Life in coral reefs is complex, intricate and sometimes looks as colorfully cryptic as a Pollock. Its interactions are so diverse that they are largely unknown and misunderstood. Surrounded by water depleted in nutrients (this is called the Darwin’s Paradox*), cooperation and symbiosis are the main conditions for survival here.

(*Glossary at the end of the page)

No coral, no zooxanthellae – no zooxanthellae, no coral. This is an essential relationship called symbiosis, challenged by recent changes in water conditions.
Zooxanthellae is a unicellular alga hosted by coral polyps, feeding on their waste, providing them food and painting the reef with multiple bright colors. Ocean temperature rise causes polyps to expel zooxanthellae. They first lose their colors, like an overexposed photograph. This is what is called bleaching. If their nourishing algae does not settle back, polyps gradually die. Dead coral then degrades to its hollow calcium carbonate skeleton. But the reef needs to keep on growing to face erosion.

If symbiosis disappears, so will the barrier.

A shrimp and a goby fish. A blind digger and an alert watcher. The perfect match. How did they find each other? This remain a mystery. But they did and their association is a perfect example of mutualistic symbiosis* for survival in the reef.
Most of the time, eyes can only catch a glimpse of a fish tail quickly disappearing in the sand. Hidden surprises are rewarded to those with patience. Down there, a pistol shrimp is settled in its hole, hunting and sheltering. There is nothing to stare at outside for this sightless animal. But taking care of its burrow means bringing gravel and sand back to the surface. There comes the watchman. A goby fish, settled at the entrance of the tunnel and keeping an eye on predators. When the shrimp comes out, it gently touches the goby with one of its antennae. As soon as the goby moves, the shrimp feels the danger and goes back to its cave, accompanied by the fish.

A shrimp and a goby fish, the unexpected perfect match.

What is to be said about clownfishes ?
Google search: Clownfishes and anemone. Result: Symbiosis. Covered with a mucous layer and therefore acclimatized to the anemone venomous tentacles, clownfish cleans and defends its home while the anemone provides nesting site, food and protection for the fish. But there is more. Like other reef fishes, the clownfish has the ability to socially change sex. Between the tentacles of the anemone, the community is composed by only one male-female dominant pair accompanied by smaller male individuals. When the female dies, the dominant male turns into a female and one of the juvenile quickly grows into a mature male, reconstituting the dominant pair.

Clownfish is a born male transgender.

Sponges are thought to be the oldest evidence of animal life on earth. Simple organism witnessing sea life evolution for 640 millions years. 
Xestospongia testudinaria, building the reef like ramshackle chimney soaring upwards to the sky. They silently witness ancient times and structure communities. In their nooks, all kinds of reef inhabitants find a commensalist* shelter. Snapper, lion fishes, moray eel, juvenile groupers, rubberlips,… And their body is even more crawling with life. Into their inner channels used for filter feeding, are hidden colonies of symbiotic bacteria that can constitute up to 60% of the sponge biomass. In return of a home, those symbionts play roles in digestion, waste removal and chemical defense for their host.

In the reef, no structure is kept empty.

Glossary :

Darwin’s Paradox : how is that coral reefs, full of life and apparently well nourished, are able to exist whereas most of the sea floor around is only a vast desert ?
Symbiosis is a close living relationship between organisms from different species, usually with benefits to one or both of the individuals involved. Symbioses may be ‘obligate’, in which case the relationship between the two species is so interdependent, that each of the organisms is unable to survive without the other, or ‘facultative’, in which the two species engage in a symbiotic partnership through choice, and can survive individually.
Mutualisms are a form of symbiosis in which both symbiotic partners benefit from the interaction, often resulting in a significant fitness gain for either one or both parties.
Commensalism is a symbiosis in which one organism benefits from, and is often completely dependent on, the other for food, shelter, or locomotion, with no obvious effect on the host.

Definitions from https://biologydictionary.net/symbiosis/

All the pictures have been taken in Sainte Marie island, Madagascar, with the collaboration of Bora Dive and Research center