Ischnomesus Richardson, 1908

Merrin, K. L. & Poore, G. C. B., 2003, Four new species of Ischnomesidae (Crustacea: Isopoda: Asellota) from off south-eastern Australia, Memoirs of Museum Victoria 60 (2), pp. 285-307 : 289-290

Ischnomesus Richardson

Ischnosoma Sars, 1868: 34 (preoccupied).

Ischnomesus Richardson, 1908: 81 .— Hansen, 1916: 56.— Gurjanova, 1932: 40.— Wolff, 1956: 88.— Menzies, 1962: 111.— Wolff, 1962: 73.— Menzies and George, 1972: 971.— Kussakin, 1988: 419 (replacement name).

Rhabdomesus . — Richardson, 1908: 81.

Bactromesus . — Wolff, 1962: 83.

Type species. Ischnosoma bispinosum Sars, 1868 (by monotypy). Diagnosis. Pereonites 5–7, pleonite 1 and pleotelson free and articulating. Remaining pleonites fused with pleotelson. Antenna 1 of at least 6 articles. Mandible palp usually absent. Maxilliped palp width at least equal with or wider than basal endite; articles 2 and 3 expanded. Carpus of pereopod 1 inferior margin usually expanded proximally. Stylet of male pleopod 2 either longer or shorter than sympod, if longer, can be whip like. Uropod long and of 2 articles (except Ischnomesus justi sp. nov. —with 1 article).

Remarks. The presence of a mandibular palp, the ancestral state in isopods, is confined to five species of Ischnomesus (namely I. justi sp. nov., I. roseus Wolff, 1962 , I. simplissimus Menzies, 1962 , I. calcificus Menzies and George, 1972 and I. carolinae Chardy, 1974 ), being absent in all other species in the family. The presence of a mandibular palp has never been considered as a character to remove these species from the family or even the genus Ischnomesus . A preliminary phylogenetic analysis placed those species with a palp within the ‘ Ischnomesus clade’ (unpubl. data).

In describing Ischnomesus, Hansen (1916) stated that the uropods are of two articles. In Ischnomesus justi the uropod is of one article. This character state has never been reported in this genus and, while such uropod characters are usually considered to be of generic significance (for example see Wolff, 1962), I. justi has free and articulating pereonites 5–7 and pleonite 1. This is the principal diagnostic of this genus. For further discussion, see Remarks under I. justi below.