Fish Anatomy
Whilst fish are vertebrates like us, they are completely different in most other respects. They're designed to live in an aquatic environment, processing oxygen from the water through their gills while balancing and moving with their fins. Fish are also cold-blooded, which means their body temperature is usually the same as their surroundings.
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Mouth
Fins
Mouth
You can tell a lot about a fish's feeding habits by looking at the shape of it's mouth. Fish are generally divided into three categories; Top-swimmer's mouths are upturned like a scoop to feed from the surface (also known as a Superior mouth). Midwater swimmers have mouths at the end of their snouts, and feed on matter in the water (also known as a terminal mouth). Bottom-dwellers have underslung mouths, allowing them to feed on algae. These fish have flattened bellies to allow them to feed from the very bottom and remain anchored ( a.k.a. Inferior mouth).
Gills
This is the fish equivalent of lungs. Water passes through layers of membrane which diffuse oxygen into the fish's bloodstream. Fish are protected from imbibing anything harmful by way of "gill-rakes", which filter out damaging matter.
Body Shape
The shape of a fish's body denotes the environment in which it lives.
Disc-shaped: Suited to slow-moving or still waters, often lives in reeds.
Deep-bellied: This belly contains muscles which allow the fish to skim over the water.
Flat-bellied: Enables the fish to anchor to rocks and feed low to the ground.
Cylindrical: These fish like fast-flowing waters
Straight-backed: This allows the fish to swim just below the surface.
Outer covering (scales)
Fish have two layers of skin. The thin outer layer is called the epidermis, and the thicker inner layer the dermis. Most fish's skin in encased in scales, which provide protection from injury and allow better movement through the water. These scales are then covered in a thin layer of mucus that protects the fish against parasites. Fish scales belong to one of two categories; Placoid scales, which are teethlike scales, as found in sharks and rays, or elasmoid scales, which are found in aquarium fish. Elasmoid scales are either ctenoid (comb or teethlike projections at the rear of the scale), or cycloid scales, which are smooth and round. Some fish have only one kind of elasmoid scales, some have both.
There are two other categories of scales. Armoured catfishes have scutes, which are two or three overlapping bony plates. There are also the so-called 'naked fishes', like the African catfish, or the Clown Loach, which has scales so tiny it looks naked. Naked fishes should be treated carefully with any medicines - usually you would have to halve the dosage because the lack of scales makes the fish vulnerable to overdose or damage.
Fins
A fish usually has seven fins; dorsal, caudal, adipose fins and a pair each of pelvic and pectoral fins.
Dorsal: Dorsal and anal fins keep the fish upright. Some fish have modified their anal fin to aid spawning, developing hooks.
Caudal: Commonly known as the tail, this is the last fin in a fish's movement pattern. It provides a strong powerful push to send the fish through the water. The fish's muscles ripple down it's body to create this power.
Adipose: Some fish carry this fin between the caudal and dorsal fins. However, its purpose is to date unknown.
Paired Fins: Fish use these for maneuvering. The pectorals are sited just behind the gills, and the pelvic or ventral fins just in front of the anal fin. These fins generally correspond to the siting of limbs on mammals.
Senses
Smell: Fish smell through their nostrils, but do not breathe through them. Usually there are two or four openings on the front of the snout. This allows fish to detect pheromones for spawning and fear-detection.
Taste: A fish's taste buds are located in its mouth, tongue and lips. They may also be present on other parts of the body, such as the pelvic fins or the barbels (whiskers) of bottom-dwellers.
Sight: Most fish have monocular vision - they can see in two directions but both eyes cannot focus on the same object at the same time. However, a few species have eyes that are present higher on the head have come binocular vision. With their eyes fish can only focus to a distance of about 45cm, but can detect things that are further away with their 'lateral line vision', which allows fish to detect vibrations and currents. The name derives from the tiny line of perforations along the lateral line of the fish, and this is considered a fish's "sixth sense." Fish have no eyelids as their eyes do not require moisture.
Hearing: Because water is very dense, sounds and vibrations are more easily detected. Therefore a fish's aural system is made up of the equivalent of a human inner ear only. Fish can hear much the same frequency range of sound as humans, but some species only hear a part of it. A fish's hearing is supported by its ability to sense vibrations, so its swim-bladder and hearing are often connected. The swim -bladder is present in most fish except a few bottom-dwellers (ie Gobies). It allows the fish to maintain it's position in the water, and in some fish acts as a sound amplifier.
Bodily Functions
Your fish's anatomy will always be beneficial for understanding the needs and habitats of your fish. It will also help you to detect any illness or problem your fish may be suffering from.
Respiration: Fish breathe through their gills, although the actual process differs between species. Anabantoids, also known as labyrinth fish, can breathe atmospheric air, whilst some suckermouths breathe through extra slits behind their head in order to continue feeding.
Sleep: Without eyelids, it is often difficult to tell whether fish sleep or not. They do require rest, and accomplish this through periods of suspended animation. You may observe your fish like this for several hours. Some marine fish bury into the sand or create mucus cocoons in which to sleep.
Excretion: Alongside usual bodily excretion, fish excrete ammonia from their gills, and some fish excrete water from their gills. Fish also deposit waste products such as guanin under their skin, which adds iridescence to the fish's colouring.
Fluid Regulation: In marine fish, the concentration of blood salts in lower in the body than in the surrounding waters. Through osmosis, these fish lose bodily water, and so have to drink a lot of water. By excreting excess salt and passing only a little urine they can maintain their fluid levels. Freshwater fish experience the opposite - the blood concentration is higher in the body than in the water. Because water is constantly absorbed into the body, the fish must excrete up to ten times their body weight in water per day, through urination and the gills.
