paints the hard intertidal rocks red

Size
Width: 10–50 cm
Lifespan
2–5 years
Diet
Photosynthetic. Draws energy from sunlight and nutrients from surrounding water.
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
Human Risk
harmless
Handling Note
marine algae, safe to handle
Conservation Note
Native red algae; not assessed by NZTCS as marine algae are outside the scope of current threat classifications.
Te Ao Māori
No recorded Māori name distinguishes the crust algae from other red seaweeds. It was likely grouped with other red seaweeds called karengo. The red stain on the rocks would have been noticed. It looked like blood like the mark of a wound like the skin of the earth showing through. It was a sign of the living rock the stone that was not quite stone. The crust algae was sometimes used as a pigment. The dried crust could be ground into a powder and mixed with oil to make a red paint used for marking tools or decorating the body.
It does not look like seaweed. 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.