Parascelidae Bovallius 1887

Family PARASCELIDAE Bovallius, 1887

Diagnosis. Body length 5–7 mm, rolled into ball, the enlarged basis of P5 & 6 completely covering other pereopoda, with telson fitting neatly into telsonic groove of basis of P6, slightly flattened dorso-ventrally. Head relatively short, with ‘beak’ between first antennae, fitting neatly between distal margins of basis of P6 when rolled up. Eyes in single, or double group, occupying most of head surface (except Euscelus). Coxae 1–6 separate from pereonites; coxa 7 almost totally fused with pereonite, with slight posterior notch between coxa and pereonite. Antennae 1 of females with 2–3 articulate peduncle, first flagellar article (callynophore) not particularly larger than peduncular articles, with two smaller articles inserted terminally. Antennae 1 of males with 1–2 articulate peduncle, and enlarged, curved first flagellar article (callynophore), with two-field brush of aesthetascs medially, and three smaller, slender articles inserted on antero-distal corner. Antennae 2 of females often reduced in size consisting of 3–5 slender articles. Antennae 2 of males with five slender articles folded back on one another underneath head and pereon, basal article the shortest, and slightly more robust than following articles, articles 2 & 3 sub-equal in length, the fourth is slightly shorter, and the fifth is slightly shorter than the fourth. Mouthparts styliform, forming buccal mass in shape of pointed cone. Mandibles with palp in males, absent in females, with narrow incisor. Maxillae 1 reduced to very small lobes, with oblique distal margin armed with few robust setae, or several in four groups. Maxillae 2 absent, or reduced to very small lobes, slightly curved with rounded bulge medially, with minute denticles on rounded distal surface. Gnathopoda simple or chelate. Pereopods 5 & 6 with basis transformed into broad operculum, always longer and larger in P6; distal articles inserted sub-terminally on basis of P5, more proximally in P6. Pereopod 6, basis with, or without, fissure, postero-distal corner with ridge-groove locking mechanism with opposing P6, distal margin with groove to lock with opposing P6, posterior margin with telsonic groove. Pereopod 7 reduced in size but with full complement of articles. Uropods 2 & 3; endopod sometimes fused with peduncle. Telson triangular, relatively broad, fused with double urosomite. Gills without folds on pereonites 2–6. Oostegites on pereonites 2–5.

Genera. Parascelus Claus, 1879; Thyropus Dana, 1852; Euscelus Claus, 1879 and Schizoscelus Claus, 1879.

Remarks. In the past this family has also been known as Scelidae Claus, 1879 (Stebbing 1888, Chevreux 1900, Chevreux & Fage 1925, Hurley 1955). However, there never was a genus Scelus and we must therefore use Parascelidae, derived from the genus Parascelus, as proposed by Bovallius (1887).

Barnard (1930) proposed the name Thyropidae for this family believing that the family name must be derived from the oldest genus, Thyropus Dana 1852. However, according to the ICZN the oldest generic name in a family need not be taken as the nomenclatural type (Opinion 141, ICZN 1950, 4: 138). Barnard’s suggested substitution is therefore unnecessary.

This family is very similar to the previous one, as already discussed under that family. Bowman and Gruner (1973) recognise three genera, Euscelus Claus, 1879, Schizoscelus Claus, 1879 and Thyropus Dana, 1852. Vinogradov et al. (1982) also recognise these genera but make a case for removing Parascelus Claus, 1879 from the synonymy of Thyropus. They also include Hemiscelus Stewart, 1913, the type species of which is regarded a juvenile form of Hemityphis tenuimanus (Platyscelidae) (Zeidler 1998).

Vinogradov (1990) described a new species of Hemiscelus, H. setosus, from the south-eastern Pacific. Unfortunately it has not been possible to borrow any specimens of this species and so it is not possible to verify the taxonomic status. Judging by Vinogradov’s description and figures, it seems to be a species of Platyscelus, near P. crustulatus (Claus, 1879), or P. serratulus Stebbing, 1888. Thus, only four genera are recognised in this review.

The four genera of Parascelidae are quite diverse in their morphology but fall into two main groups, Parascelus with Thyropus and Schizoscelus with Euscelus. Parascelus and Thyropus share the following characters that differ from Schizoscelus and Euscelus. The gnathopoda are simple; the antennae of females are linear, the first consisting of six articles and the second of five articles; the first maxillae are armed with teeth, or groups of robust setae, and the second maxillae consist of a slightly curved lobe with a rounded medial bulge. In Schizoscelus and Euscelus the second gnathopoda are chelate (also gnathopod 1 in Euscelus); the first antennae of females consist of five articles, with the terminal two articles reduced in size, and inserted dorsally on the medial corner of the callynophore; the second antennae of females are reduced and consist of only three or four articles; the first maxillae are only armed with a few setae, and the second maxillae are absent or not discernible. This grouping of these genera is also well supported by the phylogenetic analysis (Fig. 2).

The above grouping of genera might lead one to suggest that the family should be split into two, but there are other characters shared by different combinations of genera. The first antennae of males have a two-articulate peduncle in Parascelus and Schizoscelus, and a one-articulate peduncle in Thyropus and Euscelus. The basis of pereopod 6 has a fissure in Thyropus and Schizoscelus, which is absent in Parascelus and Euscelus. Thyropus also differs from the other genera in having eyes grouped into two fields on either side of the head, and in having a less elongate maxilliped, with the inner lobe split distally. Thus, the phylogeny of genera of the family Parascelidae is not clear and requires further investigation.