The Hawaiian Islands support numerous species of marine plants, including seaweeds and other algae. Various kinds of seaweed, called limu in Hawaiian, were collected for food, medicine and ritual, and used by ancient Hawaiians. They are still incorporated into the diets of Hawaii's people. Many of these marine plants have salty, spicy flavors. Limu is also very rich in minerals and vitamins A, B12, C and riboflavin. Hawaii's waters also support introduced alien marine plants, some of which have become invasive.
Green, brown and red algae are all present in Hawaii's coastal waters where sunlight can penetrate. The major difference between these groups of algae are their methods of photosynthesis, which involve specialized pigments called chloroplasts. These pigments also give the plants their respective colors. Some of these algae have interesting symbiotic relationships with various species of coral, which offer them a protected environment in exchange for nutrition derived from photosynthesis. Other algae live directly on rocks or sand and provide an important food source for marine animals, including the endangered green sea turtle.
Asparagopsis taxiformis, called limu kohu in Hawaiian, is a soft, furry seaweed with sprawling basal stems found at the edges of reefs. The upper branches of this seaweed are collected to be pounded and rolled into balls. It has a very strong flavor and is used in small quantities to season fish. Sargassum echinocarpum, or limu kala, is a tan-colored spiny seaweed with small berry-like galls found floating near shore. It has tremendous cultural significance among native Hawaiians and is still used in ceremonies and medicine today. Gracilaria parvisipora, or ogo, is a branching pink seaweed with flattened tips. It is sold commercially and relished by people of many cultures. It is illegal to collect reproductive plants of this species, which can be identified by prominent black bumps on the branches. Halophila hawaiiana is a creeping perennial herb with inch-long leaves. It is found on shallow, sandy reef flats or along the edges of ancient coastal fishponds.
In recent years, several invasive algae, including Gracilaria salicornia, Hypnea musciformis and Acanthophora spicifera, have been accidentally introduced in Hawaiian waters. Gracilaria salicornia, also dubbed "gorilla ogo" because it resembles a larger, more aggressive version of the edible ogo seaweed, forms extensive tangled mats of orange, green and purple, which literally blanket reefs. Hypnea musciformis was accidentally introduced to Hawaii in a shipment of another algae meant to be farmed for agar production. This tan-colored algae is covered with small hooks and forms dense mats which often wash ashore. Acanthophora spicifera arrived in Hawaii as an unwelcome stowaway in the bilge of a ship from Guam. This translucent, beige algae resembles a thorny tumbleweed and is very brittle.These aggressive alien species are now considered the most serious threat to Hawaii's coral reefs. Efforts to remove these invasive pests have been surprisingly successful, largely due to the invention of an underwater vacuum-like machine used to remove the algae, which has been dubbed the "super sucker."