General Impact
Acanthophora spicifera has a plastic morphology, which allows it to adapt to different conditions and invade a diversity of habitats. The brittle nature of its branches often results in fragmentation, which contributes to frequent, large free-floating populations, and its widespread distribution (University of Hawaii, 2001). It is now one of the most widespread and successful alien algae in Hawaii, and may modify native communities by smothering and outcompeting native algal species (Preskitt, 2002; Eldredge, 2003). Blooms of native A. spicifera covered by cyanobacterial epiphytes have been observed on several reefs in the Eastern Tropical Pacific, since widespread coral mortality opened substrate for colonisation during the 1997-98 El Nino Southern Oscillation. The bloom coincided with La Nina conditions, with thermocline shoaling and mixing upwards of nutrient-rich bottom water, but it persisted for several years after this (Fong et al. 2006).
Location Specific Impacts:Hawaii (United States (USA)) 
Competition: Acanthophora spicifera is often found growing next to and competing with the native Hawaiian species of Laurencia nidifica and Hypnea cervicornis. H. cervicornis may entangle in the upper branches of A. spicifera as an epiphyte (Russell, 1992; Preskitt, 2002).
Economic/Livelihoods: Five species of algae are causing major problems on Hawaiian islands: the intentionally introduced Hypnea musciformis, Gracilaria salicornia, Kappaphycus spp.; the accidentally introduced Acanthophora spicifera; and the (origin unknown) Avrainvillea amadelphia. On Maui, 20,000 pounds (over 9000 kg) of algae wash up on Kihei beaches per week, costing more than US$100,000 per year to clean. In an economic study, algal biomass costs north Kihei more than US$20 million per year in lost of rental income, decrease in property value, and clean up (Eldredge, 2003).
Interaction with other invasive species: It is possible that the introduction of algae such as Acanthophora spicifera has reduced grazing intensity on the invasive (but native) macroalgae Dictyosphaeria cavernosa, allowing the latter to overgrow and potentially displace corals in Hawaii (Stimson et al.. 2001).
Modification of natural benthic communities: Algae such as Acanthophora spicifera potentially modify native communties by smothering and overgrowth of the native biota in Hawaii (Eldredge, 2003). A. spicifera has been shown to reduce growth of Laurencia nidifica in field experiments (Russell, 1992).
Other: Acanthophora spicifera now makes up a significant proportion of the diet of the endangered green turtle (see Chelonia mydas in IUCN Red List of Endangered Species) in the Hawaiian Islands (Russell and Balazs, 1994). Waikiki Reef (United States (USA))
Ecosystem change: The Waikiki reef in Hawaii is highly degraded and is dominated by the microalgal blooms including A. Spicifera (S. Coles, pers. Comm.; in Coles and Eldredge, 2002).
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