Heteromesus Richardson, 1908

Cunha, Marina R. & Wilson, George D. F., 2006, The North Atlantic genus Heteromesus (Crustacea: Isopoda: Asellota: Ischnomesidae), Zootaxa 1192 (1192), pp. 1-76 : 10-11

Heteromesus Richardson, 1908

Heteromesus Richardson 1908: 81 ; Hansen 1916: 66; Gurjanova 1932: 44; Wolff 1956: 141; Menzies 1962: 121; Wolff 1962: 84; Birstein 1971: 200; Chardy 1974: 1543; Kussakin 1988: 472.

Type species. Heteromesus granulatus Richardson, 1908

Diagnosis Pereonite 5 freely articulated with pereonite 6, articulations not expressed between pereonite 6, pereonite 7, pleonite 1 and pleotelson. Pereonite 4 produced posteriorly but always broader than long (L/W less than 0.70 in female and 0.85 in male). Pleotelson dorsal surface axial ridge strongly vaulted, separated from lateral fields by distinct indented lines; posterolateral margin at uropod insertion produced from margin. Antennula article 1 globular, article 2 inserting dorsally; article 2 strongly curved anteriorly at proximal insertion; articles distal to article 2 reduced to 1–3 articles, distal articles altogether tiny, length less than 0.3 article 2 length. Mandible palp absent. Pereopod bases with conspicuous neck connecting shaft to coxal articulation, proximal part of shaft projecting dorsally into shoulder. Pereopod I carpus distally expanded, widest point distal to midpoint of carpus, with 1 elongate robust seta and a proximal shorter robust seta. Pleopod II female operculum with narrow proximal neck, almost circular posteriorly, with plumose setae. Pleopod III exopod with plumose setae and fringe of fine setae. Pleopod V absent. Uropod uniramous, with single article, conical, tapering distally; extending beyond posterior margin of pleotelson.

Remarks

Among the Ischnomesidae , species of Heteromesus are most easily recognised by their tiny to rudimentary antennular articles distal to article 2, a pereonite 4 that is always near the same length as the anterior pereonites, an often elongate tapering distally uropod and a typically rugose cuticle, with many short spines or tubercles. Heteromesus species lack long spines on pereonites 5–7, and pereonite 4 is never elongate as it often is among other Ischnomesidae . All species of Heteromesus that we have examined also lack pleopod V, a common trait for many Ischnomesidae . This, however, is not a general feature for the family because a small oval pleopod V is present in Haplomesus celticensis Kavanagh, Wilson & Power, 2006 .

Regarding the antennula, we prefer to not use the term “flagellum” because, technically speaking, article 3 is not flagellar but is part of the protopod. Therefore, the tiny distal articles on the antennula may not be homologous to the true flagellar articles, that is, articles distal to article 3. An additional problem emerges here as well: we don’t know the exact composition of the distal articles because podomere loss (i.e., of article 3) could be one path for reduction, but non­expression of the articulations between antennular articles could be an alternative pathway. On the basis of the evidence available here, one cannot decide. This problem requires a broader understanding of the antennular patterns across the family.