In Hawaii, A. spicifera has displaced native species, including common algae such as Laurencia spp. and Hypnea cervicornis (Russell, 1992; Smith et al., 2002). However, it is not reported to form large, monospecific nuisance blooms.
The genus Acanthophora was erected by J. V. Lamouroux in 1813 for Fucus acanthophorus, a species he had described in 1805. F. acanthophorus is a later name for Fucus spicifera Vahl (1802) and the combination Acanthophora spicifera was established by Børgesen in 1910.
The genus Acanthophora was reviewed and revised by de Jong et al. (1999), who include a complete synonymy for A. spicifera.
Plants to 25 cm, colour variable, light-pink, pale to dark-brown, green or yellow. Branches smooth, cylindrical, 0.6–3.0 mm diameter, sparingly to repeatedly irregularly radially branched, generally sparse below and more abundant above, heavily corticated; lateral determinate branchlets bearing spur-like spines to 0.5 mm long; apices of plants pointed, bearing dichotomous hair-like trichoblasts that can envelop mature apices. Holdfast is an irregularly lobed, disc-like, thickened crust from which several erect axes arise. Axes with cells of the central filament surrounded by five distinct pericentral cells and small, rounded cortical cells gradually diminishing in size toward the periphery, the central axial cells usually evident; in older axes, central axial filaments surrounded by small-celled adventitious filaments. Sporophytes and gametophytes isomorphic. Tetrasporangia in linear rows in short, determinate, spinose swollen branchlets, tetrahedrally divided, 40-50 µm diameter, 60-80 µm long. Gametophytes dioecious, spermatangial heads plate-like, on single-celled stalks near branch apices, often with sterile hairs present at base of stalk; cystocarps on adaxial sides of spines, urn-shaped, 0.5-1.0 mm diameter, solitary.
This species is widely distributed throughout the tropics into warm temperate seas, in the central eastern and western Atlantic, around the Indian Ocean, and in the central western Pacific.
Prior to its introduction to Hawaii, A. spicifera was not known from the eastern central Pacific, nor in the central Pacific Basin east of Micronesia.
The genus Acanthophora is predominantly a tropical genus, but the distribution of some species does extend into warm temperate regions. Acanthophora muscoides and A. spicifera have wide distributions, but the other species are more limited: Acanthophora aokii in the Pacific; Acanthophora dendroides in the Indian Ocean; Acanthophora ramulosa in the East-Atlantic; Acanthophora nayadiformis in the Mediterranean and Red Seas; and Acanthophora pacifica in the central and western Pacific (Kraft, 1979; de Jong et al., 1999). Apart from the occurrence of A. pacifica and A. spicifera in Hawaii, the genus is absent from the central and eastern Pacific.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
Doty (1961) first reported the Hawaiian presence of this species and suggested that it probably arrived shortly after 1950. Considering that A. spicifera is a conspicuous alga, and the number of algologists who had collected algae in the Hawaiian Islands prior to 1950, Doty considered it unlikely that it would have been overlooked. Doty also observed that older Polynesians in Hawaii had no name for Acanthophora and, when Abbott and Williamson (1974) interviewed Hawaiians familiar with seaweeds, they found several who confirmed that they had never seen this species before World War II, giving credence to the speculation that A. spicifera is an accidental introduction to Hawaiian waters.
The most likely vector was considered to be the fouled bottom of a ship arriving from the west, and possibly a barge arriving in Pearl Harbour from Guam or the Philippines. The sequence of discovery on the Hawaiian Islands was: Pearl Harbour (autumn 1952, fragments); Waikiki (April 1953); Ke’ehi Lagoon, Hau’ula (May 1953); Kaua’I (1954); Lana’I (1960); all islands (1961).
A. spicifera is the most common non-indigenous algal species in Hawaii (Smith et al., 2002). The species was found on every island surveyed by Smith et al. (2002) and its distribution was fairly uniform around all coastlines except for the island of Hawaii. It was most common in intertidal regions and in semi-protected tide-pools. Abundance has increased from 2003 to 2006, with blooms coinciding with late summer/early autumn (Dailer et al., 2006). Mature tetrasporophytes and female gametophytes were found on all islands and the species appears to release spores throughout the year, as well as having potential for dispersal by fragmentation. The broad distribution of the species through the islands is also likely to have been facilitated by hull fouling on small boats and other vessels.
