Symbiosis is defined as 'any type of close and long-term biological interaction between two different biological organisms.' There are several types of symbiosis in the natural world (and not all are reciprocal), so in honour of Valentine's Day, we've put together a list of our favourite mutualistic relationships in the big blue. These are the partnerships in which the creatures not only help each other survive, but they actually thrive, because they've found friendship across the species divide. If you love interesting behaviours and odd facts about underwater life, keep reading…

Masters of Camouflage: Decorator Crabs and Sea Sponges/Anemones

Adorned and bedazzled by all manner of sponges, corals and even anemones, decorator crabs appear to be a walking chunk of reef, going about each day with an entire ecosystem on their back. The crab becomes almost invisible via its incredibly convincing coat of camouflage, while the travelling reef on its back gains better exposure to feeding opportunities along the way. Not only that, but the living adornments offer more than just concealment for the crab; some species of anemone and coral actually provide a defence against predators, stinging and warding them off. As the crab grows, its shell does not, thus it must molt its carapace and form a new one as it expands in size. After this process, the crab will recycle the anemones, sponges and corals from its old shell, placing them safely on its new back to continue their life together.

We Glow Together: Bacteria and Bobtail Squid

Arguably the cutest creature in the ocean, the bobtail squid only grows to approximately three adorable centimetres at adulthood and has the incredible ability to not only change colour, but also to glow. While this ability is actually quite common among life in the ocean, the bobtail's vibrant luminescence comes from an extraordinary partnership with a minute bacteria that resides inside the squid called vibrio fischeri, and it reacts with the squid's bioluminescent organ to glow brighter than either could on their own.

Home Sweet Home: Clown Fish and Anemones

Possibly the most well-known underwater partnership, the anemone and its anemone fish (commonly called clown fish) are one of the prettiest and most entertaining relationships to encounter. The enthralling sway of the anemone's arms are pleasing enough, but add a shy collection of bright orange and white fish darting about within, and it becomes downright mesmerizing. The fish find safety inside the stinging tentacles, while the anemone benefits from the nutrients discarded by the clown fish. Immune to the anemone's stings, clown fish can live their days without fear of predator attacks, so long as they remain in the safety of the anemone's arms.

The Digger and the Lookout: Pistol Shrimp and Gobies

Pistol shrimp are delicate, nearly blind crustaceans that spend their days digging and maintaining their tunnel home while their goby counterparts keep a keen eye out for danger. This unlikely pair rely on one another, the shrimp establishing a hiding place for both, while the goby alerts the shrimp to any predators that come within striking distance, which gives them both time to dart down to the safety of their hideaway. At night, cuddled up together, these bottom-dwelling buddies bunk in their burrow on the sandy seabed to rest.

Hunting Buddies: Grouper and Moray Eels

The ultimate hunting partners, moray eels and grouper have worked out a clever strategy in chasing down prey that plays to both their strengths. Grouper will cruise the reef's edge stalking their prey, which, in an attempt to elude the predator, will inevitably seek refuge in the cracks and crevices of the reef. Ready and waiting, the moray eel can squeeze through the labyrinthine coral system either catching their own dinner, or forcing the fish back out into open water, where the grouper waits for its own meal.