Arachnidium fibrosum Hincks, 1880

(Fig. 2)

Arachnidium fibrosum Hincks 1880: 511, pl. 71, figs 6, 7; Marcus 1938: 51, pl. 12, fig. 29A, pl. 11, fig. 29B; Marcus 1941: 27; Prenant & Bobin 1956: 226, fig. 99; Hayward 1985: 82, fig. 24; Vieira et al. 2008: 10; De Blauwe 2009: 60, figs 35–37.

Material examined. MACN-In 32532, Monte Hermoso, September 23, 1981, collected by Claudia Bremec. MACN-In 42251, Ingeniero White harbour, September 4, 1984, 5 m, collected by Daniel Martínez. MACN-In 43869, Ingeniero White harbour, June 22, 2016, collected by Sandra Fiori.

Description. Colony uniserial, adnate, branching, forming a dense network on the substrate. Autozooids expanded distally, irregular in outline, with a slender, tubular, proximal portion, producing up to eight buds. Orifice subterminal, on a papillate peristome. Buds consist of long, bifurcating cylindrical stolons which anastomose irregularly to form a network, sometimes giving rise to stellate bodies that seem to be aborted autozooids. Short, characteristic tapering filaments are common around the margins and on the frontal surface of the autozooids.

Remarks. Due to its disjunct geographic distribution (see below), this species could be regarded as cryptogenic, i.e. a species that is not demonstrably native or exotic (Carlton 1996) in Argentina. This study expands its geographic range southwards by about 13 degrees of latitude, from Parana State (Brazil) to Monte Hermoso (Buenos Aires Province).

Distribution. Europe (Hincks 1880; Prenant & Bobin 1956; Hayward 1985; De Blauwe 2009), Atlantic coast of United States (Maturo 1968), Brazil (Marcus 1938, 1941; Vieira et al. 2008), Buenos Aires Province (this study).