurants
where the how the fish looks is very important to
customers. Deformed fish should therefore be
graded or culled out.
7. Fins
Fins are another key part of the external anatomy
of fish and serve a a variety of functions including:
● Locomotion and movement
● Stability, manoeuvrability and maintaining
balance inn the water column,
● Reproduction (ie Livebearers).
Modern fish have rayed fins, with fin rays (cartilage
rods) supporting a web of skin. In some species,
the fins have little or no colour but in some species
the fins can be brightly coloured and can play a
major role in both courtship and mating behaviour
8. Fins
There are two (2) types of fins, these
include:
● The median or unpaired fins such
as the tail, dorsal, anal, ventral,
adipose fins
● The paired fins such as the pectoral
and pelvic fins
Fin arrangements vary considerably
between species, which is due to
adaptation to their environment.
9. Fins
Fins can be fairly delicate and easily damaged by a range of factors such as:
● Rough surfaces such as nets and tank walls when densities are high
● Fish may also ‘nip’ the fins of other fish.
● Poor water quality will damage fins in fry during early growth periods.
Suboptimal levels of pH and ammonia are a particular problem. Infection
problems in fry may also result in deformed fins.
Physical damage to fins can often lead to bacterial infections, which can further
damage the fins. The fins may repair to a certain extent damaged fins usually
do not repair the damage start the body base. (but slowly and only if cause is
fixed). Damage to fins may make the fish unmarketable (ie ornamentals, whole
fresh fish). Therefore care must be taken when managing or handling fish to
prevent damage.
10. Mouth
The mouth is another important structure of fish. The
mouth shape and position will also vary depending
on the environment off the fish, with bottom feeders
having a mouth located towards the bottom of the
head, top feeders have a mouth located more
towards the top of the head and mid-water species
mouths are more centrally located
The type, size and number of teeth will also vary
depending on what the fish eats. Carnivorous fish
tend to have larger sharp teeth for grasping prey and
tearing flesh, whereas some fish that eat algae etc
may have little or no teeth
11. The Opercula or Gill Cover
The opercula or gill covers, are movable boney plates that cover and
protect the gills. They are also important in generating water movement
over the gills for gaseous exchange this action is synonymous with
breathing in land animals. The opercula provide a physical barrier to
physical damage of the delicate gill structures, as well to disease
organisms such as parasites. Damage to the opercula can be caused
by:
● Disease
● Poor nutrition
● Genetics
● Rough handling
● Poor water quality, particularly during early development
Damage and disfigurement can reduce marketability (ie ornamentals,
whole fresh fish). Disfigurement may also lead to a new market in
ornamentals (ie blushing angels have visible gills).
12. The Skin
The body of fish is covered by skin which is a complicated and important organ. It
consists of 2 layers:
● Epidermis: outer layer, very thick, contains mucous cells, also taste buds in some
species.
● Dermis: underlying skin, contains pigment cells, and the scales
The skin contains a range of different cell types which preforms a variety of
functions. Mucous cells produce mucous which protects fish from disease and aids in
movement. This mucous continually sloughs off - removing any disease organisms
with it. This makes the fish slippery and difficult to handle. Use of wet hand or soft
gloves and soft net materials protects slime coat of fish. The skin also contains
pigment cells which produce colour in the fish (discussed later in this section) as well
as helping to control water and salt balance in fish.
13. Scales
Scales are small hard plates that grow out of the skin
and form an overlapping protective layer across the
skin. The size, shape and colours vary widely with
different species and can be used to help identify the
species of fish. There are different types of scales
including cycloid, , ctenoid and ganoid but most
aquaculture species have either cycloid or ctenoid
scales.
Scales can be dislodged from the skin during handling,
and this leaves holes in the skin that can be infected
with bacteria, fundus or parasites. While they can
regenerate it is important to be careful when handling
fish not to damage the scales and skin to prevent
infection
14. Colour in Finfish
Fish come in a variety of colours, many
species are brightly and ornated coloured,
while other species have more subdued
colours that allow them to camouflage
themselves and blend into the background.
Colour therefore plays a major part in
aspects such as camouflage themselves
against attack, reproduction, life stage
changes or become dark to indicate stress.
While there are huge variations, in general,
fish are dark on top and light beneath to
blend into the environment.
15. Colour in Fish
Pigmentation and patterns are genetic but can often be controlled by the fish:
● Colours can be altered to match background,
● May change during breeding season,
● Fish often become darker due to stress or disease.
These colour pattern changes can be either morphological with changes in cell
types, cell numbers and is more of a long term adaptation as seen in life stage
changes; or physiological with changes in pigment distribution in cells, a short term
adaptation that may be sued during reproduction of to indicate the ‘mood’ of the fish
and be an indicator of stress etc. There are many types of colour cells or
chromatophores – these vary in chemical composition and reflective properties:
● Xanthophores: yellows, reds
● Melenophores: blacks - darkens colours
● Iridophores: reflective - blues, greens
16. Colour in Fish
Fish need chemicals such as carotenes to
produce red/yellow colours in skin (also effects
colour of the flesh) with amino acids such as
guanine are also being important. These
chemicals come naturally from the feeds that the
fish eat in nature, but in aquaculture are often
added to feeds to produce these colours in either
the skin or flesh of the fish
Sex hormones such as testosterone may
increase numbers of iridophores and are
sometimes used to colour ornamental fish,
however, this may effect their breeding behaviour
or even sex reverse the fish.
