Body shape and fins
A fish has a streamlined body: narrow at both ends and wide in the middle. Water slides past it with little push-back. A slippery skin with scales helps too.
- Tail fin: pushes the fish forward.
- Pectoral and pelvic fins (side and belly fins): steer, turn, brake.
- Dorsal fin (on the back): keeps the fish upright.
A line of sense cells along each side, the lateral line, feels water movement.
How fish breathe
Water has a little oxygen dissolved in it. A fish takes water in through its mouth and sends it over its gills. Gills are red because they are full of tiny blood vessels. Oxygen goes from water into the blood. Carbon dioxide goes the other way. The water leaves through the gill slits. Bony fish have a gill cover (operculum) that protects the gills and helps pump water.
A fish out of water cannot breathe because its gill sheets collapse and stick together.
Swim bladder and floating
Bony fish have a gas-filled bag called the swim bladder. If the fish adds gas to it, the fish becomes lighter overall and rises. If the fish lets gas out, it becomes heavier and sinks. This lets it stay at any depth without much effort. Sharks have no swim bladder, so they must keep swimming or sink, but they have an oily liver that helps them float.
Cartilaginous and bony fish
| Cartilaginous fish | Bony fish | |
|---|---|---|
| Skeleton | Soft cartilage | Hard bone |
| Examples | Shark, ray | Rohu, catla, tuna, seahorse |
| Gill cover | No, open gill slits | Yes |
| Swim bladder | No | Yes |
| Scales | Tooth-like, rough | Thin, flat, overlapping |
| Fertilisation | Inside the body | Mostly outside |
Both groups are cold-blooded (body heat follows the water) and have a two-chambered heart.
Fishing and fish farming
People catch fish with nets, lines and traps. The hole size (mesh) matters. A large mesh lets small young fish escape so they can grow and breed. Catching too many fish, or young fish, is called overfishing and it lowers the number of fish.
Fish farming (aquaculture) grows fish like rohu and catla in ponds with clean water and feed, so there is less pressure on wild fish. Fish give protein, vitamins and omega-3 oils.
Try it
In the 3D: on the swim bladder step, slide Gas to the top and bottom. Where does the fish go? On the net step, slide Mesh until the small fish escape.
At home: look at a whole fish at a market or a picture. Find the tail, the back fin, the gill cover and the scales. Name each job.
Key formulas and definitions
- Fish = streamlined body + fins + gills + scales + two-chambered heart + cold-blooded
- More gas in swim bladder = rise. Less gas = sink.
- Cartilaginous: shark, ray (cartilage skeleton). Bony: rohu, catla (bone skeleton).
- Bigger net mesh = young fish escape = more fish in future
Worked examples
1. A fish swims up from the bottom of a pond without moving its tail. How?
It adds gas to its swim bladder. This makes it lighter overall, so it floats upward.
2. Name one difference and one similarity between a shark and a rohu.
Difference: shark has a cartilage skeleton while rohu has a bone skeleton (also: rohu has a gill cover and swim bladder). Similarity: both breathe with gills and are cold-blooded vertebrates.
3. A farmer stocks 2,000 rohu fingerlings in a pond. 10% die. How many survive?
10% of 2000 = 200 die. 2000 - 200 = 1800 survive.
Common mistakes
- Saying sharks have bone. Their skeleton is cartilage.
- Thinking fish drink oxygen gas from air. Gills take oxygen dissolved in water.
- Saying all fish have a swim bladder. Sharks and rays do not.
- Calling fish warm-blooded. They are cold-blooded.