What does it look like?
Dead man’s fingers can grow up to about a metre long and weigh as much as 3.5 kilograms. At low tide, it often looks like a fuzzy clump of soft, tubular “fingers” hanging down from rocks, which is how it gets its name.
Each of these finger like branches can be up to 1 centimetre wide and sometimes more than 30 centimetres long. There are several native Codium species in New Zealand that look very similar, including another subspecies (Codium fragile subsp. novaezealandiae), which can make identification tricky.
Dead man’s fingers prefer sheltered marine environments, particularly the intertidal and subtidal zones of harbours, estuaries, and bays. It can also survive in tide pools on more exposed shores.
It grows on a wide range of surfaces, including natural hard substrates like rocks, boulders and pebbles, as well as on both living and dead shellfish. It’s also commonly found on artificial structures such as wharves, jetties, ropes, mussel lines and oyster racks.
This seaweed can tolerate large changes in salinity and temperature, although growth appears to be fastest in warmer conditions, around 24°C.
Why is it a problem?
Dead man’s fingers is native to Japan but has spread widely around the world. It’s now found along coastlines in Europe, New Zealand, Australia, Chile, the Mediterranean, and the east coast of North America. Its ability to tolerate a wide range of temperatures and salinity means it can establish and spread quickly in many different marine environments.
Once established, dead man’s fingers can have significant impacts on seabed (benthic) communities. It can dominate the areas it invades, crowding out other species and changing the structure and function of marine habitats.
This seaweed spreads in several ways. It reproduces sexually by releasing eggs and sperm into the water, and asexually by producing free swimming “swarmers” or through fragments that break off and grow into new plants. These spores and fragments are easily carried by water currents, wind, and waves.
Human activity can also help spread dead man’s fingers. Fragments and spores may be transported in ballast water (seawater pumped into ship tanks) or hitch a ride on boat hulls and marine equipment. When the seaweed grows on small objects like shells, its buoyancy can cause it to break free and be carried to new locations by currents and wave action.
Control methods
You can help prevent the spread of marine pests by:
- Regularly cleaning your boat’s hull –keep fouling growth to no more than a light slime layer.
- Applying a thorough coating of antifouling paint and keep it in good condition.
- Ensure your hull is clean and free of fouling before you travel to a new area.
- Clean and dry any marine equipment (e.g. ropes, lines and pots) before using in a new area.
- Inspect areas on your boat that retain water for signs of marine life.
- Check for aquatic weeds tangled around anchors, trailers, and other equipment.
