Habitat 
Depth 
Group 
Role 
Average sizeWHAT DOES IT LOOK LIKE?
The “coins” of sea cactus (Halimeda tuna) flatten into discs or little paddles. They overlap and branch in one plane, joined by narrower areas.
Their colour is green, sometimes pale. Calcification, the deposition of calcareous mineral material within the segments, gives them stiffness and emphasises their appearance as separate little elements.
WHERE DOES IT LIVE?
Sea cactus attaches to rock and other firm surfaces, particularly shaded areas on steep underwater walls and in coralligenous habitats. It can persist where light levels are relatively low.
Records extend to roughly 75 metres, although the depth range differs between sites.
WHAT DOES IT EAT?
Its green tissues carry out photosynthesis, using sunlight to produce organic material from carbon dissolved in seawater. Mineral nutrients come from the surrounding water. Calcium carbonate deposited within the segments gives them much of their stiffness.
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