In Hawaii, fragmentation of G. salicornia is considered to be the primary mode of reproduction and any sort of physical disturbance can generate fragments (Smith et al., 2004). Although the fragments are relatively heavy, and tend to settle quite rapidly limiting the distance of dispersal, currents and other hydrodynamic events such as large swell can aid dispersal. As new populations emerge, more biomass is available for fragmentation and subsequent dispersal. The size and the density and thickness of mats (5-10 cm) leads to the species monopolizing substrata (Smith et al., 2004).
The genus Gracilaria was established by R.K. Greville (1830) and redefined by J. Agardh (1852) (Tseng and Xia, 1999). It is considered to be one of the largest genera of seaweeds, with possibly more than 150 species, and is economically important as a major source of agar.
Under Gracilaria salicornia, along with the synonyms tabulated herein under non-preferred names in the identity table, Abbott (1995) also listed Gracilaria canaliculata (Kützing) Sonder, following the review of G. salicornia by Xia (1986). Withell et al. (1994) describe forms conforming to both G. salicornia and G. canaliculata from western and northern Australia and considered the two as separate taxa, although retaining the latter as a form of the former.Molecular studies have shown G. salicornia and G. canaliculata (including Gracilaria crassa) to be separate entities (Lim et al., 2001; Gurgel and Fredericq, 2004; Iyer et al., 2005).
Thalli prostrate to semi-erect, up to 17 cm long, occurring as solitary individuals or forming loose, tufty, entangled clumps to 30 cm or more across; holdfast irregularly discoid, giving rise to a single axis, or many aggregated holdfasts forming an expanded basal disc. Thalli of articulated segments, with the segment bases attenuated and the apices obtuse, often dilated or club-shaped, with secondary attachments here and there; segments generally di- or trichotomously arranged, but sometimes single or umbellate with up to six segments; segments 2 to 5 mm in width. Fronds in transverse consisting of many layers of thin-walled cells, 150-400 µm in diameter, with a cortical layer of two to four cells with abundant “gland” cells; the transition of cells from medulla to cortex is abrupt. Tetrasporangia cruciately divided, 25-30 x 37-45 µm in diameter, scattered over surface of thallus. Spermatangial conceptacles oval (‘verrucosa-type’), single or in groups of two or three cavities. Cystocarps globose, non-rostrate, slightly constricted at bases, 0.8-2.0 µm in diameter; gonimoblasts consisting of many small cells, with a persistent fusion cell; pericarp thick, consisting of two kinds of cells, six to eight elongate cells in outer layer, five to eight rounded cells in inner layer; absorbing filaments lateral and upper; carpospores spherical, 16-24 µm in diameter.
Yellow to bright orange in clear water, dark brown in muddy turbid water.
Sphaerococcus salicornia was first described by C Agardh (1820) from specimens collected by Chamisso during a voyage on the Russian exploring ship Rurik. These specimens were subsequently attributed to Unalaska in the Pacific subarctic, although Chamisso himself was uncertain of their origin. When Dawson (1954) collected specimens matching the original description in Manila Harbour in the Philippines, he decided that this was most probably the type locality, since the Rurik had spent six weeks there and G. salicornia is a tropical species unrecorded from cold-temperate waters.
G. salicornia is widely distributed throughout the tropics in warm temperate seas around the Indian Ocean, and in the central western Pacific.
It is not known from the Atlantic and prior to its introduction to Hawaii, it was not known from the central Pacific Basin east of Micronesia.
Although species of Gracilaria are distributed throughout the world’s warmer waters, the highest species richness is found in the western Pacific, where 46 of the 110 published species are known to occur (Oliveira and Plastino, 1994). Nine species of Gracilaria are known in the Hawaiian flora, of which six are native (Abbott, 1999).
Withell et al. (1994) recorded G. salicornia from western, northern and eastern seaboards of Australia. Re-examination of the Western Australian records led Millar and Xia (1997) to conclude that these represented Gracilaria blodgettii.
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.