Areas at risk of invasion are those islands and coastlines in the central and eastern tropical Pacific that do not fall within the natural distribution of the species. Boat and vessel traffic is the most likely means for dispersal to new regions.
This species is saxicolous, epiphytic, or epizoic; intertidal and subtidal, or free-floating. Often a dominant intertidal species on calm shallow reef flats, in tidepools and rocky benches swept by small waves (Hawaii Biological Survey, 2008).
DNA sequencing (based on a variable region of the nuclear large subunit ribosomal RNA gene, and the mitochondrial cox 2-3 spacer region) revealed no variation in plants from Hawaii, or from other areas of the Pacific and Australia (O’Doherty and Sherwood, 2007). In contrast, these authors found fragment techniques (Inter-Simple Sequence Repeats [ISSRs]) to reveal highly structured Hawaiian populations with a substantial range of both within- and among-population variation, with individual plants forming discrete clusters corresponding to geographic locality.
The sexual life history of A. spicifera has a triphasic alternation of generations, with isomorphic tetrasporophytes and dioecious gametophytes. The thalli are also easily fragmented by wave action, and fragments have the potential to re-attach after 2 days (Kilar and McLachlan, 1986).
Skelton and South (2002) quote an autecological study of A. spicifera carried out in the Fiji Islands, which demonstrated that sexual phases were absent and that fragmentation was the main mode of propagation (Skelton, 1998). This was considered to confirm earlier reports (Mshigeni, 1978; Kilar and McLachlan, 1986) that reproduction in this species is often predominantly asexual.
Across the Hawaiian Islands in a 2000 survey, tetrasporophytes and female gametophytes containing mature carpospores were found on all islands except Hawaii (Smith et al., 2002). However, in 2005, O’Doherty and Sherwood (2007) found no spermatangial or carpogonial plants in their collections at nine locations. In regular collections between February and late October 2005, no reproduction was visible until the final week of May, after which tetrasporangial plants were present through the remainder of the study.
Physiology and Phenology
A. spicifera stores both nitrogen and phosphorus in response to enhanced nutrient supply (Fong et al., 2001; 2003). In a study in Honduras, artificially elevating nitrogen and phosphorus levels increased grazing by herbivores across habitats when compared to controls (Boyer et al., 2004).
Survival of A. spicifera can be enhanced on reefs by association with dense aggregates of other algal species that are more tolerant of wave action and are able to retain water when exposed to air (Smithsonian Marine Station, 2008). A. spicifera benefits by being shielded from direct sunlight and insulation from dessication. In the Caribbean, Laurencia papillosa is one associated species. In some habitats, A. spicifera is able to outcompete, but not exclude, L. papillosa. The relative success of both species is considered to be heavily dependent on the duration and types of habitat disturbance and the ability of each species to maintain space during competition, reproduction, and vegetative growth (Smithsonian Marine Station, 2008).
An association refuge is also reported to sometimes occur when A. spicifera grows in association with the soft coral Sinularia sp. with fish predation on A. spicifera decreasing with its proximity to Sinularia (Kerr and Paul, 1995; Smithsonian Marine Station, 2008). Littler et al. (1986) similarly reported that A. spicifera suffers much less grazing damage when it grows near the toxic brown algae Stypopodium zonale.
The maximum primary production for A. spicifera has been reported to occur at a water temperature of 25°C (Kilar and Norris, 1988). In the Caribbean, the species has been observed to disappear during mid-winter and this has been used to speculate that it may have a lower temperature limit of 23.5°C (Taylor and Bernatowicz, 1969). However, the wide distribution of the species through the tropics and subtropics suggests a broader temperature tolerance (Smithsonian Marine Station, 2008). In Hawaii, abundance is reported to peak in late summer/early autumn (Dailer et al., 2006).
Feeding preference experiments undertaken in Guam found that the crab Menaethius monoceros preferred A. spicifera to the green alga Bryopsis pennata, the brown algae Padina tenuis and Sargassum cristaefolium and two cyanobacteria (Cruz-Rivera and Paul, 2006). Amphipods in the trial were not selective, saccoglossans preferred Bryopsis and gastropods preferred the cyanobacteria. In Australia, both wild and captive-bred rabbitfish (Siganus fuscescens) preferred to consume A. spicifera over seagrasses, brown and green algae, and cyanobacteria (Capper et al., 2006).
