Fish are aquatic creatures found all over the world. There are about 32,000 species of fish, making them the most diverse group of vertebrates when it comes to species!
Did You Know?
Fish have the distinction of being the first vertebrates on Earth. They were the ones who provided the very basic blueprint for all kinds of body structures which have evolved since then (of course, with a lot of changes to suit the needs of each respective species)!
The recorded evolution of fish dates back almost 500 million years, when the first fish-like creatures made their appearance. These were a prequel to the actual fish according to experts, because they lacked a proper bony or cartilaginous backbone. From these fish-like creatures the first accepted fish developed – the jaw-less fish! From then on, there was no looking back for these unassuming creatures, as today there are more than thousands of varieties of fish, and they all owe their existence to these jaw-less ancestors. And come to think of it, today only two varieties of jaw-less fish exist – the hagfish and the lampreys.
The jaw-less fish evolved into 3 groups of jawed fish:
- Bony Fish
- Plated Skin Fish
- Cartilaginous Fish
The first group, the plated skin fish, did not survive for long and eventually became extinct after a few years. Other than the hagfish and the lampreys, all remaining fish belong to the remaining two categories – boneless and cartilaginous. There are over 32,000 known species of fish in the world. Out of them, almost 27,000 species include bony fish. 970 species out of the remaining include rays, sharks, and chimeras, and 108 are the lampreys and hagfish.
Classification According to Species
The American Fisheries Soceity gives the following 3 classes of fish:
- Agnatha: This is the most primitive class of fish. They have no jaws, and undeveloped backbones. They are also known as Jawless Fish. Only two groups are still surviving in this class – hagfish and lampreys.
- Chondrichthyes: This class of fish has jaws, and is characterized by their cartilaginous skeletons, and the lack of bones in their body. This class is divided into three types – sharks, skates, and rays.
- Osteichthyes: These too, are jawed fish, and have partly or wholly bony skeletons and no cartilage. This class is subdivided into two subclasses –
1. Sarcopterygii – which are the lobe-finned fish, and
2. Actinopterygii – which are the ray-finned fish.
A few of the ray-finned fish left their water abode and ventured onto the land, thus becoming the ancestors for all terrestrial vertebrates.
Agnatha (Jawless Fish)
These jawless creatures are very primitive in nature. Not only do they lack jaws, most of the species in the two surviving groups also do not have a defined stomach, fins, or the ability to regulate their own body temperature (i.e. they are ectothermic). There is not much information available about their mating or reproductive habits, but fertilization is believed to be external. While hagfish feed mainly on dead animals, lampreys mainly feed off other fish.
Chondrichthyes (Cartilaginous Fish)
As the name says, these fish possess a cartilaginous skeleton. The young are born with a notochord, which is gradually replaced by cartilage. They have scales, gills, and paired fins. Although they have a skeleton, they lack a ribcage; thus, if taken out of water, they will die, because their body weight will crush their internal organs. They also lack bone marrow. A heterocercal (where the top lobe is larger in size than the bottom lobe) tail in sharks helps them in locomotion.
Osteichthyes (Bony Fish)
Fish in this class possess a bony skeleton, gills as well as an operculum which enables them to breathe when they are resting. Many species have swim bladders (a gas-filled organ which helps the fish control buoyancy). Other than a few exceptions, the fish in this class are ectothermic. Although the primary organ for respiration is gills, most of the species are also able to breathe through either lungs/swim-bladders/stomach/skin or intestines!
Based on their feeding patterns, osteichthyes are classified into the following 4 groups:
Other examples of Osteichthyes are Oarfish, Flying fish, and Milkfish.
Did You Know?
One of the main ingredients used in the manufacturing of lipsticks is fish scales! Majority of the brands use it for making their lipsticks to get that shimmery effect, and it is something you may want to keep in mind the next time you apply a generous layer on your lips!
Classification According to Habitat
According to habitat, there are again three classes of fish namely:
- Marine Fish
- Freshwater Fish
- Migrating Fish.
A few examples of each of these classes are given below, although they are by no means all the fish in that particular category.
Freshwater fish are adapted to survive in very low levels of less than 0.05% level of salinity. These fish have to do some extra work as compared to their saline water counterparts, because they have to utilize the very low levels of salt found in freshwater optimally, to maintain their internal body salt balance.
Their main physical adaptations are scales on their body, and very well-developed kidneys. The scales help reduce water diffusion through their bodies, while the kidneys help to retain salts as much as possible. Quite a few of the freshwater species are now endangered due to water pollution caused by humans.
Marine fish are typically adapted to water which has an average pH level of 8.2. Usually, these fish are characterized by brilliant colors and many different physical adaptations, which makes it easier for them to protect themselves from their predators. Marine fish are classified into three types, based on their feeding habits:
The other examples of Marine Fish are Hawk Fish, Grouper, Squirrelfish, and Nurse Shark.
Fish migrate for various reasons, but according to research the top reasons are reproduction and diet. Migrating fish can travel huge distances of thousands of kilometers. Some forage fish migrate in triangles, in the sense that they have three stops (they migrate from stop 1 to stop 2, stop 2 to stop 3, and stop 3 back to stop1), which are generally at the tips of a triangle shape.
Migrating fish can be divided into 2 types:
- Catadromous: These are the fish which are born in marine/saline water, but migrate to freshwater sources to spend the major part of their lives.
- Anadromous:These fish follow the exact opposite pattern – they are born in freshwater, but migrate to salt water bodies to spend most part of their adult lives. They return to freshwater bodies when they want to reproduce.
Few more examples in this category are Dolphin, Char, and Trout.
