crust algae painting intertidal rock in vivid green and grey
- Size
- Width: 10–50 cm
- Lifespan
- 2–5 years
- Diet
- Grows on rocky shores in intertidal and shallow subtidal zones. Requires clean water, stable rock surfaces, and good water flow. Tolerates wave action, sun exposure, and temperature fluctuations. Forms a thin, flat, reddish-brown crust that paints the rocks like a layer of dried paint.
- Habitat
- Grows on rocky shores throughout New Zealand, from the Three Kings Islands down to Stewart Island, in the intertidal and shallow subtidal zones. A creature of the hard surfaces, the smooth rocks, the places where other seaweeds struggle to hold on. Forms a thin, flat, reddish-brown crust that paints the rocks like a layer of dried paint. Almost impossible to peel off.
- Range
- Found throughout the North and South Islands on rocky shores in intertidal and shallow subtidal zones. Most common on exposed and semi-exposed coastlines. Also found in temperate seas worldwide.
- Endemism
- Native
- Main Threats
- None significant. This species is common and widespread on rocky shores. Localised threats include coastal development, pollution, sedimentation from land clearance, and climate change affecting water temperature and clarity.
- Population
- Not Threatened. Crust algae is common on rocky shores throughout New Zealand, often forming extensive red crusts on intertidal rocks. It is a widespread species found in temperate seas around the world.
- Conservation Status
- Not Threatened
The one that does not look like seaweed has no fronds, no branches, no floats. It is a thin, flat crust, pressed tight against the rock, coloured a deep reddish-brown. From a distance, it looks like a stain or a patch of discolouration. Up close, it is a living organism, a layer of cells that has taken over the rock, claiming it for its own.
What makes it special is the adhesion. The crust algae grips the rock with a strength that seems impossible. You can scrape at it with your fingernail and it will not come off. You can scrub it with a brush and it will hold on. It is the glue of the intertidal zone, the thing that will not let go, the living paint that refuses to be removed.
The crust algae grows slowly, adding a thin layer of cells each year. A patch of crust algae may be decades old, a living record of the rock's history. It is a slow, patient organism, content to spread a millimetre at a time, to claim the rock inch by inch. Under the microscope, the crust is made of countless tiny cells packed together, each one contributing to the strength of the whole.
Biologically, the crust algae is a red alga. It reproduces by releasing spores from specialised structures embedded in the crust. The spores are released into the water, carried by the currents, and settle on nearby rocks to grow into new crusts. It also reproduces asexually, fragments of the crust breaking off and growing elsewhere.
The crust algae provides a surface for other organisms. Small snails graze on its surface. Barnacles and tubeworms settle on top of it. It is the foundation of the intertidal community, the base layer on which other things grow. Without the crust algae, the rock would be bare, a sterile surface in a world of life.
To find crust algae is to find the red stain on the rock. It is not beautiful. It is not dramatic. It is just there, covering the stone, holding on, refusing to be peeled off. It is the flat one, the patient one, the one that proves that you do not need to stand tall to survive, that the most successful strategy is sometimes just to hold on and never let go.