In contrast, in feeding preference experiments undertaken in south-east India, A. spicifera was not one of the five species of twenty offered that were grazed by herbivorous fishes (Ganesan et al., 2006). Similarly, A. spicifera was not a preferred alga consumed by the sea urchin Tripneustes gratilla in Hawaii (Stimson et al., 2007). This was considered possibly due to the long thin branches of A. spicifera, which may be more difficult for the urchin to handle and ingest than algae with rigid morphologies.
The initial introduction to the Hawaiian Islands was attributed to hull fouling, most likely on a barge arriving in Pearl Harbour from Guam or the Philippines (Doty, 1961). The species then appears to have radiated in all directions from the initial site of reproduction; locally by the release of spores and fragmentation, and more broadly by hull fouling on small boats and other vessels (Smith et al., 2002).
A. spicifera has become a significant component in the diets of green turtles (Chelonia mydas) in Hawaii, and appears to provide an important source of energy (Arthur and Balazs, 2008). Although many algal species were identifiable in faecal pellets, no A. spicifera was observed, indicating that the species was completely ingested and assimilated.
Impact on Biodiversity
In Hawaii, A. spicifera is now often a dominant intertidal species and can outcompete native algae, including the native Laurencia nidifica (Russell, 1992; Schaffelke and Hewitt, 2007). The abundance at some sites has increased total algal biomass and productivity. The competitive balance between Laurencia papillosa and A. spicifera varies with wave exposure (Kilar and McLachlan, 1989). Throughout most of the fore-reef zone, L. papillosa is the competitive dominant, as defined by its ability to occupy and dominate space, but as wave exposure subsides, A. spicifera can dominate.
Stimson et al. (2001) have hypothesized that the introduction of A. spicifera has led to a steady gain in the abundance of the less palatable green alga Dictyosphaeria cavernosa on coral reefs.
A. spicifera is eaten as a vegetable in parts of the Pacific including Fiji (South, 1993) and Vanuatu (Dixon and Kraft, 2007). In India, culture and utilization of A. spicifera has been explored (Ninwe, 1997), including culture as a source of additional income for local fishermen during the monsoon (Mohammed, 2000).
A. spicifera is one of a number of marine macroalgae studied to show that they could be utilized as a source of natural antioxidant compounds on the basis of crude extracts and fractions exhibiting antioxidant activity (Ganesan et al., 2008). This species has also been used in anti-cancer research/drug development (Sithranga Boopathy and Kathiresan, 2010).
A. spicifera has been shown to be useful as a bio-indicator of nutrient enrichment near coastal shrimp farms (Lin and Fong, 2008). Isotope ratios were the most sensitive indicators.
Seven species of Acanthophora are recognized: Acanthophora aokii, Acanthophora dendrooides, Acanthophora muscoides, Acanthophora nayadiformis, Acanthophora pacifica, Acanthophora ramulosa and A. spicifera.A. spicifera differs from other terete species in being sparingly branched and, apart form the occasional solitary spine, generally lacking spines on the main axes (de Jong et al., 1999). In A. spicifera, spines are also mostly at the apices of branchlets and smaller in size toward the apices of indeterminate axes.
The genus Acanthophora is most similar to the genus Chondria, but differs in the form of the branchlets. In Chondria, thebranchlets are club- or spindle-shaped, and are constricted or tapered at their bases, whereas in Acanthophora, the branchlets are spine-like (de Jong et al., 1999).
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Site-specific control methods are proposed as a means to manage the spread and expansion of invasive populations (O’Doherty and Sherwood, 2007). Populations that are not fully established are considered to have less genetic variation and therefore be less likely to resist control measures such as large-scale removal or introduced herbivores. It is therefore recommended that control measures target locations that have strong potential to recruit new populations through the frequent production and dispersal of propagules. Exposed reefs, rather than bays, lagoons and tide pools, would be expected to produce and disperse more propagules because of higher wave action, so it is suggested that removal efforts target these sites. Removal efforts should also be timed to precede spore production, possibly in late spring in Hawaii (O’Doherty and Sherwood, 2007).
Fouling on vessels is considered to be the vector for both the initial introduction of A. spicifera to Hawaii and its spread between islands. Public awareness and more stringent regulations of vessel fouling and ballast water are therefore proposed to be necessary to prevent or slow the recruitment of new populations (O’Doherty and Sherwood, 2007).
Of the invasive macroalgal species introduced to Hawaii, A. spicifera is the most widespread and successful (Smith et al., 2002). Eradication is not considered possible (O’Doherty and Sherwood, 2007).