Fishes
0 of 4 hands-on
01 Introduction
Fishes are the animals most divers come to see, and they are the most varied group of vertebrates. Their shapes, colours and habits are all answers to the question: how do I find food, avoid being eaten and raise the next generation?
Objectives
- Describe the main groups of fishes
- Explain how a fish breathes, swims and senses its surroundings
- Give examples of fish adaptations and behaviours that divers can see
02 Watch
Built to swim
5 min · Video in production
- 1
Show a fish in cross-section: gills, swim bladder, muscles, fins and the lateral line.
- 2
Show a school of fish turning in unison, with a predator approaching.
- 3
Show a cleaning station, where a small wrasse removes parasites from a larger fish.
- 4
Show a shark and a ray gliding past, with skeletons made of cartilage.
03 Learn
Three main groups
| Group | Skeleton | Examples |
|---|---|---|
| Bony fishes | Bone. The great majority of fish species | Tuna, groupers, parrotfish, clownfish, seahorses |
| Cartilaginous fishes | Cartilage, a flexible tissue (not bone) | Sharks, rays, skates |
| Jawless fishes | Cartilage, with no jaws. Few species | Hagfish, lampreys |
How a fish works
- Gills: water is taken in through the mouth and passed over feathery gills, where oxygen passes into the blood. Water and blood flow in opposite directions, which captures more oxygen.
- Fins: pectoral and pelvic fins steer and brake, the dorsal and anal fins stabilise and the tail fin (caudal) propels.
- Swim bladder: most bony fish have a gas-filled organ that adjusts buoyancy. It works on the same Boyle's Law principles as a diver's BCD.
- Lateral line: a line of sensors along the side that detects vibrations and pressure changes in the water.
- Scales and mucus: protection and a smooth surface that reduces drag.
Predict first
How is a fish's swim bladder like a diver's BCD?
Sharks and rays
Sharks and rays have skeletons of cartilage and no swim bladder. Many sharks must keep moving to push water over their gills, although others can rest and pump water. Sharks have a keen sense of smell and detect electric fields with special pores near their snouts. Rays are flattened relatives, with wing-like fins, and range from bottom-dwelling stingrays to giant filter-feeding mantas.
Shapes and strategies
| Shape or behaviour | What it does | Example |
|---|---|---|
| Torpedo shape | Fast swimming in open water | Tuna, barracuda |
| Flattened body | Lives on the bottom and hides | Flatfish, rays |
| Camouflage and counter-shading | Dark on top and light below, to be hidden from predators above and below | Many open-water fish |
| Schooling | Safety in numbers, confuses predators | Sardines, jacks |
| Cleaning stations | Small cleaners remove parasites from bigger fish | Cleaner wrasse, cleaner shrimp |
| Changing sex | Some fish change sex in their lives | Clownfish (male to female), many wrasses and parrotfish (female to male) |
Reading fish behaviour
Watching what a fish is doing tells you what it needs. Fish gathered at a cleaning station are waiting to be cleaned. A fish that stays close to a crevice is protecting a nest or a hiding place. A shoal that suddenly tightens or races off has probably seen a predator, and you may see one yourself if you look around.
Take a guess first
A tuna, a fast open-water predator, can swim at a cruising speed of roughly 5 body lengths per second. A 1 m tuna, how far does it travel in 10 seconds?
04 Explore
Try it yourself
Match each fish feature with its job.
Match each feature with its job.
- Gills
- Swim bladder
- Lateral line
- Caudal (tail) fin
- Cartilage skeleton
05 Apply
Scenario
At a reef, a diver notices a larger fish hovering with its mouth open while a tiny striped fish nips at its skin and gills. The diver wonders if the little fish is attacking it.
What is the best explanation?
06 Check
07 Summary
Key points
- Fishes are bony, cartilaginous or jawless. Most are bony.
- Gills extract oxygen, fins steer and propel, and the swim bladder adjusts buoyancy.
- The lateral line senses water movement, and shape and behaviour show what a fish does.
- Sharks and rays are important, shy of divers, and should be given space.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.