Chroodactylon ornatum Basson 1979

Chroodactylon ornatum (Figs 1 A–B)

Conferva ornata Agardh (1824: 104).

Thallus erect, filamentous, uniseriate, having pseudodichotomous ramifications, greenish colored, up to 1.7 mm in height, fixed by a lobed basal cell. Filament diameter (25–)30(–35) µm in the basal region, (15–)21(–25) µm in the median region, and 15 µm in the apical region. Vegetative cells square to sub-square, (10–)13(–15) µm in diameter, each with a star-shaped chloroplast with a conspicuous central pyrenoid; wrapped in a mucilaginous sheath 2.5–5.0 µm thick. Apical cell cylindrical with rounded apex, longer than width, (15.0–)18.5(–25.0) µm long and 10.0– 12.5 µm in diameter.

Representative specimens examined:— BRAZIL. Bahia: Ilha de Itaparica, Vera Cruz, Praia da Penha, 25/ XI/2007, Santos (HUEFS 130889).

Geographic distribution along the west coast of Atlantic Ocean:—North Carolina, Florida, Isla Mujeres, Yucatán Peninsula, Aruba, Bonaire, Bahamas, Bermuda, Cuba, Grenada, Hispaniola, Virgin Islands, Venezuela (Howe 1920, Taylor 1960, Vroman & Stegenga 1988, Ganesan 1989, Schneider & Searles 1991, Suárez 2005, Mendoza-González et al. 2007, Dawes & Mathieson 2008) and Brazil (this study).

Comments:— Chroodactylon Hansgirg (1885: 14) is a widely distributed genus in tropical marine and temperate environments, estuaries, as well as fresh-water areas (Brodie & Nielsen 2005). Both the genus and the species C. ornatum are reported here for the first time as occurring in the American South Atlantic. The species had previously been recorded from the coast of Connecticut (USA) to Venezuela, including the Caribbean Islands (Wynne 2011, Guiry & Guiry 2011).

Chroodactylon ornatum has characteristics very similar to Stylonema alsidii (Zanardini) Drew (1956: 72), including a filamentous thallus, uniseriate, ramified, and a star-shaped chloroplast with central pyrenoid; it differs, however, in terms of the dimensions and shapes of its cells and the thickness of the mucilaginous sheath. Chroodactylon ornatum has generally square cells, large (ca. 15 µm in diameter), with a thinner sheath (2.5–5.0 µm) (Fig. 1B), while S. alsidii has spherical or elliptical cells, rarely square, smaller (7.5 µm diameter) and with thick sheaths (up to 25 µm) (Santos 2010). Some authors consider the color of the thallus to be a useful character for distinguishing between the two species, for the thallus of C. ornatum is greenish and that of S. alsidii is pink to vinaceous (Taylor 1960, Schneider & Searles 1991); this characteristic is easily lost, however, in fixed material (Børgesen 1915). According to Abbott (1999), the characteristic greenish coloring of C. ornatum is due to the predominance of phycocyanin in the plastids.

The specimens analyzed here had dimensions very close to those described by Howe (1920) for the Bahamas, and by Basson (1979) for Saudi Arabia. Taylor (1960), Schneider & Searles (1991) and Dawes & Mathieson (2008) described the thalli of C. ornatum as being up to 10 mm high.

Descriptions in the literature (Børgesen 1915, Howe 1920, Basson 1979, Schneider & Searles 1991, Abbott 1999, Dawes & Mathieson 2008) indicated that the branching patterns and the cell shapes of C. ornatum are variable. The thalli are described as ranging from simple filaments to densely ramified, unilateral, dichotomous, pseudo-dichotomous or irregular; the cells are described as having diverse shapes (oval, elliptical, elongated, globose, square, or sub-square), although they all are essentially the same size. The material analyzed here had pseudo-dichotomous ramifications (Fig. 1A) and filaments with squared cells, except the apical cells that were cylindrical with rounded apices.

No reproductive structures were observed in the specimens analyzed; however, according to Howe (1920), Børgesen (1927), Taylor (1960), Schneider & Searles (1991) and Dawes & Mathieson (2008), C. ornatum reproduces by way of monospores or by subglobose or ellipsoidal akinetes with thick cell walls that arise from vegetative cells in the filaments that are liberated laterally by the rupturing of the mucilaginous sheath.

Chroodactylon ornatum was considered uncommon in the present study area as it was only found once on the beach at Penha. According to Dawes & Mathieson (2008), this species is common in Florida and grows on marine phanerograms. This alga has been found as an epiphyte on many different macroalgae, including Ceramium sp., Corallina sp. and Ganonema farinosum (J.V. Lamouroux) Fan & Wang (1974: 492) (Børgesen 1915, Howe 1920, Børgesen 1927, Basson 1979, Vroman & Stegenga 1988, Schneider & Searles 1991, Abbott 1999).