Hexapleomera
Creators: name: Bamber, Roger N.
Publisher: name: Zenodo
Year: 2012
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Tanaidacea
Tanaididae
Hexapleomera

description: Species within Hexapleomera
Unfortunately, the type-material of H. robusta is lost (Sieg 1980). Sieg (1980) redescribed what he referred to as this species from material either from the Atlantic coast of Brazil, or from the Galapagos Islands, unfortunately without specifying which (presumably as he considered the specimens conspecific).
Edgar (2008) described male and female material which he attributed to H. robusta from Queensland, Australia, discussing the issues with Sieg’s material, and pointing out a number of differences in the setation, inter alia, between the Australian material and Sieg’s description, but was justifiably uncertain whether these features represented inter-specific or interpopulation differences. Taking Sieg’s (1980) analysis of interspecific differences in Zeuxo Templeton, 1840 as a reasonable yardstick for criteria within the Tanaidae (and in the light of the other material examined herein), I consider Edgar’s material to represent a distinct species, diagnosed below.
Edgar (2008) also described as new Pancoloides moverleyi, based only on males, discussing that it appeared to have attributes of both Pancoloides Sieg, 1980 and Hexapleomera, and assigning it to the former on the selfadmitted “arguably trivial grounds” of its being free-living. A significant distinction defined by Sieg (1980) between these two genera was the presence of a reduced distal uropod segment in Pancoloides but not in Hexapleomera; as Edgar’s species is without such a reduced segment, and in the light of free-living specimens of Hexapleomera found in the Eastern Mediterranean (Bamber et al. 2009 and below), this species is herein transferred to Hexapleomera.
Significant morphological features of these taxa, as well as of material from the Eastern Mediterranean and the new species from Plymouth, UK, are shown in Table 1. Of these features, the number of uropod segments (for convenience referred to here as the basis plus ramus) in adults of the Tanaidae is stable and species-specific (e.g. Bamber 1990; Edgar 2008). As all previous descriptions of Hexapleomera robusta have demonstrated four uropod segments in total, the presence of five uropod segments in adults of two of the taxa analyzed herein is considered significant. Equally, the number of inner setae on the pleopod endopod is stable and species-specific, one seta in all but one of the present taxa, but two in H. robusta sensu Edgar 2008. The third pleopod of Hexapleomera shows slight reduction in setation: the inner seta of the basis is absent in most taxa, but present in both Australian taxa herein.
Taxon H. robusta H. robusta H. robusta H. robusta Pancoloides sensu Moore, sensu Sieg, sensu Bamber sensu Edgar, moverleyi 1894 1980 et al., 2009 2008 Edgar, 2008 The lack of an apophysis on the coxa of pereopod 1 is cited as genus-diagnostic by Sieg (1980), and used by him as a species-characterizing feature of Zeuxo (inter alia); however, such an apophysis is clearly present in three of the present taxa.
The distal setae on the maxillule palp are prone to detachment on dissection, but all recently-described taxa herein have only four or five such setae, compared with eight in H. robusta sensu stricto, a difference too consistent and too great to be attributable to occasional detachment. The distal setae on the maxilliped coxa and basis are not so prone to detachment, and the differences found here (three or two, and two or one respectively) are considered to be real. The presence or absence of small spinules on the maxilliped endite is also a relevant character.
Edgar (2008) found the number of aesthetascs present on antennulae to be a useful distinguishing feature in this family: they are considered here as supporting characters, although I know of no study of intraspecific variation in this feature.
While the length:width ratio of pereonites 1 to 3 together is diagnostic of the genus, some of the present taxa are less compact than others (although all have this ratio <1).
The tooth-like proximal apophysis on the fixed finger of the chela of the male is shown as conspicuous by Sieg (1980; see also Dollfus 1898; Dudich 1931), who also indicates such an apophysis in the female; no other Hexapleomera taxon is shown to have such a conspicuous apophysis. Interestingly, Moore (1894) gives an elaborate description of the distal end of the cutting edge, but neither describes not figures such a tooth-like apophysis in either gender. Conversely, spinulation on the cutting edge of the chela dactylus is shown by all descriptions except for that of Pancoloides moverleyi (vide Edgar 2008), and is considered herein to be significant for that species.
description: Published as part of Bamber, Roger N., 2012, A re-assessment of Hexapleomera Dudich, 1931 (Crustacea: Tanaidacea: Tanaidae), with designation of three new species, pp. 51-70 in Zootaxa 3583 on pages 52-54, DOI: 10.5281/zenodo.283096