Macrostylis G.O. Sars, 1864

Riehl, Torben & Brandt, Angelika, 2010, Descriptions of two new species in the genus Macrostylis Sars, 1864 (Isopoda, Asellota, Macrostylidae) from the Weddell Sea (Southern Ocean), with a synonymisation of the genus Desmostylis Brandt, 1992 with Macrostylis, ZooKeys 57, pp. 9-49 : 10-13

Macrostylis G.O. Sars, 1864

Macrostylis Sars 1864, p.13; 1899, p. 120; Beddard 1886b, p. 173; Hansen 1916, p. 75; Barnard 1920, p. 411; Wolff 1956, p. 99-106; 1962, p. 91-93; Menzies 1962, p. 127-133; Birstein 1963a, p. 95-106; Mezhov 1988, p. 60-69; Brandt 1992, p. 74-78; Vey & Brix 2009, p. 358

Vana Meinert 1890, p. 195

Desmostylis Brandt 1992, p.70, Figs 11-13

Composition.

See Table 2.

Type species.

Macrostylis spinifera Sars, 1864

Diagnosis.

As of the family.

Remarks.

Some characters are absent or poorly illustrated in original descriptions and could not be analysed thoroughly during this study. Therefore these have not been included in the family diagnosis (e.g. presence of exopod of pleopod 5, setation patterns and setal substructures).

Following the original description ( Brandt 1992), Desmostylis can be separated from Macrostylis by the following characters: absence of dorsal triangular expansion on the pereopod 3 ischium, lack of dactylus on pereopod 4 and of claws on pereopods 5-7. However, after comparisons of different species of Macrostylis , and re-examination of the holotype of Desmostylis obscurus Brandt, 1992 these characters have been found not to be delimitating Desmostylis from Macrostylis . Shape and extension of pereopod 3 ischium varies greatly between species of Macrostylis . For example Macrostylis galatheae Wolff, 1956 (p. 101, Fig. 17) has a strong and acute extension and no extension is present in Macrostylis abyssalis Brandt, 2004 (p. 28, Fig. 15), very similar to Desmostylis obscurus . However, another described species of Desmostylis , Desmostylis gerdesi Brandt, 2002 shows a strongly fossorial pereopod 3 bearing a prominent dorsal extension on the ischium. Thus, the condition of this character in Desmostylis lies within the interspecific range of variation in Macrostylis . Therefore, this character is not usable to maintain the genus Desmostylis .

The absence of a pereopod 4 dactylus was another generic character of Desmostylis . However, the dactyli in Desmostylis obscurus are not absent but have been overseen in the original description and they were distorted in the pereopod illustrated of the holotype. The value of differences in setal counts or occurrence of types of setae on pereopod 3 ischium for discriminating between two genera is not known. The genus Desmostylis was erected on basis of a juvenile specimen. Using ontogenetically variable characters of a juvenile type specimen to erect a new genus is problematic.

Dactylar setae fit the definition of claws ( Wilson 1989): terminal/subterminal modified setae on pereopodal dactyli with pronounced sclerotisation and a sharp tip. This definition was to the authors’ knowledge not narrowed since then. Taking into account the incomplete documentation of dactylar claws in the literature on the one hand, and insufficient knowledge about plasticity of setae and their substructures on the other, a differentiation of genera exclusively based on such characters seems problematic. Due to the above listed reasons we consider the genus Desmostylis synonymous with Macrostylis .