Anatanais Nordenstam, 1930

Bird, Graham J, 2008, Untying the Gordian Knot: on Tanais novaezealandiae Thomson (Crustacea, Tanaidacea, Tanaidae) from New Zealand, with descriptions of two new Zeuxoides species, Zootaxa 1877, pp. 1-36 : 6-7

Anatanais Nordenstam, 1930

Species included ( New Zealand and Australasian species in bold): Anatanais lineatus (Nordenstam) , A. pseudonormani Sieg.

Anatanais novaezealandiae sensu Sieg (1980) is incertae cedis with respect to Anatanais or Zeuxo but new material is required to help establish a formal new name and classification.

Remarks: Anatanais was established by Nordenstam (1930) as a subgenus for those species of Tanais with a six-segmented pleon (i.e. five free pleonites and pleotelson), although this became a synonym of Hexapleomera Dudlich, 1931 , erected for the same reason. Its first use as a full genus was by Stephensen (1936) while Zeuxo (an old name from the 19th Century) was brought back into use by Lang (1967) as a senior synonym for Anatanais and Hexapleomera . The last genus now resides in the tribe Pancolini , principally because of the reduction in size of the pleopod-3.

As used by Sieg (1980) Anatanais was re-established as a full genus but was restricted to species whose antennular article-1 was no more than twice as long as article-2. However, this was not strictly adhered to by Sieg, whose taxon A. novaezealandiae has a ratio of 2.5. This so-called diagnostic character is naturally highly dependent not only on the length and proportions of the first article but also the second. While it is clear that it is article- 2 in A. lineatus and A. pseudonormani that is relatively longer and more slender than in most Zeuxo species, in A. novaezealandiae sensu Sieg the first article is relatively short, as is article-2. Of the Zeuxo species described (see below), the ratio in females actually ranges between 1.46 and 3.67 (based on measurements from published drawings) and only four species have a ratio of ca. =3.0 ( Z. angua , Z. cloacarattus , Z. coralensis and Z. holdichi ). A distinct coxal process on pereopod-1 is also present in Anatanais species although they are not restricted to this genus and are found in various other species of Zeuxo (Zeuxo) , Zeuxoides and Synaptotanais (sub-family Tanainae ). A further given distinction of Anatanais is that the species exhibit weak sexual dimorphism.

Allometric effects are also closely relevant and morphometric analysis of the Zeuxo species linked to Tanais novaezealandiae Thomson (see below) shows that the antennule article 1:2 ratio changes with both growth and sexual differentiation, moving it from ‘ Anatanais’ to ‘ Zeuxo’ using the antennule ratio. Using this character, it appears as if A. novaezealandiae sensu Sieg should be classified within Zeuxo , leaving the two other species of Anatanais as the only representatives of this questionable genus.