17. Internal Anatomy of Finfish
The internal anatomy of fish also varies considerably with species. Many of these
differences are associated with adaptations of their digestive systems to different
food types they have evolved to eat in their environment. While the size, colour and
form of organs may vary, the basic function remains the same. The following
section provides a general introduction to the internal anatomy of fish.
18.
19. Skeleton
The skeleton is a major part of the internal anatomy of fish and forms the ‘scaffold’
for all the internal organs and muscles to attach on and determines the basic form
of the fish. The skeleton of most aquaculture species is made form bone, whereas
sharks and rays have a cartilaginous skeleton.
20. Skeleton
The Skeleton is consist of the main vertebrae or back-bone, a
skull for the head with rays extending above and below the spine
into the muscle tissue and extending to the fins. However, except
for the tail the skeleton has no direct connections to the fins.
The bones are similar to other vertebrates in that they are consist
of matrix of calcium and proteins however, they are less dense
than land animals to reduce the weight so fish are able to
maintain buoyancy more effectively. Some species such as
seahorses may also hard an exoskeleton formed by modified
scales that fuse together
21. Muscles
Fish have two major types of muscles: white and red,
with the red muscles being aerobic while the
white muscle mostly anaerobic. They can be relatively
easily distinguished by their colour with the majority of
muscle in fish being white muscle, and the red muscle
generally located along the side of the fish.
Muscles are formed in blocks called Myomeres which
are commonly zig-zag, "W" or "V"-shaped muscle fibers.
The myomeres are separated from
adjacent myomere by connective tissues and often
easily seen as the ‘flakes’ of fish that the flesh breaks
into when cooked.
22. Digestive System
The digestive system processes food so that materials can be absorbed into the
body and used for various metabolic functions, with waste materials excreted. The
most prominent internal organs are associated with eating and digestion such as
mouth, intestine, stomach, anus, spleen, liver, etc. The digestive system processes
food so that materials can be absorbed into the body and used for various
metabolic functions, with waste materials excreted.
The digestive tract and mouth parts will vary with species, and feeding behaviour:
●Carnivores -short gut, large teeth for catching, retaining prey.
●Herbivores - long gut, small or no teeth.
●Omnivores - compromise between carnivore and herbivore.
23. Digestive System
Most fish have a muscular mouth, with a movable jaw. Teeth are generally present
on the jaws and/or tongue, but can be highly modified in some species. The mouth
will also contain salivary glands and taste buds. A muscular tube behind the mouth
called the pharynx is also present in most fish and grinds or crushes food to aid
digestion before passing into the oesophagus which leads to the stomach. Partly
digested food then passes to the intestines, with faeces excreted from the anus.
24. Digestive Organs
The structure and function of the fish gut is grossly similar to that in land animals,
with the major organs including:
● The stomach works to breakdown food particles with the aid of digestive enzymes
and acidic gastric secretions.
● Pyloric caeca are not present in all species, and are thought to aid in fat digestion
and adsorption of nutrients.
● The intestines breakdown food particles and absorb nutrients through enzymes
and bacterial action.
● The liver serves a range of functions including enzyme production, detoxification
and protein synthesis.
● The spleen acts as a blood filter and red blood cell generation and the immune
system
25. The Swim Bladder
This consists of 1 or 2 connected chambers of air, which is species dependant. The
swim bladder is primarily for buoyancy, and is connected to the oesophagus via a
pneumatic duct or the gas gland, that allows the fish to adjust the air volume and
thus its buoyancy.
The swim bladder is also be used for sound generation, hearing, and in some
species oxygen transfer. The swim bladder develops in larvae and a critical
development stage for ,many species is ‘swim-up’ when fry are first free-swimming
and swim to the surface to take a gulp of atmospheric air to inflate the swim bladder.
During this stage poor water quality and unsuitable temperatures can affect inflation
and the future survival of fry. .
26. Kidneys
There is an anterior (head) and a posterior kidney (hind). They are dark red in colour,
and situated along the spine. They can vary in size and shape with species and may
not be separate in some species. The kidneys serve the same function as in other
animals and filter metabolic wastes from the blood. The urine contains: creatine,
creatinine, various organic compounds, some urea and ammonia.
The kidney also regulate water balance (excrete it if it is too high or reabsorb it if the
level is low). This is discussed further in the section on excretion and
osmoregulation.
27. Heart and Circulatory System
Fish have a two chambered heart compromising of a single atrium and ventricle.
However, there are two accessory chambers that enable the heart to function more
like a mammalian heart than that of 3 chambered reptile or amphibian hearts.
Blood flows through the body through a network of arteries, capillaries and blood
vessels. The circulation of blood is defined as single cycle circulation flowing from
the heart to the gills where it is oxygenated, then on to the bodies tissue and
organs, then back to the heart.
