Is there such a thing as a peaceful reef life? No, not by a long chalk. Let me explain.
In an earlier post – War of the coral worlds! – wrote about corals jostling for space. This is the same story, but with different combatants: bubble-tip sea anemones (Entacmaea quadricolor) and Montipora corals.
Both belong to the phylum Cnidaria, and both come armed. Corals are genuinely sessile – they grow where they’ve settled. Bubble-tips are usually anchored in one spot too, but they can move if conditions turn against them. When they don’t move, it’s because they’re holding a patch worth keeping.
Reef real estate is finite, which makes the best nooks and ledges valuable. Neither anemones nor corals are passive tenants. Their ‘right’ to a spot is negotiated at the boundary – day after day, year after year.
Below are three photographs taken at different times of the same anemone–coral formation.
16 August 2020: A bubble-tip anemone sits in a crater surrounded by Montipora. On a reef, space is currency – and both sides treat the border as a live edge, not a backdrop.
7 November 2020: Same anemone, same coral, same standoff. The anemone holds the centre by making contact costly (stinging cells and venom); the coral holds the perimeter by building a raised rim – growth and calcification as a slow-motion barricade.
11 January 2026: Six years on, the line still holds, so this is a straight cnidarian-versus-cnidarian boundary: anemone chemistry and reach versus coral growth and persistence.
Bubble-tips defend themselves with nematocysts and venom – not just for feeding, but for deterring anything that presses too close. Corals have their own ways of fighting back, which vary by species and circumstance. I can’t tell you exactly which mechanisms are in play here, but the story is visible: the anemone holds the crater, and the surrounding coral maintains a thickened rim – a slow-motion wall that limits where the anemone can spread.
I’ll keep returning to this spot. It’s easy to miss what’s happening underwater when it happens in coral time as this border holds, shifts, and is continually tested.
Below are a few more examples of this underwater tension at play.
