Microxina charcoti Topsent, 1916 (Fig. 7).
Synonymy. Microxina charcoti Topsent, 1916: 170; 1917: 73, pl.1, fig. 3, pl. 6, fig. 17; [in part] Burton, 1932b: 271, pl. 50, fig.1 (under M. benedeni (Topsent, 1901a)).
Material examined. Lectotype (here designated): MNHN LBIM DT 226-Antarctica, near Terre Alexandre, Baie Marguerite, 200-297 m. Paralectotypes: MNHN LBIM DT 392, 216, 217, 223, 316, 317 (BMNH 1926.10.26.338 slides) – stn. 8, Man-O-War Anchorage, Funchal, Madeira. 0-25 m, ‘Terra Nova’ collection. Other comparative material. Holotype of Microxina benedeni (Topsent, 1901a)(Fig. 7D): IRSNB 24 POR MOD II- Antarctica, 70°20’ S, 83°23’W, 29.11.1898, ‘Belgica’ 1897-99, no. 725.
Description. Tubular and ramified from a narrow fibrous base (Fig. 7A), with a deep central aquiferous cavity ending in a large oscule of the same diameter. Surface strongly spined by long surface spines up to 10 mm long (Fig. 7B). Ectosomal skeleton is a tangential network of oxeas interrupted by the terminal tufts of longitudinal primary fibres subdivided to form the surface spines (Fig. 7B). Choanosomal skeleton with large and irregular meshes delimited by fasciculate, longitudinal primary multispicular fibres, containing more than 15 spicules, packed compactly (Fig. 7C) and issued from the base of the sponge, 296-426-800 μm in diameter. Primary fibres may be irregularly split to form secondary fibres, 41-90-148 μm in diameter. Meshes contain abundant microscleres and free oxeas. No visible spongin. Microscleres: microxeas, 45-73-95 μm long x 3-4 μm in diameter.
Remarks. Topsent (1917) included his new genus Microxina in the Gelliinae, and made a comparison between Microxina and Strongylophora. This genus was included in Gelliinae by Dendy (1905), and Topsent (1917) considered that bother genera were close due to the common possession of microxeas, and because of the fibrous character of their skeletons. At the same time he also remarked that both genera should probably be included with the axinellids. Later, in the same work, he gave up this idea because Strongylophora was (in his opinion) less fibrous than Microxina, and because of the existence of other Gellinae with a spined surface (e.g., Gelliodes). We consider that both genera are sufficiently different in their skeletal structures to be valid. Burton (1932b) considered that Gelliodes benedeni and Microxina charcoti were subjective synonyms, due to their external form and spiculation, with the only notable difference being that G. benedeni had only sigmas (and additional raphides in var. fortior). By comparison, Microxina had only microxeas although Burton (1932b) also observed raphides. Wiedenmayer (1989) regarded Hemigellius as a subjective synonym of Microxina. We also compared both genera and observed significant differences between their respective type species, such as the presence (in Hemigellius rudis, sensu Kirkpatrick, 1908c) of paniculate primary fibres and subectosomal ‘embryonary holes’, and differences in the continuous fibres or tracts observed in Microxina charcoti. In addition, we found that the structure of the choanosomal skeleton was compact and disorganized in Hemigellius rudis, but clearly fibrous in Microxina charcoti. The external morphology, consistency and surface skeleton are also different: Hemigellius is massively lobate or thickly subcylindrical, whereas Microxina is tubular, to massive or globular (e.g., M. benedeni; Fig. 7D). Hemigellius has a clear surface palisade or a finely hispid surface, whereas in Microxina there is a fine membrane between the solid surface spines producing a strongly spiny surface.
(Hooper and Van Soest, 2002: 885-886)
