the filamentous brown alga of NZ's sheltered shores
- Size
- Microscopic filaments, visible as brownish film on rocks
- Lifespan
- 1 years
- Diet
- Grows on rocky shores, intertidal rocks, shells, and larger seaweeds. Requires clean, moving water and stable rock surfaces. Tolerates wave action, sun exposure, and variable temperatures.
- 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 surfaces, the rocks, the shells, the larger seaweeds. Forms thin, brownish coatings over everything, a subtle film that you only notice when you stop to look. The subtle one, the background noise, the thing that is everywhere but seen by no one.
- Range
- Found throughout the North and South Islands on rocky shores, intertidal rocks, and larger seaweeds. Most common in intertidal and shallow subtidal zones. Also found in temperate and cold seas worldwide.
- Endemism
- Native
- Main Threats
- None significant. This species is one of the most common and widespread seaweeds in the world. Localised threats include coastal development, pollution, and climate change affecting water temperature and clarity.
- Population
- Not Threatened. Brown film algae is one of the most common and widespread seaweeds in the world, found on rocky shores from the tropics to the poles. In New Zealand, it is abundant on intertidal rocks and often grows as an epiphyte on larger seaweeds.
- Conservation Status
- Not Threatened
The one you do not see grows as a thin, brownish coating on rocks, shells, and other seaweeds. It is not leafy. It is not branching. It is just a film, a stain, a subtle discolouration. From a distance, you might think it is just a dirty rock. Up close, it is a living organism, a forest of microscopic filaments. It is the alga of the invisible forest, the one that lives in plain sight.
What makes it special is the ubiquity. The brown film algae is everywhere. It grows on every rocky shore, on every pier piling, on every shell that sits still long enough. It is the background noise of the intertidal zone, the thing you never notice because it is always there. It is the alga of the background, the one that holds the ecosystem together without anyone knowing.
The brown film algae is a brown alga, a member of the Ectocarpaceae family. Its filaments are microscopic, a few cells wide, a few centimetres long. They form a tangled mat on the surface of the rock, so thin that you can see the rock through them. The colour is a pale brownish-green, blending in with the rock and the sand. Under a microscope, the filaments are beautiful, each cell a perfect cylinder, the branches forming at precise angles.
Biologically, the brown film algae reproduces by releasing spores from specialised structures on its filaments. The spores are released into the water, carried by the currents, and settle on nearby surfaces to grow into new films. It also reproduces by fragmentation, a piece broken off can grow into a new mat.
To find brown film algae is to look closely at the rock. See that brownish stain? That is not dirt. That is a living organism, a forest of microscopic threads, a world invisible to the naked eye. It is the subtle one, the background noise, the thing that is everywhere but seen by no one, the one that proves that the most successful organisms are often the least noticeable.