Two of the most popular “fish”, the jellyfish and starfish do not actually belong to the fish family, irrespective of what their names say!
Other Categories – Aquarium Fish
Aquarium fish are becoming exponentially popular by the day as more and more people want to keep aquariums in their homes. But it is not as easy as getting fish from the pet store and keeping them in a bowl. Aquarium fish are also further classified into three categories, based on the kind of environment they need to be able to survive and thrive. These three categories are described below.
Freshwater Aquarium Fish
Freshwater aquarium fish require very little maintenance once their basic needs are looked after. They need a minimum level of salinity to survive. Hence addition of salt is quite unnecessary, but there are varied opinions on the topic. The surface area should be decided based on how many fish are going to be kept in the aquarium so as to provide enough oxygenation to the water. It is necessary to provide some furnishings in the aquarium to make it interesting for the fish and also to stimulate them.
For Freshwater Aquarium Fish, temperature of the water should be maintained between 24ºC to 28ºC.
Marine Aquarium Fish
Marine aquarium fish have slightly stricter requirements in terms of the water pH, alkalinity, temperature, etc., and hence, the cost of maintaining marine fish species is more than what is required for freshwater species. All marine aquarium fish species require a pH level of 8.2 to 8.4 to be able to survive. Also, regular filtration is required and in case of marine aquariums, it is a complicated process. Usually these fish prefer natural sunlight or fluorescent light.
Marine fish require a temperature range of 10ºC to 24ºC, while tropical marine fish require a temperature range of 24ºC to 28ºC.
Brackish Aquarium Fish
Brackish fish species need a temperature range of 22ºC to 28ºC. Their main requirement is that salinity levels can be varied depending on which species are being kept, but all brackish fish species are well adapted to tolerate varying levels of salinity quite efficiently! One interesting thing to keep in mind is that brackish aquarium fish species tend to jump, and hence, the tank must be kept covered at all times to prevent them from jumping out of the tank! Varying the salinity on a regular basis will help in maintaining good health of the fish.
Other examples of Brackish Aquarium Fish are Sailfin Molly, Indian Glassy Fish, Orange Chromide, and American Flagfish.
Did You Know??
Although records are varied, the pre-historic fish, called Leedsichthys are believed to be the largest fish that ever lived. While some paleontologists say that it was only around 30-40 feet in length and weighed approximately 10 tons, there are others who argue that it was more than 70 feet in length and could weight as much as 50 tons! These humongous creatures had more than 40,000 teeth, but you’ll be surprised to know that their diet was mainly, and probably only, plankton. The closest in size to these giants, amongst the existing fish, are whale sharks, which are almost 65 feet in length and weight around 38-40 tons.
Other Categories – Food Fish
Fish are not only delicious to taste, they offer scores of health benefits which can do you a world of good in the long term! Consumption of fish on a regular basis can help in prevention of various types of cancers, cardio-vascular diseases, skin, and hair problems, and they also help in keeping the brain stimulated. But behold, because this is just the tip of the iceberg. There are many more reasons why a diet rich in fish is good for you, some of them being:
- Firstly, fish is one of the richest sources of omega-3 fatty acid which helps in lowering blood pressure, as well as maintaining a normal heart rate.
- Fish have a low content of fats and calories, while being a rich source of proteins.
- Fish has fatty acids, which aid in faster growth of hair, contribute to making hair stronger and shinier, and also aid in the production of collagen which helps in reducing wrinkles and thus delaying the appearance of old age.
- Fish are an amazing source of minerals and vitamins. They contain calcium, which helps in maintaining healthy teeth and bones, help in growth and metabolism, and also help in keeping the eyes healthy.
Again, these are just a few of the health benefits of consuming fish regularly. They help in so many ways, why not make them a regular in your diet? The fact that they are delicious to taste only adds to their appeal!
Shown below are a few of the most popular food fish.
Other examples of edible fish are Walleye, Sturgeon, Conger Eel, Monkfish..
Although it sounds incredulous, fish can drown! Fish need a certain amount of oxygen in the water that they are living in, and if they do not get the required amount of oxygen, they can drown!
Most of the fish included in this category have self-explanatory names, while some have developed weird physical traits, while still others have developed such excellent camouflaged bodies that you wouldn’t know they were fish if you weren’t told so! The fish in this category deserve a lot of credit for doing their best to come tops in the ‘survival of the fittest’, and you can see the lengths to which some of them have gone to, in terms of physical structure and adaptations, to help themselves. Take a look at some of these fascinating creatures!
Facts About Seahorses
Seahorses are fascinating creatures. Here are 4 reasons why…
1. They have excellent camouflage ability and can change their color to match their surroundings.
2. Seahorses are one of only two classes of fish that can swim vertically, the other being shrimpfish/razorfish.
3. Seahorses have eyes that can move independently of each other. So while one eye looks back, the other can look in the front or sideways!
4. In seahorses, it is the male who gets pregnant! The female deposits her eggs into the pouch of the male, where he fertilizes them, and carries them till they are ready to hatch. The male seahorse provides prolactin to the eggs, while the pouch regulates the temperature and salinity of water and also provides oxygen to the eggs.
Amongst all types of fish, freshwater fish seem to be the most vulnerable when it comes to being endangered. Overall, the IUCN Red List has listed more than 1000 species of fish which are on the verge of extinction. The reasons are, of course, water pollution, habitat destruction, overfishing, and sometimes introduction of species in different habitats than the ones that they are used to. Rigorous measures need to be undertaken to ensure that these listed species are soon out of the list, and safe and sound in their natural habitats.