The ocean covers almost three-quarters of the Earth’s surface and is one of the most important animal habitats. It is home to around 230,000 recorded species, and many more that are still to be discovered and named.
Biologists estimate that the total number of species living in the world’s oceans may be over 1,000,000!
Crabs are members of the animal group Crustacea. Other crustaceans include lobsters, crayfish, krill and barnacles. Most crustaceans live in the sea, but some – such as woodlice – have adapted to life on land.
A typical crab has five pairs of limbs. The front pair is usually modified into pincers, while the other four pairs are used for movement. Most crabs walk sideways rather than forwards.
Crabs are protected by a hard exoskeleton. As the crab grows it periodically molts, removing its entire body – including its legs – from its old shell. Its new shell is soft and expandable, giving the crab enough room to continue to grow.
Dolphins are marine mammals. Although their ancestors were land animals, dolphins are entirely aquatic. Their bodies are streamlined and fish-like, although, being mammals, dolphins still have to come up to the surface to breathe.
Most dolphins are members of the family Delphinidae, a group otherwise known as the oceanic dolphins. With 30 species, Delphinidae is the largest dolphin family.
As the name suggests, oceanic dolphins are ocean animals. Members of the other three dolphin families live in freshwater or brackish (a mixture of fresh and salt water) habitats.
The brains of dolphins are among the largest – in relation to body size – of any animal. Dolphins are highly-intelligent animals, capable of teamwork, teaching / learning, and problem solving.
Jellyfish, like sea anemones and corals, are members of the invertebrate group Cnidaria.
Jellyfish in the free-swimming, adult stage of their development have round, jelly-like bodies (known as bells) with several trailing tentacles. A jellyfish in this stage of development is known as a medusa.
The tentacles of a jellyfish are equipped with stinging cells.
Although jellyfish stings are painful, they are usually harmless to humans. However, in some cases contact can be fatal. Box jellyfish (a type of jellyfish identified by their square-shaped bells) are the most dangerous to humans.
Jellyfish have a complex life-cycle. They begin life as larvae. At this point they are free-swimming. The larvae then attaches itself to a suitable surface and becomes a polyp.
The polyp then begins to produce buds. The buds float off and eventually become medusae.
Sea turtles are a group of seven reptiles that, unlike other Testudines (turtles) have adapted to a life spent mainly in the sea. In fact, once a male sea turtle enters the ocean as a hatchling, it will never again walk on land.
Female sea turtles will periodically return to land – often to the very beach on which they hatched –in order to lay their eggs.
The limbs of sea turtles have evolved into flippers. Although able to spend long periods of time under the water, sea turtles always have to return to the surface in order to breathe.
Sadly, sea turtles occasionally get caught up in the nets of fishing boats and drown. Sea turtles are also vulnerable to pollution, mistakenly eating pieces of plastic they find floating in the sea.
This, combined with the loss of many of their traditional nesting areas, means that sea turtles are in great danger.
Three sea turtle species are rated Vulnerable, one is rated Endangered, and two are rated Critically Endangered.
Seahorses are small fish, named after the land mammals that they resemble. There are 45 species of seahorse.
Seahorses are not powerful swimmers. They spend most of the time holding onto marine vegetation with their prehensile tails. (Prehensile means ‘able to grip’.)
Male seahorses have pouches into which the female deposits eggs. The couple’s eggs develop inside the pouch and the young emerge as tiny seahorses.
Sharks are among the best known – and most feared – of all ocean predators. There are over 500 species of shark. Although not all are dangerous, species such as the great white shark, tiger shark and hammerhead shark are all apex predators; top of the food chain wherever they are found.
Sharks are fast, silent, and most species are armed with a mouthful of sharp teeth that are replaced continuously throughout the animal’s lifetime.
All sharks have special electroreceptive organs in their heads. Using these, a shark can sense the electrical fields produced by other ocean animals. This ‘sixth sense’ is used to locate prey.
Sharks are cartilaginous fish. Whereas the skeletons of bony fish (the other main fish group) are made of bone, those of cartilaginous fish are made of a flexible substance called cartilage.
The world’s largest fish – the whale shark – and the second-largest fish – the basking shark – are both sharks. Unlike their smaller, but deadlier cousins, these two giants are filter feeders. Their diet consists of small marine organisms such as krill.
Starfish / Sea Star
Starfish (also known as sea stars) are invertebrate ocean animals in the order Asteroidea. (The name comes from the Greek for ‘star-like’).
Starfish live on the sea bed, where they feed on small mollusks and other marine invertebrates.
Most starfish have five arms, but some species can have over 50. If a starfish loses one or more of its arms, it can re-grow them!