PLATYSCELIDAE Bate, 1862

Zeidler, Wolfgang, 2016, A review of the families and genera of the superfamily PLATYSCELOIDEA Bowman & Gruner, 1973 (Crustacea: Amphipoda: Hyperiidea), together with keys to the families, genera and species, Zootaxa 4192 (1), pp. 1-136 : 99-101

Family PLATYSCELIDAE Bate, 1862

Diagnosis. Body length up to 25 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 ( Tetrathyrus ), or double groups, occupying most of head surface. Coxae 1–6 separate from pereonites; coxa 7 almost totally fused with pereonite (except Platyscelus ), with slight posterior notch between coxa and pereonite. Antennae 1 of females with 2–3 articulate peduncle, first flagellar article (callynophore) with slight bulge proximally ( Tetrathyrus ), or distally, with two much narrower, 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 with three smaller, slender articles inserted on antero-distal corner.

Antennae 2 of females absent ( Tetrathyrus ), or of five slender articles, the terminal one often very small. 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; articles 4 & 5 are also sub-equal in length but distinctly shorter than previous pair, article 5 sometimes slightly shorter than article 4. Mouthparts in form of short, broad cylinder. Mandibles with 3-articulate palp in males, absent in females; incisor relatively broad, with slightly enlarged tooth laterally. Maxillae 1 reduced in size, consisting of small, elongate lobes, with oblique distal margin armed with few robust setae, or two, or four, bifid teeth. Maxillae 2 reduced to small, rounded or pointed lobes, slightly curved with rounded bulge medially. Gnathopoda simple, sub-chelate, 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 for P6. Pereopod 6; basis with, or without ( Tetrathyrus ), fissure, postero-distal corner with ridge-groove locking mechanism with opposing P6, posterior margin with telsonic groove. Pereopod 7 reduced in size, usually consisting of basis and 1–2 tiny articles, rarely with full complement of articles (abnormal). Uropod 3; endopod fused with peduncle, rarely articulated, or endopod of U2 also fused ( Tetrathyrus ). Telson triangular, relatively broad, fused with double urosomite. Gills with, or without, folds on pereonites 2–6. Oostegites on pereonites 2–5.

Genera. Platyscelus Bate, 1862 ; Hemityphis Claus, 1879 ; Paratyphis Claus, 1879 and Tetrathyrus Claus, 1879 .

Remarks. In the past this family has also been known as Typhidae Dana, 1853 ( Claus 1879, Stebbing 1888, Chevreux 1900, Chevreux & Fage 1925) and Eutyphidae Bovallius, 1887. The name Typhidae is derived from the genus Typhis Risso, 1816 which, although being an older name, is preoccupied by a genus of molluscs ( Montfort 1810). Consequently Bate (1862) proposed the name Platyscelidae , derived from the next oldest, available generic name, Platyscelus Bate, 1861 . Claus (1879) incorrectly proposed a new genus Eutyphis to encompass a number of older generic names including Platyscelus . Bovallius (1887) following Claus, proposed the family Eutyphidae but incorrectly attributed it to Dana (1853). Thus, Platyscelus is the oldest available generic name and the family should be known as Platyscelidae , as proposed by Bate (1862).

This family is very similar to Parascelidae , but is distinguished by the shape of the mouthparts, which are in the form of a broad, rounded cylinder (sharp, pointed cone in Parascelidae ), and pereopod 7 which is usually reduced to the basis and a few vestigial, terminal articles. Also, the basis of pereopod 5 is more elongate in Platyscelidae , and in Parascelidae the basis of pereopod 6 is more prominently narrowed for the distal half. The presence of a fissure on the basis of pereopod 6 can also be a useful character to distinguish the genera of the two families. In the family Platyscelidae a fissure is only absent in Tetrathyrus , which is readily distinguished by the morphology of the gnathopoda, and the shape of the basis of pereopod 6. In all other genera of Platyscelidae , a fissure is present on the basis of pereopod 6, and is located just below the notch for the first urosomite (beginning of telsonic groove). In the family Parascelidae a fissure is absent in Parascelus and Euscelus , but is present in Thyropus , where it is located well above the telsonic groove, and in Schizoscelus where it is very long and extends above and below the notch for urosomite 1.

Members of this family are usually found rolled into a ball; the enlarged basis of pereopods 5 and 6 completely covering the gnathopoda and other pereopoda, with the telson fitting neatly into a telsonic groove on the basis of pereopod 6 ( Fig. 49 B). Although the ability to roll into a ball has evolved in other families of the superfamily Platysceloidea, none have perfected the habit to the extent found in the families Platyscelidae and Parascelidae . The morphological characters that allow individuals to achieve this have not been described in any detail previously. These characters vary slightly, in detail, at the generic and specific level but are basically the same for all members of the two families as follows. The basis of pereopod 5 is articulated proximally with the basis of pereopod 6 by means of a small knob, located medially, near the proximal anterior corner of the basis of pereopod 6, which articulates in a groove on pereopod 5, produced by an elongate process on the medial surface of the coxae and a small, adjacent knob on the basis ( Fig. 52 ). The basis of pereopod 5 has a bevelled anterior margin that fits under the coxae (1–4), and distally in a groove under the head, next to the second antennae. The posterior margin of the basis of pereopod 5 overlaps with the anterior margin of the basis of pereopod 6, which is bevelled to ensure a neat overlap. The posterior margin of the basis of pereopod 6 has a distinct notch (or shoulder) for urosomite 1, followed by a telsonic groove, which neatly accommodates the double urosomite and the telson. Distal to the telsonic groove is a locking mechanism for opposing pereopoda, consisting of a ridge/groove arrangement, similar to the funnel locking mechanism found in squid, with the left usually overlapping the right, suggesting they are predominantly ‘left handed’ (in 160 specimens of Platyscelus ovoides examined only three had the right overlapping the left). In addition, the distal margin of the basis of pereopod 6, distal and anterior to the locking mechanism, is grooved so that the margins of opposing pereopods can overlap thus assisting in holding them together. The head has a central ‘beak’, which fits neatly with the antero-distal corner of the basis of pereopod 6 (see Fig. 49 B). Finally the epimeral plates have lateral ridges, which fit neatly against the proximal part of the posterior margin of the basis of pereopod 6, when the animal is curled. Thus, these animals are able to achieve an almost smooth, tightly packed, globular shape. The reasons for adopting this habit are not known. A spherical shape would sink more rapidly, and this may assist in the avoidance of predators, or in diurnal migrations.

The family Platyscelidae has not been reviewed since the works of Claus (1879, 1887) and all genera are in need of thorough taxonomic revision.

Prior to this review Amphithyrus was included in this family, but this genus differs significantly from the other genera of Platyscelidae , and is accommodated in the new family, Amphithyridae, together with Paralycaea and Amphithyropsis , with which it shares a number of significant characters. Thus, only four genera are recognised in this family.

Figures

FIGURE 49. Platyscelus ovoides (Risso, 1816), female (17.5 mm) and male (15.2 mm), Tasman Sea, SAMA C 4224. A – C, habitus, ventral view when rolled up, and anterior view of head of female. Scale bars = 2.0 mm (A – C), 1.0 mm (G 1 & 2), 0.2 mm (remainder).

FIGURE 52. Hemityphis tenuimanus Claus, 1879, male (6.8 mm), Tasman Sea, SAMA C 5363. Scale bars = 0.1 mm (Mxp), 0.5 mm (remainder).