Didymo

What does it look like?

Didymosphenia geminata, commonly known as didymo or “rock snot”, is a freshwater alga belonging to a group of single celled plants called diatoms. Individual cells are microscopic and cannot be seen with the naked eye until they form large, dense colonies, known as algal blooms. Didymo cells are distinctive in shape and sit on long, mucous‑like stalks.

Diatoms have cell walls made of silica (sand), which is why didymo feels gritty when touched.

Under favourable conditions, didymo produces large amounts of a sticky, mucus‑like substance that firmly anchors it to rocks and other submerged surfaces. This growth can form thick brown mats that smother rocks, plants, and streambeds. Long trailing strands often develop, resembling wet cotton wool or tissue paper, and may turn white at the ends.

Any sightings of didymo in North Island waterways should be reported immediately to Ministry of Primary Industries or to the relevant regional council. If you think you may have encountered didymo, take care not to spread it - thoroughly check, clean and dry all gear before using it in another waterway.

Why is it a problem?

 Didymo attaches to stream, river and lake beds by long stalks, forming thick brown mats that cover rocks, submerged plants and other surfaces. It often develops flowing ‘rat‑tail’ strands that can turn white at the ends and resemble tissue paper, becoming paler as they lengthen.

In flowing waters, didymo replaces naturally complex communities of plants and invertebrates. By smothering the riverbed, it reduces food availability for fish and some bird species. Fish spawning sites and juvenile fish habitat can be affected, along with adult fish species like tuna (eel) and trout.

While didymo poses no known risk to human health, it significantly degrades the appearance of waterways. During blooms, detached clumps can be seen drifting through the water, further impacting water quality and recreational values.

Control methods

Report any sightings of didymo in Hawke’s Bay to MPI’s free 24-hour emergency hotline on 0800 80 99 66.

CHECK, CLEAN, DRY

The Check, Clean, Dry method is the most effective way to prevent freshwater pests from spreading between waterways and is recommended nationwide by the Ministry for Primary Industries (MPI).

Check

• Remove all visible plant material, mud and debris from gear.
• Leave removed material at the site or dispose of it in the rubbish.
• Do not wash plant material down drains.

Clean any gear that has come into contact with water. Allow extra time for absorbent items. Use one of the following methods:

• Soak or spray with a 5% dishwashing detergent solution (500 mL detergent in 10 L water) for at least 1 minute
• Soak or spray with a 2% bleach solution (200 mL bleach in 10 L water) for at least 1 minute
• Soak in water above 60°C for at least 1 minute
• Soak in water above 45°C for at least 20 minutes
• Freeze until solid

Dry

• Dry all gear completely, then leave for at least 48 hours before using it in another waterway.

Most items can be cleaned using the methods above, but some gear has specific cleaning requirements. 

More information

Related links

• Check, Clean, Dry – Prevent the Spread of Freshwater Pests (MPI NZ)
• Freshwater Invasive Species of New Zealand (2020) – NIWA (PDF)
• Didymo and Other Threats – DOC NZ
• Didymosphenia geminata (Didymo) – NZ Plant Conservation Network
• Didymo – An Introduced Nasty (Manaaki Whenua Landcare Research)