Scientists suspect the algae are a hybrid or even a new species, unintentionally created in a marine aquarium environment. Unlike its tropical cousins, the new form is able to endure waters where winter temperatures would be expected to kill it. It is also triple the size of any other known members of the species. One of the aquariums where it has been seen sits just across the beach from the water's edge where the alien's first Mediterranean sighting was documented, says Alexandre Meinesz, a Caulerpa expert at the University of Nice-Sophia Antipolis in France.  

The algae were first spotted in 1984 on about a square yard of seafloor off the coast of Monaco, according to Science News. By 1990, the interloper had reached France. Two years later, it was well established along the shores of the Spanish island of Majorca and, a year after that, along the coast of Sicily. By 1994, it was growing in the Croatian Adriatic.  

Like its tropical counterparts, the algae can spread asexually, regenerating a new plant from barely visible fragments of itself. This has rendered all but useless any attempt to pull up the algae either by hand or by using the underwater equivalents of plows. In fact, the plant appears to be spreading throughout the Mediterranean, primarily from fragments dispersed by fishing nets and anchors, reports Science News. 

"Over the eight years that we have observed this Caulerpa in the Mediterranean, we have never seen evidence of sexual reproduction," says Meinesz. The only reproductive cells it releases are male, fostering a suspicion that all C. taxifolia in the Mediterranean are clones of a single aquarium plant. 

Science News reports that there are efforts underway to harness a natural predator, a mollusk, to control the voracious algae since other strategies have proved to be too expensive or toxic to try on a large scale. Meinesz is leading the team of scientists working toward a biological control effort. They are using sea slugs whose mouth structures allow them to eat only Caulerpa algae. The big question is whether the control organisms will make a dent in the algae's invasion, because these predators eat at a snail's pace--literally, says Meinesz. He presented his analyses at a recent meeting sponsored by the Redmond, Wash.-based Marine Conservation Biology Institute.  

Though biological control is becoming quite common in agriculture and gardening, it has never been attempted in a marine environment. Andrew Cohen of the San Francisco Estuary Institute in Richmond, Calif. questions the wisdom of turning so quickly to biological control. "This stems from my sense that we are not very good at predicting how that [control] organism will act in a novel environment," he says. 

According to Science News, Cohen is drafting a letter, to be endorsed by marine scientists, that will highlight the Caulerpa taxifolia algae threat and accompany a formal request that the United States prohibit the sale, possession, or transport of the alien species. Members of a United Nation's workshop on invasive Mediterranean Caulerpas made much the same recommendation two months ago. They cited the 1992 Convention on Biological Diversity, which asks ratifying nations to "regulate the intentional or accidental introduction of non-indigenous species" and advised that European nations prohibit the aquarium trade or anyone else from selling or housing the invasive algae.

Introduced species can wreak havoc on an ecosystem. Look at what the sea lamprey did to the Great Lakes, cheatgrass to the Great Basin, and camels to the outback of Australia.