Creeping Chinese seaweed threatens to choke Venice

The waterways of Venice have been invaded by a fast-breeding giant Chinese seaweed.

"The killer seaweed has almond eyes," said La Repubblica newspaper. "It's brown, tall, flexible, invading and a bully... No one expected that after glassware, lace and bags, the Chinese would invade the Gulf of Venice."

The weed, Undaria pinnatifida, grows up to three metres long, dwarfing the somewhat smelly but far smaller weed native to the Venice lagoon. It grows quickly, at a rate of up to one centimetre per day, matures after 40 to 60 days, produces millions of spores and can wipe out smaller, local weeds with ease. It also has a great talent for hitch-hiking on the hulls of boats; the only sure way of preventing it spreading, experts say, is to keep the undersides of boats immaculately clean.

According to Venice's Museum of Natural History, the weed apparently arrived from France in the ballast of a ship - the weed's other preferred way of getting around. It established itself in the outer parts of the city, such as Giudecca, Arsenale and San Marco where the flow and replenishment of water is faster. But it is feared that the weed is moving into the narrower canals in the heart of the city, which could end up being choked by it.

Undaria is the second oriental alien to colonise the city's waters. The first was the Philippine clam Tapes philippinarum, known locally as "caparozzolo", deliberately introduced to the Venice lagoon in 1983 to revive the city's fishing industry, badly damaged by industrial pollution. The clams adapted readily to their new surroundings, and have become the mainstay of the local fishermen. But even the benign vongole (Italian for clam) is not without its environmental downside: to maximise their catch, fishermen are in the habit of using illegal mechanical rakes to haul them off the seabed, destroying much else that is down there as well. A low-level war is under way between the local authorities and the rake-wielding fishermen.

Undaria is described as one of the world's 100 most threatening invasive species, and has already established itself around much of the coast of New Zealand, Argentina and other parts of Europe.

Undaria arrived in New Zealand around 1987, probably on the hull or in the ballast water of a ship from Asia. "It is an unwanted organism under the Biosecurity Act 1993," says the New Zealand government. "This pest seaweed has the potential to change the natural character of our coastline and rocky shores. Coupling this with its ability to spread quickly in water currents and on boat hulls, the odds appear to be stacked against marine biodiversity."

Yet Undaria is not all bad news. Known as wakame in Japan, it is one of the staples of the Japanese diet, frequently found in miso soup. It is also eaten with Japanese salads and soba (buckwheat) noodles. It is rich in protein, calcium, iodine, magnesium, iron and folate, and is sold in pill form as a vitamin supplement.

There is an obvious solution to the Undaria inundation. Given the enormous number of Japanese tourists who visit Venice and their penchant for eating food with which they are familiar, all the local fishermen need to do is to start harvesting it.

It would not be the first time that Venice has learnt from the East: the cathedral of San Marco betrays a strong Byzantine influence, and spaghetti, Italy's national dish, is widely believed to be an imitation of Chinese noodles.