During the Reef Renewal course, Lars raised an eyebrow. ‘If,’ he said, ‘you plant out two different strains of coral together, they’ll often look exactly the same. But they won’t fuse together. They just won’t.’

I stared at him, chin in my hands. Huh. Growing coral really seems like a mix between embryology and gardening.

Why don’t they join together? In coral speak, they look exactly the same. And no offence but they’re coral. Not exactly performing brain ops.

Well, apparently it does matter to coral. A huge football field of staghorn might have a single genotype. What happens if another genotype comes up from next door? And says, ‘let’s make this official.’

No way. Corals have standards. The first piece of coral’s immune system goes all rejection. Considering that coral’s survival is under a serious, global threat at the moment, you’d think the coral might be a little less picky about who they hung out with. But silly me. I’m just at the beginning of learning the rules.

There was another thing Lars said which grabbed my attention. I’d never realised when coral gets stressed – as it does with warming sea temperatures – it expels the algae which live inside it. I knew algal desertion causes corals to bleach. I just always thought the algae left of their own accord. I imagined them getting too hot and performing a fan-waving mass exodus. Saying, ‘Let’s go see if the brain coral’s got some ice cubes.’ Anthropomorphism is slightly addictive. Especially as it was so frowned upon at Uni.

When you think about it though – by letting the algae live inside them in the first place, corals must be specifically not rejecting the algae. Which is strange when you compare this to the way they treat other stony corals. You’d think they’d consider joining calcium carbonate-building forces, when they allow a whole other organism to live inside them.

This is the same animal which, on the outside, looks and behaves so simply. The whole of Barbados’s handsome Parliament building is made out of coral (albeit dead coral). Whole islands are made out of coral. Tropical beaches are made out of coral. But this behaviour, well, this doesn’t seem simple at all.

Corals don’t mix genotypes and fuse together unless they are from exactly the same strain. But they’ll let algae live inside of them in a friends-with-benefits arrangement, unless they get too hot. Then they use their rejection process and expel the algae. It often causes the coral itself to die. The algae photosynthesises up to 90% of the coral’s food supply.

I don’t know what’s going on. Lu and I just visit the corals – to garden. Lars on the other hand. Lars must know more.