Horn coral

Underwater you recognise the horn coral (Hydnophora microconos) by its surface at first glance: the cushion-shaped or encrusting colony is covered in tiny, regular cones – so-called hydnophores – as if the whole thing had been textured with a fine rasp. It is most often pale green, cream or grey-green, and up close it looks almost velvety. Those cones are not the polyp cups: they form where the walls of neighbouring corallites fuse together, while the polyps sit in the valleys around the cones.

Like every reef-building hard coral, the horn coral lives in partnership with zooxanthellae: the single-celled algae housed in its tissues produce most of its energy through photosynthesis, receiving shelter and nutrients in return. The colony builds its limestone skeleton slowly, a few millimetres to centimetres a year, and reproduces by mass spawning: on summer nights it releases bundles of gametes into the water, where fertilisation takes place and the larvae settle on fresh reef surface. Hydnophora is also notoriously aggressive: at night it deploys long stinging sweeper tentacles to knock back neighbouring corals that grow too close, winning itself space on the crowded reef.

Around Hurghada you will find it on sunlit reef tops and upper slopes at roughly 1–15 metres – the coral gardens of the Giftun Islands and the El Fanadir plateau are good examples. It is easy to observe while snorkeling too, but as with every coral: never touch it and never stand on it. The coned surface is fragile, and a single fin kick can destroy decades of growth – hover above it with neutral buoyancy and the reef will still be there for you next time.