Cuttle fish

The cuttle fish is small medium sized molluse that is found throughout the ocean waters of the world. In the same way as their squid and octopus relatives, cuttlefish have a large, elongated body with tentacles surrounding their mouths.

Cuttlefish are found in large numbers throughout the world's ocean waters from the warm, tropical shallows to the cold depths of the deep ocean. Cuttlefish are well known for the "flashing" colours that are displayed on their bodies during fighting and mating. In the same way as squid and octopuses, the cuttlefish also has an ink sack which ejects ink in order to fool oncoming predators.

There are 120 known species of cuttlefish found across the globe which vary in size from just 15 cm to the Australian giant cuttlefish which is often half a meter in length (not including its tentacles) and weighs more than 10kg.

The cuttle fish is a carnivorous animal that primarily preys on small crustaceans such as shrimp and crabs, but the cuttlefish also eats alot of fish. The cuttlefish uses its ability to change body colour to hide itself, before catching its prey with the sucker-pads on the end of its long tentacles which bring the prey into the sharp beak of the cuttlefish.

During the mating season, male cuttlefish change their body colours in order to successfully attract a female. Some male cuttlefish have even been known to make themselves look like female cuttlefish in order to trick the dominant males and stealing their mates. The female cuttlefish lays around 200 small eggs and sadly dies soon afterwards.

Crown of thorns starfish

The crown of thorns starfish (Acanthaster planci) is native to coral reefs in the Indo-Pacific region.

On healthy coral reefs, the coral-eating starfish plays an important role, as it tends to feed on the fastest growing corals such as staghorns and plate corals, allowing slower growing coral species to form colonies. This helps increase coral diversity.

However, outbreaks of this venomous invertebrate pose one of the most significant threats to the Great Barrier Reef.

According to research by the Australian Institute of marine science, coral cover on surveyed reefs has dedeclined by about 50 per cent over the past 30 years. Crown-of-thorns starfish were responsible for almost half of this decline.

The research estimates that if crown-of-thorns starfish predation had not occurred over the past three decades, there would have been a net increase in average coral cover.

Cyclic outbreaks of the crown-of-thorns starfish occur approximately every 17 years. There have been four documented outbreaks on the Great Barrier Reef since the 1960s, with the latest starting in 2010

Crown-of-thorns starfish spawn during the warmer months (around October to February), with large females capable of producing up to 65 million eggs over the spawning season.

Predators of adult crown-of-thorns starfish include the giant triton snail, the hump head Maori wrasse, starry puffer fish and titan trigger fish. Predators of the starfish in its younger life stages are less known.

Coral

It may shock you to know that the hole-filled rock like substance, known as coral, is actually an animal and a vital part of the marine world. Coral is a similar animal species to the sea anemone and almost appears to be half animal and half plant when you look into what coral does.

There are thought to be around 70,000 different species of coral that are found throughout the oceans of the world but are in abundance in the southern hemisphere due to the warm and tropical climates.

Coral species are generally divided into two subspecies, depending on how many tentacles the coral individual has. Those coral species with eight tentacles are known as Alcyonaria, which includes soft coral, sea fans and sea pens. Those coral species with more than eight tentacles are known as Zoantharia, which includes the coral species that are found in coral reefs.

The coral catches small fish and animals such as plankton using stinging cells on their tentacles, although these animals obtain most of their nutrients from algae. This means that most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60 m (200 ft). There are however a number of coral species that have adapted to inhabiting the oceans at depths of up to 3000 m.