This week’s Nova, Deep Sea Invasion, on PBS describes the spread of a heinous form of macroalgae in the Mediterranean, the California coast, and Australia. Caulerpa taxifolia, commonly known as Notched Caulerpa, was developed at the Wilhelmina Zoo in Stuttgart, Germany, and introduced into the Mediterranean at the Oceanographic Museum of Monaco while Jacques Cousteau was director. This variety of caulerpa overwhelms indigenous plants and destroys the coastal ecosystem because it produces a toxin that prevents fish and other predators from eating it, and it thrives in the temperate waters of the most colorful marine ecosystems. 
Growing at the rate of an inch a day, and reproducing by cloning, at can’t be simply picked out of the oceans and removed: it has to be poisoned, at the expense of all the other flora and fauna in the area, or eaten by its one predator, a tropical sea slug that can’t live in all the waters where caulerpa grows.
The infestation of the Franco/German menace was first discovered in 1989 by Alexandre Meinesz when it only covered one small bed a few feet square, and had it been dealt with at the time he sounded the alarm, it would have been no big deal to eradicate, but French oceanographers connected with the Monaco Institute said it was no big deal, thinking it was simply an ordinary tropical plant like many others that have been seen in the Mediterranean since the Suez Canal was opened. Thing is, it’s not simply a wild plant, it was selectively bred for its hardiness by people looking for cool aquarium plants in Germany, and it’s really a killer weed outside your hobbyist’s tank.
You can buy the stuff today for $10 a bunch from a number of aquarium suppliers, and it’s in tanks all over the world already. This is one Franco-German bio-terror hazard that has to be stopped before it gets any worse. Write you congressman to get this stuff banned, and if you have any in your aquarium, kill it.
Here’s an aquarium-keepers’ discussion of the show.
UPDATE: More information here.
Another update: Caulerpa has been banned in California since Jan. 1, 2002 by AB 1334:
CALIFORNIA CODES
FISH AND GAME CODE
SECTION 23002300. (a) No person shall sell, possess, import, transport, transfer, release alive in the state, or give away without consideration the salt water algae of the Caulerpa species: taxifolia, cupressoides, mexicana, sertulariodes, floridana, ashmeadii, racemosa, verticillata, and scapelliformis.
(b) Notwithstanding subdivision (a), a person may possess, for bona fide scientific research, as determined by the department, upon authorization by the department, the salt water algae of the Caulerpa species: taxifolia, cupressoides, mexicana, sertulariodes, floridana, ashmeadii, racemosa, verticillata, and scapelliformis.
(c) In addition to any other penalty provided by law, any person who violates this section is subject to a civil penalty of not less than five hundred dollars ($500) and not more than ten thousand dollars ($10,000) for each violation.
The ban extends beyond the taxifolia species because it’s difficult to distinguish it from the others enumerated in the law, in the opinion of Fish and Game’s scientists. The author of this bill was Tom Harman, an enviro-Republican from Huntington Beach, one of affected areas.