Fish
Fish are cold-blooded, aquatic vertebrates. They have been on Earth for over 500 million years — longer than any other group of vertebrates. There are more than 30 000 species of fish, making them the largest group of vertebrates.
Key features of fish:
- Gills for breathing dissolved oxygen from water
- Scales covering the body
- Fins for swimming and steering
- Cold-blooded (body temperature matches the surrounding water)
- Most lay eggs in water
Fish are divided into three main groups: jawless fish (e.g. lampreys), cartilaginous fish (e.g. sharks, rays — their skeleton is made of cartilage, not bone), and bony fish (e.g. tuna, salmon, goldfish — the most common group, with a true bone skeleton).
The largest fish in the world is the whale shark, which can reach 12 metres long — yet it feeds only on tiny plankton, not large prey. The smallest fish is the Paedocypris from Sumatra, less than 8 mm long.
Amphibians
Amphibians are cold-blooded vertebrates that live part of their life in water and part on land. The word "amphibian" comes from the Greek meaning "double life". There are about 8 000 known species.
Key features of amphibians:
- Moist, smooth skin without scales — they can absorb oxygen and water through it
- Breathe with lungs as adults; larvae breathe with gills
- Cold-blooded
- Lay jelly-coated eggs in water (eggs have no shell)
- Go through metamorphosis — changing shape dramatically as they develop
The three main groups of amphibians are:
- Frogs and toads — most common; powerful hind legs for jumping
- Salamanders and newts — four short legs and a long tail, often found in damp woodland
- Caecilians — legless, worm-like; live underground in tropical regions
How to tell a frog from a toad: frogs have smooth, moist skin and longer back legs for jumping; toads have dry, bumpy skin and shorter legs for walking. Both are amphibians.
Comparing Fish and Amphibians
| Feature | Fish | Amphibians |
|---|---|---|
| Breathing | Gills (whole life) | Gills (young) → Lungs (adult) |
| Skin | Scales | Smooth, moist |
| Eggs | In water, no jelly coat | In water, jelly-coated |
| Habitat | Water only | Water and land |
| Limbs | Fins | Four legs (adults) |
Real world: Fish in a fish tank
Pet goldfish and tropical fish in aquariums breathe through gills — you can see them opening and closing their mouths to push water over the gills. The fins help them steer and balance. Goldfish are bony fish, sharing their basic plan with salmon and cod at the fish counter.
Real world: Frogs in garden ponds
Garden ponds often fill with frog spawn in early spring — masses of jelly-coated eggs laid in still water. Tadpoles hatch, grow gills, and swim like fish; over weeks they develop legs, lose their gills, and become air-breathing adult frogs that can hop onto land. This is the amphibian double life you can watch in your own garden.
Real world: Salmon life cycle
Atlantic salmon hatch in freshwater rivers, swim out to the ocean to grow, and return years later to the same river to spawn. They are bony fish that spend their whole life in water, breathing through gills — a stark contrast to amphibians that move from water to land as they mature.
Where do you see this in real life? Visit a garden pond in spring to see frog spawn and tadpoles showing the amphibian life cycle in action. In a supermarket, compare fresh fish (gills, scales, fins) with frogs' legs sold in some countries — spot the differences between the two vertebrate groups.