Connected creatures of the Great Barrier Reef
Issued: 14 Jun

Spanning an area bigger than Victoria and Tasmania combined, the Great Barrier Reef is not only recognised as a World Heritage Area for its outstanding values, but it is also Australia’s addition to the seven natural wonders of the world. Covering 348,000 square kilometres this phenomenal seascape hosts the world’s largest coral reef ecosystem.

We’ve made a list of five stunning critters that call the Great Barrier Reef and its surrounding national and marine parks home for you to dive into.

Nudibranch

When you think about the staggering 6,000 varieties of molluscs the reef supports, you probably don’t think of colourful little slug-like creatures, but that’s exactly what nudibranchs can look like. Coming in a sensational array of shapes and shades, these colourful critters are what’s known as benthic animals, meaning they are found along the sea floor.

They feed predominantly on other invertebrates—particularly sea anemones—though some have developed specialised tastes for unlikely delicacies such as the bluebottle jellyfish. While some nudibranchs can reach lengths of over 60cm, others can be as small as 4mm, so you need to keep your eyes peeled if you’re hoping to catch a peek of their unique appearance.

Clownfish

Not to be outdone in nomination for dad-of-the-year, the small but mighty seahorse takes things a step further. Mating for life, the females use an ovipositor (a tube-like organ) to deposit the eggs within the male’s brood pouch. As the eggs develop, he will regulate the chemistry of the fluid within the pouch and slowly acclimatise the growing young to the salt water their parents dwell in. Depending on the species, this gestation period can take anywhere between 10 days and six weeks.

After the young have hatched, the father pushes them out of the pouch with a convulsing motion. Once born, the fry (baby seahorses) are left to fend for themselves, while the male is ready for a new brood of eggs almost immediately. Assuming they manage to avoid natural predators like crabs and larger fish, the fry will eventually reach a full size anywhere between 2cm and 35cm.

Box jellyfish

Box jellyfish are among the most infamous residents of the Great Barrier Reef. Recognised for their nearly transparent, box-shaped bell, they can grow up to 60 tentacles that stretch to 3m in length. Each tentacle is lined with thousands of microscopic cnidocytes, which are specialised cells that deliver potent venom capable of subduing prey—or causing serious harm to humans. While they are often feared for their painful and potentially lethal stings, box jellyfish play a crucial role as apex predators in their ecological niche. Preying on small fish and crustaceans, they are capable of hunting at speeds up to four knots.

Found most commonly during the summer months, these translucent travellers are incredibly necessary contributors to life within the reef as both predator and prey. Hunted by several species, including turtles - which are immune to the jellyfish's sting – they play a crucial role in the careful balance of the reef.

Green turtle

We couldn’t talk about the wonders of the reef without including green sea turtles. These gentle marine reptiles are omnivores as juveniles, feeding on whatever they can find, including jellyfish. As adults they’re herbivores, primarily grazing on seagrasses and algae. Their feeding habits are crucial for maintaining healthy seagrass beds, which are important coastal habitats providing nursery grounds for fish and helping to stabilise the ocean floor. By feeding on the seagrass, green sea turtles crop the grass ensuring that it stays healthy. Much like mowing a lawn encourages new growth.

Throughout their life cycle, green sea turtles utilise a variety of habitats within the Great Barrier Reef region, from the open sea as juveniles, to the shallow reef waters as adults. As the world’s largest known rookery for green turtles, Raine Island plays a key role in their conservation and protection. This variety in habitats highlights the interconnectedness of marine ecosystems; what happens in one part of the reef can have significant impacts elsewhere. Green turtles themselves are a source of biodiversity, providing habitat for a range of epibiotic species, such as barnacles and algae, which live on their shells. By maintaining a balance in the ecosystems they inhabit, and participating in complex food webs, green turtles are indispensable contributors to the biodiversity of the Great Barrier Reef. Their conservation is critical, not just for the preservation of the species itself, but for the continued health and resilience of the entire reef ecosystem.

From algae to zebra lionfish, the reef relies on the creatures great and small to support its vast ecosystem. Be sure to check Caring for the Reef before you visit to find out what you can do to keep this international treasure around for years to come.