Haplomesus Richardson, 1908

Kavanagh, Fiona A., Wilson, George D. F. & Power, Anne Marie, 2006, Heterochrony in Haplomesus (Crustacea: Isopoda: Ischnomesidae): revision of two species and description of two new species, Zootaxa 1120, pp. 1-33 : 5-7

Haplomesus Richardson, 1908

Haplomesus Richardson, 1908: 81 ; Hansen, 1916: 59; Gurjanova, 1932: 42; Birstein, 1960: 6; 1963: 59; 1971: 209; Menzies, 1962: 117; Wolff, 1962: 86; Menzies & George, 1972: 973; Kussakin, 1988: 445.

Not Haplomesus . Merrin & Poore, 2003: 286 .

Type species: Haplomesus quadrispinosus ( Sars, 1879) .

Species included: Haplomesus angustus Hansen, 1916 ; H. bifurcatus Menzies, 1962 ; H.

biscayensis Chardy, 1975; H. brevispinis Birstein, 1960 ; H. concinnus Birstein, 1971 ; H. consanguensis Mezhov, 1980 ; H. corniculatus Brökeland & Brandt, 2004 ; H. cornutus Birstein, 1960 ; H. formosus Mezhov, 1981 ; H. gigas Birstein, 1960 ; H. gorbunovi Gurjanova, 1946 ; H. insignis Hansen, 1916 ; H. orientalis Birstein, 1960 ; H. modestatenuis Menzies & George, 1972 ; H. modestus Hansen, 1916 ; H. ornatus Menzies, 1962 ; H. profundicola Birstein, 1971 ; H. quadrispinosus ( Sars, 1879) ; “ H. quadrispinosus” sensu Brandt, 1992 ; H. robustus Birstein, 1960 ; H. scabriusculus Birstein, 1960 ; H. tenuispinis Hansen, 1916 ; H. thomsoni ( Beddard, 1886) ; H. tropicalis Menzies, 1962 ; H. zuluensis Kensley, 1984 . Excluded species: H. franklini Merrin & Poore, 2003 (incertae sedis).

Diagnosis. Pereonites 5–7, pleonite 1 and pleotelson lacking intersomite articulations. Pereonites 5–7 narrowing posteriorly. Antennulae with 5 or 6 articles, distal flagellar articles at least twice as long as wide. Pereopod I carpus without ventral expansion of palm. Maxilliped palp narrower than basal endite, articles 2 and 3 expanded. Uropods uniramous, single segmented, distally tapering. Mandible palp absent.

Remarks. Many Haplomesus species have thin, attenuated bodies, although a few species such as H. robustus Birstein, 1960 are rather more heavy bodied and Heteromesus ­ like. Species in the genus exhibit a great variety of spination, and none is completely lacking spines anywhere on the body. A few other characters may be characteristic of the genus but are not illustrated in most species. For example, the species that we have examined have a distinct thin neck between the pereopodal articulation of the basis and coxa and the shaft of the basis, with the basis neck and shaft forming an approximate right angle. This character, but with an added spinose shoulder, is also found in several species of Heteromesus ( Cunha & Wilson personal communication). We exclude H. franklini Merrin & Poore, 2003 from the genus because its uropod does not match the current diagnosis, in being elongate and biarticulate. Merrin & Poore (2003) state that the presence of biarticulate uropods is treated as a specific autapomorphic character, but this view argues for an ad hoc reversal of a more general character. The uropod is more parsimoniously interpreted as plesiomorphically biarticulate, with the transition to the uniarticulate conical form as a synapomorphy of the genus Haplomesus . The inclusion of H. franklini into the genus, therefore, substantially weakens its definition. They also indicate that the fusion of pereonites 5–7 with the pleonites and pleotelson, and the stylet of male pleopod II not extending beyond the sympod are key synapomorphies for the genus. The stylet character is not likely to be apomorphic, given the variation seen in the other species of the family (both long and short forms can be found). Ultimately a cladistic analysis could arbitrate the apomorphic status of these characters, but none is published to support these assertions. On the pleotelson of H. franklini , the uropods project from a raised part of the posterolateral margin, whereas the uropods of all Haplomesus species sit in a concavity, and the pleonite 1–2 region of H. franklini is longer and somewhat more inflated than seen in Haplomesus . In the absence of an empirical test of these assertions, H. franklini should be not included. Nevertheless, we use a broad definition of Haplomesus ; the missing last pereonite alone is not sufficient evidence or justification for creating a new genus (see discussion below).

A few Haplomesus species have been given broad distributions by some authors, despite most species being known from fairly narrow ranges. Notably Menzies (1962) ascribed a Mediterranean­Caribbean distribution to his species H. tropicalis (discussed below). More recently, H. quadrispinosus was reported in the South Atlantic near Antarctica, but Brandt's (1992) illustrations show that this is a different species from Sars' species. The Brandt species should be compared with H. bifurcatus Menzies, 1962 and other species with an indented pleotelson axis. Although most ocean basins have insufficient sampling, our experience (e.g. the undescribed species shown in Fig. 1) shows that each species have distributions limited to basins or smaller regions.