When you see different colours in a single Paragoniastrea australensis colony – say, a soft green next to a deep brown or pale grey – it might look like one coral with a fancy paint job. But often, it’s not one coral at all. It’s two (or more) colonies that fused together early in life.
Coral larvae settle on the reef as tiny individuals, often near each other. If they land close enough, and they’re not too genetically different, they can merge and grow as one. This fusion creates what looks like a single coral, but with distinct colour zones that reveal their separate origins. It’s a natural form of cooperation – each section maintaining its own identity.
These colour differences come down to a few things. Most obviously, the tiny symbiotic algae (zooxanthellae) living inside the coral tissue can vary between colonies. Different strains of algae can produce different hues, especially under changing light or nutrient conditions. On top of that, the coral animal itself produces pigments, which add another layer of variation. So even if two colonies fuse, they might still ‘wear’ different colours.
Sometimes the boundary between two colonies is smooth, almost imperceptible. Other times, there’s a sharp seam or ridge – like a join line in a sculpture. This might be because the colonies are only partially compatible, or they’ve simply grown differently over time. Either way, they’ve worked out a kind of peaceful cohabitation.
It’s worth noting that not all corals are this amicable. Some will compete for space with their neighbours, using stinging tentacles or chemical warfare to keep rivals at bay.
From a reef-watching perspective, these patchwork corals are fascinating – a multi-coloured coral mosaic, grown from individual lives, now joined together.