In Hawaii, two populations of G. salicornia were known to exist on the island of Hawaii, in Hilo Bay and Kapoho, before 1950 (Smith et al., 2002). The origin of these populations is uncertain, but a possibility is unintentional transport by shipping into Hilo Bay from the Philippines in the early twentieth century (Smith et al., 2004). In April 1971, the species was transported intentionally from Hilo to Waikiki, and in September 1978 to Kaneohe Bay on Oahu for aquaculture projects that were later abandoned (Russell, 1992; Smith et al., 2002). In the 1980s, the species was translocated from Oahu to Puko’o fishpond on the island of Molokai, where a second Gracilaria species, Gracilaria parvispora, was being cultivated (Smith et al., 2002).
Smith et al. (2002) comment that, in Hawaii, G. salicornia had the most discontinuous distribution of all the non-indigenous and invasive macroalgae they examined. At most sites where it was found, it was highly dominant over a distinct area. For example, at the time of their study, it was very common in the south of Kaneohe Bay, but absent from the north, and at Waikiki it was dominant in front of the Aquarium, but absent from adjacent sites such as Al Moana Beach Park or at Kahala. Smith et al. (2002) suggest that, once introduced, G. salicornia may have the ability to spread laterally within a site and become locally dominant, but does not have great success at dispersing over larger distances or between sites or islands, at least within the two decade timeframe from first introduction. However, subsequent to these 1999 surveys, new populations have been documented suggesting that the species may be becoming increasingly successful on Hawaii reefs (Smith et al., 2004).
Vessel traffic is the suspected vector for the original introduction of G. salicornia to the Hawaiian Islands from the central western Pacific. There are no reported observations of the species as fouling on vessel hulls, but it is feasible that the species itself could grow on a heavily fouled structure. Movement between Hawaiian Islands has been a consequence of human movement for aquaculture.
Gurgel et al. (2006) compared rbcL-rbcS (chloroplast) and cox2-cox3 (mitochondrial) spacers in G. salicornia from Hawaii with populations from a wide geographic range across the Western Pacific Ocean. They found that the Waikiki invasive population appeared to have multiple sources of independent introduction with haplotypes also present in the other countries surveyed: Malaysia, Micronesia, Indonesia, Thailand, Japan, Guam and the Philippines. However, results from both markers showed a high frequency of haplotypes from Guam, the Philippines and Japan in the Waikiki population. The authors suggest that this could represent the formation of a genetic “super invasive”, where invasive populations have higher genetic diversity than native populations as a consequence of multiple events of introduction from isolated sources.
The sexual life history of G. salicornia has a triphasic alternation of generations, with isomorphic tetrasporophytes and monoecious gametophytes.
Sexual reproduction was not observed in the field in Hawaii, and vegetative growth and reproduction by fragmentation was therefore considered the primary mode of reproduction. The species fragments easily, and any sort of physical disturbance can generate fragments, including wave action, fish bites and trampling (Smith et al., 2004). However, fragments are quite heavy and tend to sink rapidly (Smith et al., 2002), but can remain viable after more than 6 hours dessication, enabling beach wrack to disperse and regrow if washed back off the beach (Smith et al., 2004). Vegetative growth and reproduction would facilitate localized, lateral spread of the species, with the short dispersal range of fragments limiting wider spread (Smith et al., 2002). Fragments may also be spread by transport through the guts of herbivorous fishes, urchins, or sea turtles (Russell and Balazs, 1994).
Physiology and Phenology
The unique mat-forming morphology of G. salicornia is considered to provide physiological adaptations that allow the species to tolerate a wide range of light environments while also allowing monopolizing nutrients that may seep from underlying sediments (Beach et al., 1997; Larned, 1998; Smith et al., 2004).
Tests in Hawaii demonstrated that herbivorous fishes preferred the native Gracilaria coronopifolia as a food source over the non-indigenous G. salicornia (Smith et al., 2004). All of the species of acanthurids tested showed a consistent preference for the native Gracilaria species, but juvenile scarids, although consuming far less algal biomass than any other group, did not demonstrate a preference for either alga.
Of the nine aldelphoparasitic species from the genera Congracilaria, Gracilariocolax and Gracilariophylla reported from various regions of Asia, five have been found growing on G. salicornia (Terada et al., 1999).
