Caulerpa is a genus of seaweeds in the family Caulerpaceae (among the green algae). They are unusual because they consist of only one cell with many nuclei, making them among the biggest single cells in the world. A species in the Mediterranean can have a stolon more than 3 metres (9 ft) long, with up to 200 fronds.
Some species (especially Caulerpa lentillifera and Caulerpa racemosa) are eaten under the names sea grape or green caviar or umi-budo in Okinawa. They have a peppery taste. Seagrapes are eaten in Indonesian cuisine, sometimes fresh, and othertimes coated in sugar. They are raised in Cebu, for domestic consumption in the Philippines as well as export to Japan. Unconfirmed reports claim that the alkaloid Caulerpin found in seagrapes can sometimes cause poisoning.
Another species, Caulerpa taxifolia, has become an invasive species in the Mediterranean Sea, Australia and southern California (where it has been eradicated). It is thought that Caulerpa species capable of surviving in temperate waters are freed from predators, in part contributing to invasive growth. Most Caulerpa species evolved in tropical waters, where herbivores have an immunity to toxic compounds within the alga. Temperate water herbivores have no natural immunity to these toxins, allowing Caulerpa to grow unchecked if introduced to temperate waters. C. racemosa has recently been found in waters around Crete, where it is thought to have contributed to a significant reduction in fisheries. The alga has invaded the area from the warmer waters of the Red Sea.
In U.S. waters, the Mediterranean strain of Caulerpa taxifolia is listed as a federal noxious weed, under the Plant Protection Act. The Aquatic Nuisance Species Taskforce has also created a National Management Plan for the Genus Caulerpa. The state of California also prohibits possession of nine different species of Caulerpa.
Caulerpa is common in the aquarium hobby as a nitrate absorber because of its rapid growth under relatively adverse conditions. It may also be used in refugiums for a long term nitrite absorber. Many introductions of invasive Caulerpa to the wild are thought to have occurred via aquarium dumping although there is no proof of it.