Fish are vertebrate animals which means that they are animals who possess a backbone or a spine. The backbone aids the fish by supporting the bodies tissues and anchor muscles, this allows the fish to move and function using the spinal cord system.
Fish are classified into three main categories: –
Agnathan
These are called jawless fish, the two examples of jawless fish, are lampreys and hagfish.
They are unable to move their teeth which makes it difficult for them to eat. The fish have no distinct stomach, but they do have a long body.
The agnathan fish are unable to regulate their own temperatures, ectothermic, this means that the metabolism of these fish is slower in cold water environments which impacts the digestion of the food they do eat.
When it comes to eating the lampreys feed mostly on other fish and mammals, injecting them with an anticoagulant fluid, stops the blood from forming clots, to result in more blood being in their prey.
Whilst the hagfish tend to be known as a scavenger fish, feeding on animals that are already dead.
There has not been much research on the hagfish, however the lampreys have been known to lay eggs to reproduce. This is usually done in freshwater riverbeds where the eggs are buried just under the bottom of the river. There does not appear to be any knowledge of the parent lampreys staying with their young. From the eggs hatching to adulthood for a lamprey this is known to take approximately four years.
Elasmobranchs
These fish tend to be made up of cartilage as opposed to bone.
A few examples of elasmobranchs are sharks, rays, and skates.
Elasmobranch fish usually lay large heavy eggs or give birth to live babies, these species of fish practice internal fertilisation. This is partly due to the females of the species having no visible reproduction organs whilst the makes have two on the pelvic fin, these are called claspers and are used to internally fertilise the eggs within the female. The eggs that are laid are called mermaid’s purses. This is known as oviparity where the baby develops inside if the egg cases gaining the nourishment from within the egg case. In some cases, after these egg cases have been laid, they can be found washed ashore on neighbouring beaches.
In the species where the baby is grown in viviparity (inside the mother) they may be pregnant for periods up to two years, this is dependent on the species of the parents.
As with the agnathan classified fish. There is an apparent lack of parental care one the young are born, or the eggs have hatched.
There are several characteristics which separate the elasmobranchs from other classifications, for example they do not possess a swim bladder.
They have pairs of gill clefts, these gill clefts come in either five, six or seven pairs and open individually to the rigid dorsal fins.
The upper jaw of these species is not merged to the skull which allows it to move without the movement of the head.
The teeth come in several rows which are continuously replaced, with a new row of teeth being just behind the current set ready and waiting to be needed and will move forwards within the gum line as teeth are lost.
The scales of these fish have the same consistency as teeth, with enamel outer part. These are called denticles, these scales are shaped like curved teeth and are extremely tough. They function as a protective layer, but they also have a rough exterior texture which can be used as an additional way to injure the desired prey.
The scales design means that they are all streamlined leaning back from the head to the tail, this allows the fish to swim faster and move more efficiently through the water.
However, due to having stiff fins combined with the lack of a bony spine and muscle control this means that these fish can only swim forwards.
Osteichthyes
These fish are characterised as bony fish and make up approximately 90% of all known species so far. There are more than 32,000 species of fish which have been identified and are documented.
The main difference between Osteichthyes and other classifications of fish is the endoskeleton.
These fish are known as bony fish and have skeletons made up of collagen and calcium.
The skin of these fish is usually covered in one of two types of scales, cycloid or ctenoid. Apart from flounders who have one side covered in cycloid scales and the other has ctenoid.
The cycloid scales are round and flexible with a smooth edge as opposed to the ctenoid which has spines or needle-like teeth along the free edge. The outer layer of these scales is made of calcium and has an inner layer of connective tissues. Both scales continue to grow with the fish, as they grow the scales gain rings as a pattern, these rings much like with trees can be used to gain an approximate age of the fish.
The Osteichthyes have four pairs of gill slits which are protected by operculum and aid the fish to intake oxygen and dispose of carbon dioxide efficiently to enable the cells of the fish to be adequately oxygenated.
Dependent upon the type of the type of the fish, the diet varies. Most of these fish are in fact omnivorous, they eat both plant matter and meats that are available in the environment. There are some that are herbivores and live solely on plants or are carnivores and have a completely meat-based diet.
This is all dependent upon the breed of the fish.
Bony fish are primarily oviparous. This means that the females lay eggs, and the males fertilise them externally. The fertilised eggs then hatch, and the cycle begins again. As with most fish there is an apparent lack of parental care however there are a few species where the young are guarded by their parents. This is not considered to be the norm amongst fish. And in some cases, the paternal fish provides this care more than the maternal fish.
A few common examples of Osteichthyes in the United Kingdom are: –
This list is only a select few examples as more than 27% of all known species originate in the United Kingdom.
