Gambierdiscus toxicus
Adachi et Fukuyo, 1979
Plate 20, Figs. 1-6

Species Overview: Gambierdiscus toxicus is an armoured, marine, benthic dinoflagellate species. It is a toxic species that was discovered attached to the surface of brown macroalgae in the Gambier Islands, French Polynesia.

Taxonomic Description: Species in this genus are anterio-posteriorly compressed and are observed in apical or antapical view. The epitheca and hypotheca are not noticeably different in size. A distinguishing feature is the shape and size of the apical pore complex (APC) (Fig. 1) (Faust 1992).
Cells of Gambierdiscus toxicus are large, round to ellipsoid (Figs. 1, 2, 4, 5), and flattened anterio-posteriorly. The epitheca and hypotheca are nearly equal in height. The cell surface is smooth with numerous deep and dense pores (Figs. 1, 3). Thecal plates are very thick. Cells range in size from 24-60 µm in length, 42-140 µm in transdiameter, and 45-150 µm in dorso-ventral depth (Adachi & Fukuyo 1979).

Nomenclatural Types:
Holotype: Gambierdiscus toxicus Adachi and Fukuyo, 1979: figs. 1-7
Type Locality: South Pacific Ocean: Gambier Islands, French Polynesia
Synonyms: Diplopsalis sp. Yasumoto et al., 1977

Thecal Plate Description: The plate formula of Gambierdiscus toxicus is: Po, 3', 7'', 6c, 8s, 5''', 1p, 2'''' (Faust 1995). The apical pore plate (Po) is oval to ellipsoidal with a characteristic fishhook shaped apical pore (Figs. 1, 3), the opening of which is always oriented ventrally. Apical plate 2' is subrectangular and is the largest of the three apical plates (Figs. 1, 6) (Adachi & Fukuyo 1979). The epitheca is slightly indented ventrally (Figs. 1, 4). The hypotheca is deeply excavated ventrally (Figs. 2, 5, 6) (Adachi & Fukuyo 1979; Fukuyo 1981; Taylor 1979).
In the hypotheca the postcingular plate 1''' is triangular; its right corner extrudes, curves inside, and contacts antapical plate 1'''' (Figs. 2, 6) (Adachi & Fukuyo 1979; Fukuyo 1981). The posterior intercalary plate (1p) is broad and pentagonal (Figs. 2, 6). During cell division the sutures widen, the 1p plate changes its shape to rhomboid (Fukuyo 1981).
The cingulum is circular, narrow and deeply excavated, and ascends slightly (Adachi & Fukuyo 1979; Bagnis et al. 1979; Taylor 1979). The cingular wall consists of six plates and measures nearly 5 µm in width. It is bordered by a low, thick ridge which is made up of the folding of pre- and postcingular plates (Figs. 1, 4) (Adachi & Fukuyo 1979).
The sulcus is short, deeply concave and pouch-like, and is oriented to the right (Figs. 2, 5) (Adachi & Fukuyo 1979; Bagnis et al. 1979; Taylor 1979). Along the sulcal margin, an overhanging ridge continues along the edge of postcingular plate 5''', and antapical plates 1'''' and 2'''' (Fig. 2) (Adachi & Fukuyo 1979).

Morphology and Structure: G. toxicus is a photosynthetic species with yellow to golden-brown chloroplasts and a large crescent-shaped nucleus (Fig. 5) (Adachi & Fukuyo 1979).

Reproduction: G. toxicus reproduces asexually by binary fission.

Ecology: Cells of G. toxicus are frequently found as epiphytes on macroalgae and dead coral. Different strains apparently exhibit a preference for certain algae; e.g. the Hawaiian strain prefers the red alga Spyridia filamentosa (Shimizu et al. 1982). Cells readily attach to substrates via mucoid strands originating from the sulcal area (Steidinger & Tangen 1996).

Toxicity: G. toxicus is known to produce the following toxins: ciguatoxin (Yasumoto et al. 1987; Murata et al. 1990; Yasumoto et al. 1993); gambieric acid (Yasumoto et al. 1993); and maitotoxin (Yasumoto et al. 1977; 1993; Yokoyama et al. 1988).

Species Comparison: This species resembles Heteraulacus in tabulation, but differs by its right-handed girdle torsion, large apical closing plate, and a pouch-like sulcal depression (Taylor 1979). Gambierdiscus toxicus shares a number of characteristics with G. belizeanus. They both have the same plate formula, and have similar apical pore, cingulum, sulcus, general cell shape (lenticulate and antero-posteriorly compressed), and golden brown chloroplasts. However, they differ in a number of distinct features. Architecturally, both species have similar epithecal plates, but differ in thecal surface morphology: G. toxicus has a smooth surface with scattered fine pores, whereas G. belizeanus has a deeply areolated surface. G. toxicus is considerably larger than G. belizeanus. And plate 1p is broad in G. toxicus, whereas it is long and narrow in G. belizeanus (Faust 1995).

Etymology: The genus 'Gambierdiscus' was named after the Gambier Islands from which it was discovered and also the discoid shape of the cell. The species name 'toxicus' is derived from the toxin-producing nature of this species.

Habitat and Locality: This species was identified from tropical reefs in the Pacific Ocean (Adachi & Fukuyo 1979; Fukuyo 1981), the Indian Ocean (Quod 1994), and the U.S. Virgin Islands (Carlson & Tindall 1985). Populations have been found in tidal pools and lagoons, as well as in colored sand, in the Caribbean (Faust 1995). In the United States, G. toxicus has been collected in waters around Hawaii (Taylor 1979; Shimizu et al. 1982) and the Florida Keys (Bergmann & Alam 1981; Besada et al. 1982; Loeblich & Indelicato 1986).

Figure 1: Morphology of a Dinoflagellate

Glossary

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