Algae tough, fleshy, firm; up to 2 m tall. Thalli coarse, with axes and branches 1 - 2 cm diameter; heavy, with major axes relatively straight, lacking secondary branches near apices. Frequently and irregularly branched, most branches primary, secondary branches intercalated between primary branches or mostly lacking.  Shiny green to yellow orange.

Morphologically plastic: variable thalli forms, from gnarled forms with few small branches in shallow areas to large, intricately tangled, fleshy mats in deeper waters.

Reef flat and reef edge, 1 to 17 m deep. Loosely attached to broken coral, or unattached fragments floating in shallow and deep waters. Can form large, moving mats of unattached thalli.

Reef flat and reef edge, 1 to 17 m deep. Loosely attached to broken coral, or unattached fragments floating in shallow and deep waters. Can form large, moving mats of unattached thalli.

The two Kappaphycus species, K alvarezii and K. striatum, are some of the largest tropical red algae, with extremely high growth rates, doubling in 15-30 days. Since their introduction in Kaneohe Bay in 1974, they have spread at the rate of 260 m/yr. The high growth rate, plastic morphology, and extremely successful vegetative regeneration make the two Kappaphycus species potentially destructive invasives in Hawaiian waters.

Until recently in Hawaii Kappaphycus sp. have  only recently was Kappaphycus species found to reproduce sexually.  Since its introduction, observations and studies have reported reproduction by vegetative fragmentation. At the tip of each branch is a cluster of apical cells potentially high in regenerative capabilities that are able to regenerate a new thallus after breaking off. A broken tip can grow into full-sized thalli in a short period of time. This species has been highly successful at Kaneohe Bay, dominating the sandy spur and grooves on the reef flat. It inhabits barren sandy grooves where it does not appear to compete with native algal species. In other locations, K. alvarezii has been found to provide habitat for a diverse invertebrate population and reef fishes. This red alga was found to have a higher index of species diversity than neighboring reefs without the alien. It is highly grazed and can be found to be the primary food item in gut contents of common herbivorous fishes. The alga’s large growth form can be detrimental to corals by shadowing or smothering, thus causing coral death.

The red alga’s dispersal is thought to be constrained by size and weight, as it appears to become trapped in depressions and channels. The species is also constrained by high herbivory. K. alvarezii has managed to spread to neighboring reefs with supportive physical factors and little grazing, where it is dominating the changing marine ecology.

K. alvarezii has been introduced throughout the warm tropics for commercial cultivation. It is a major producer of kappa-carrageenan, which is used for medicinal purposes and as a homogenizer in milk products, toothpaste and jellies.