threads through the shaded reef surfaces
- Size
- Length: 5–20 cm
- Lifespan
- 1–2 years
- Diet
- Photosynthetic. Draws energy from sunlight and nutrients from surrounding water. Requires clean water and good flow.
- Habitat
- Shaded crevices, low-light reef surfaces, and as an epiphyte on larger seaweeds in intertidal and shallow subtidal zones.
- Range
- Found throughout the North and South Islands on rocky shores, in rock pools, and on other seaweeds in intertidal and shallow subtidal zones.
- Endemism
- Native
- Main Threats
- None significant. Localised threats include coastal development, pollution, and climate change affecting water temperature.
- Population
- Not Threatened. Common on rocky reefs throughout New Zealand, often growing as an epiphyte on larger seaweeds or on shaded rock surfaces.
- 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 red thread algae from other red seaweeds. It was likely grouped with other red seaweeds called karengo. The fine, red threads would have been noticed by those who looked closely. They were a sign of healthy water. Of clean rock. Of a reef that was alive in all its small details. The red thread algae was sometimes used as a dye. The red pigment could be extracted. Used to colour fibres or to paint the skin. Today the red thread algae still grows on the rocky reefs of New Zealand. Visible if you look closely. A fine red tangle in the shadows. Drifting in the current. Barely visible. The quiet one. The hidden one. The one that proves the reef is made of small things as well as large.
It is almost missed. An alga that hides in the shadows.
Polysiphonia stricta is the one that is almost missed. It grows thin, red, thread-like fronds. A few centimetres long. Branching occasionally. But mostly just drifting in the current. It does not demand attention. Or wave its arms. It just hangs there. A fine red tangle in the shadows. Visible only if looked at closely. An alga that does not want to be seen.
What makes it special is the fineness. The fronds are only a few cells thick. A single layer of cells in a tube-like structure. Delicate. Almost invisible. Easily torn by a careless hand. But also tough. Able to survive in the low-light conditions of the shaded reef. A paradox of fragility and resilience. Often found growing on other seaweeds. An epiphyte. A plant that lives on another plant. It clings to the fronds of larger kelps. Using them as a ladder to reach the light. It does not harm its host. It just hitches a ride. An alga that is a passenger. Reproduction happens by releasing spores from specialised structures on the fronds. Spores are carried by currents. And settle on nearby surfaces to grow into new plants. Red thread algae provides habitat for tiny invertebrates. Microscopic crustaceans hide among its threads. To find red thread algae is to look closely at the reef. The reef is shadowed. The red thread hangs. Fine and red. Drifting in the current. A magnifying glass reveals the threads. The tiny crustaceans hiding among them. The alga does not know it is beautiful. It does not know it is a micro-forest. It just wants to catch the light. Not Threatened. Common on rocky reefs throughout New Zealand. Often growing as an epiphyte on larger seaweeds or on shaded rock surfaces.