Marine reptiles and mammals
0 of 4 hands-on
01 Introduction
Turtles, sea snakes, dolphins, whales and seals all breathe air, yet they live in the sea. Because they share our need to surface, they offer a useful lesson in how bodies cope with pressure and cold. They are also some of the most memorable animals a diver can meet.
Objectives
- Name the main groups of marine reptiles and marine mammals
- Explain how air-breathing animals are adapted to life in the sea
- Describe how to behave around turtles, dolphins, whales and other large animals
02 Watch
Breathing air in the sea
4 min · Video in production
- 1
A green turtle rising to the surface for a breath, then settling to graze on seagrass.
- 2
A humpback whale and calf, with a diver at a respectful distance.
- 3
A sea lion colony playing around divers over a kelp bed.
- 4
A graphic showing the dive response in a seal: slow heart rate, blood directed to the brain and heart.
03 Learn
Reptiles of the sea
| Animal | Key facts |
|---|---|
| Sea turtles | Seven species. They breathe air, can rest underwater for long periods and return to land beaches to lay eggs. Most species are threatened. |
| Sea snakes | Air-breathing and venomous, but usually placid. They have paddle-like tails and can stay down for long periods. |
| Marine iguana | Found only in the Galapagos. It feeds on algae and basks on rocks to warm up. |
| Saltwater crocodile | A large predator of estuaries and coasts in parts of Asia and Australasia. Not a diving animal: heed local warnings. |
Turtles lay their eggs on beaches, and the temperature of the sand affects whether the hatchlings are male or female. Young turtles face many hazards and few survive to adulthood. Plastic bags look like jellyfish to a turtle, and entanglement in nets and lines is a major cause of death.
Mammals of the sea
| Group | Examples | Notes |
|---|---|---|
| Cetaceans | Whales, dolphins, porpoises | Spend their whole lives in the water. Toothed whales and dolphins use echolocation. Baleen whales filter plankton. |
| Pinnipeds | Seals, sea lions, walrus | Feed in the sea but breed and rest on land or ice. |
| Sirenians | Dugongs, manatees | Slow, gentle herbivores that graze seagrass. Threatened by boat strikes and habitat loss. |
| Others | Sea otters, polar bears | Sea otters have the densest fur of any animal and need it to stay warm. |
All mammals breathe air, give birth to live young and feed their young with milk, and most are warm-blooded. Marine mammals keep warm with a layer of blubber, or in the case of sea otters, very dense fur.
Built for breath-holding
Marine air-breathers hold their breath efficiently. They store a lot of oxygen in their muscles and blood, slow their heart rate and send blood mainly to the brain and heart. This is the mammalian dive response, and humans have a weaker version of it, which is why cold water on the face slows your heart rate. Their lungs can collapse safely under pressure, so they largely avoid the problems divers face with gas under pressure.
Predict first
Why can a seal dive deep without decompression sickness, when a diver breathing from a cylinder cannot?
Echolocation and song
Dolphins and toothed whales send out clicks and listen to the echoes to build a picture of the world around them, even in murky water. Whales communicate over long distances with calls and songs. Noise from ships, sonar and construction can interfere with these abilities.
Meeting large animals
- Let the animal decide. Never chase, surround, corner or swim between a mother and her calf.
- Do not touch, ride or feed turtles, dolphins, whales or seals.
- Keep a respectful distance, and follow local rules. Some places set minimum distances and limits on time spent with an animal.
- Control your buoyancy so you do not crash down onto resting turtles or sleeping animals.
- Do not use a flash or lights close to animals, and back away if they show stress, such as sudden tail slaps or a change of direction.
- Never block a turtle's route to the surface. It needs to breathe.
Take a guess first
A green turtle resting on the bottom surfaces to breathe about every 30 minutes. How many breaths might it take during a 3-hour rest?
04 Explore
Try it yourself
Match each animal with its group.
Match each animal with its group.
- Green turtle
- Bottlenose dolphin
- Sea lion
- Dugong
- Banded sea krait
05 Apply
Scenario
A diver is hovering at 12 m when a turtle swims slowly past. Another diver in the group kicks up after it, hoping to hold onto its shell for a photograph. The turtle begins to flap hard and heads for the surface.
What is the best action for the first diver?
06 Check
07 Summary
Key points
- Marine reptiles include turtles and sea snakes. Marine mammals include cetaceans, pinnipeds and sirenians.
- All breathe air and have adaptations such as blubber, oxygen storage and the dive response.
- Noise, plastic, entanglement and boat strikes are major threats.
- Never chase, touch, ride or feed large marine animals, and let them decide.
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.