In Hawaii, G. salicornia can monopolise space and displace native species (Schaffelke and Hewitt, 2007). In Waikiki, G. salicornia became the single-most dominant benthic species in an area that before invasion supported a community with more than 60 species of macroalgae (Doty, 1969; Smith et al., 2004).
G. salicornia is an agarophyte so is potentially a commercial source of agar. However, G. salicornia collected from rocky shores in Malaysia was found to have poor quality agar of 345 g/cm2 gel strength and maximum agar yield of 10% dry weight (Phang, 1994).
In the Philippines, G. salicornia is eaten in salad form, as well as dried as raw material for agar extraction. Although very common, it is not a major contributor to the harvestable biomass (Trono et al., 1983). In Hawaii, when there is a shortage of cultured Gracilaria species for food (G. parvispora, G. tikvahiae) and relatively no “wild” G. coronopifolia available, G. salicornia is often used as a substitute (Abbott, 1999). Abbott comments that it might be used more widely because many like its “crunchiness” but, despite its spread on Oahu, it was still not common enough to have become one of the choice edible species. In late 1997 it was found under the common name “robusta” in a seafood store in Honolulu.
The total dietary fibre content measured in Gracilaria salicornia from Hawaii was ~36 %, toward the lower end of a total dietary fibre values (ranging from 23.5 to 59.8%) measured in 26 macroalgal species eaten by people and marine herbivores in Hawaii (McDermid et al., 2005). Further nutritional analyses found G. salicornia to have protein values above 10% of dry weight and the highest crude lipid levels and beta carotene concentration of the four Gracilaria species analysed (McDermid and Stuercke, 2003).
Subgenera of Gracilaria are largely defined by the location of spermatangia and structure and distribution of spermatangial conceptacles. In this regard, G. salicornia falls within the subgenus Gracilaria, in which spermatangia cover the entire inner surface of deep pot-shaped conceptacles (Tseng and Xia, 1999). Analysis of the rbcL gene in Gracilariaceae aligned G. salicornia with G. canaliculata and G. arcuata and this group was nominated as sub-group IV of the Gracilaria sensu stricto (Gurgel and Fredericq, 2004).
Within both the genus and subgenus, G. salicornia is one of very few species in which branches are articulated, and the species is unique in having claviform articulations, varying in width along their length. In G. articulate,the articulations are cylindrical and of uniform width (Tseng and Xia, 1999). G. canaliculata does not have articulated branches and the thallus surfaces have many whitish translucent spots caused by clusters of hair cells (Withell et al., 1994). The colour is light to dark rose, unlike the yellow to orange of G. salicornia, and branching is subdichotomous (Iyer et al., 2005).
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.
Manual removal trials (Smith et al., 2004) suggest that eradication is not possible unless the very first colonizing plants are detected and removed.
Manual removal was trialled as a means of control in Hawaii (Smith et al., 2004). Before removal G. salicornia represented close to 48% of the benthic cover on the reef. Immediately after removal the cover was reduced to <1%, but increased to almost 13% within two weeks and over 20% after 4 weeks. Removal took an average of 415 minutes/m2. When clearing plots, only about one tenth of the time is spent removing the large conspicuous biomass, with the remainder removing small basal attachment points with tweezers.
In a volunteer-based removal effort at Waikiki, over 20,000 kg of G. salicornia was collected during 5 clean-up efforts, each lasting 4-5 hours (Smith et al., 2004). Over 400 volunteers contributed approximately 2000 person-hours to these clean-ups.
Manual removal was considered by Smith et al. (2004) to be the most feasible control strategy available. However the technique is extremely time-consuming (6.9 person-hours/m2) and the alga regrows rapidly from any small basal parts that are not removed. Removal activity can also generate fragments that can disperse and regrow.
Only extreme temperature and salinity reduced growth rates of G. salicornia in experiments conducted in Hawaii (Smith et al., 2004). Growth rates were positive at temperatures of 8, 16, 27 and 34oC and salinities of 0, 17 and 34 ppt, but negative at 41oC and at 50 and 75 ppt. Both high and low herbicide, and high algicide treatments reduced growth rates, but not low algicide treatment.
However, implementation of any of these treatments in the field are considered impractical because of the collateral damage that would occur in other reef organisms, especially corals (Smith et al., 2004).