Holophryxus richardi, KOEHLER, 1911

Huys, Rony, Savchenko, Alexandra S. & Kosobokova, Ksenia N., 2023, Discovery of a new species and host record of Holophryxus Richardson, 1905 (Isopoda: Dajidae) from the central Arctic: a model of enhanced descriptive standards for epicaridean isopods, Zoological Journal of the Linnean Society 198 (2), pp. 592-649 : 635-636

HOLOPHRYXUS RICHARDI KOEHLER, 1911

Synonym: Holophryxus sp. ( Ho. Richardi Koehler 1911 ) sensu Stephensen (1912).

Original description: Koehler (1911 —as Ho. Richardi ): 23–25, figs 15–17 (immature ♀).

Additional descriptions: Stephensen (1912): 108– 109, figs 9–10 (subadult ♀). Stephensen [1912 —as Holophryxus sp. ( Ho. Richardi Koehler ?)]: 109–112, figs 11–12, 13 (bottom), 14 (adult ♀). Stephensen (1915): 24 (adult ♀). Rustad (1935): 9–30, figs 1–18 (subadult and adult ♀♀ and ♂).

Host: Eusergestes arcticus (Dendrobranchiata, Sergestidae ).

Distribution: North Atlantic Ocean ( Table 2 ).

Differential diagnosis: Holophryxus . Body length 3.7 mm (immature ♀; N = 1) ( Koehler, 1911); 9 mm (subadult ♀; N = 1), 14 mm (adult ♀; N = 1) ( Stephensen, 1912); 8 mm (adult ♀) ( Stephensen, 1915); 4.1–12.9 mm (immature ♀♀; N = 6), 11.2–16.5 mm (adult ♀♀; N = 11), 1.64–2.00 mm (♂♂; N = 3) ( Rustad, 1935); 8.6–16.4 mm (adult ♀♀; N = 4) ( Greve & Johannessen, 1981); 6.55–13.86 mm (adult ♀♀; N = 23), 9.66–12.18 mm (ovigerous ♀♀; N = 2) ( Jones & Smaldon, 1986). Colour of ♀ pale yellow in alcohol ( Stephensen, 1912), from yellow to red in live specimens ( Rustad, 1935; Greve & Johannessen, 1981); unknown in ♂.

FEMALE: Body spindle shaped, about 2.0 times as long as maximum width. Cephalon demarcated from pereon in dorsal aspect by constriction bilaterally and transverse surface fold dorsally; dorsal prominence with emarginate (bilobed) anterior margin; only small part of ventral cephalic ridge discernible in dorsal aspect, slightly arched, about 0.45 times as long as maximum width, anterior margin rounded, without paired pit organs; posterolateral corners of cephalic ridge with pointed processes, representing coxal plates of incorporated first pereonite. Maxilliped present, bilobate. Median sternal plate with paired posteriorly directed appendices. Pereon with five pairs of incubatory plates; oostegite 1 slightly bilobate, with medial projection on proximal lobe; oostegite 5 unilobate, covering about 60–65% of ventral side of body, posterior margin with 11–13 spinular extensions. Coxal plates of pereopods 2–5 expressed as pointed processes on lateroventral margins of pereon, separated by transverse folds. Posterior part of pereon with pair of ventrolateral spinous papillae (vestiges of sixth coxal plates) anterior to outer distal corners of fifth incubatory plates. Pleon short, demarcated from pereon by weak constrictions laterally and occasionally deeper fold (in preserved specimens) ventrally; about 13% of total body length; broadly conical.

MALE: Body elongate, about 2.0–2.7 times as long as maximum width, dorsoventrally flattened. Cephalon completely fused to first pereonite, forming the cephalothorax; not displaying any evidence of original segmentation; distinctly wider than long, narrower than pereon; anterior margin rounded. Antennule unconfirmed. Antenna not extending to posterior margin of first (incorporated) pereonite; segmentation unconfirmed. Pereonites 2–6 completely separated, pereonite 7 almost completely fused dorsally with pleon; pereonites 2 and 7 narrowest; lateral margins of pereonites 2–7 rounded. Pleon oblong-conical, markedly narrower than pereonite 7, representing about 38% of body length; posterior margin rounded.

Variability: Considerable variation has been recorded in the colour of live females. According to Rustad (1935), it can range from yellow, over different nuances of orange, to red, possibly reflecting different levels of maturity ( Rustad, 1935). Greve & Johannessen (1981) found that the largest was light orange, tinged with yellow, whereas the smaller specimens were bright red with a faint yellow tinge dorsally and yellow ventrally. Rustad (1935) reported individual variation in the timing of disappearance of the sixth pereopods during development, claiming that they are no longer expressed once females attain a length of about 11–13 mm. In addition to the small protuberances representing the vestigial coxal plates of the sixth pereopods, subadult females also exhibit paired groups of three small papillae further posterior on the ventral surface of the pleon (probably representing remnants of the seventh pair of pereopods). Stephensen (1912: fig. 11) recorded a slightly asymmetrical female while Rustad (1935) observed variation in body shape, particularly in the length:width ratio and in the shape of the pleurotergites of the pereonites, in the two males he examined.

Remarks: Holophryxus richardi belongs to a group of three closely related species in which the adult females possess (1) a cephalon with a slightly arched ventral ridge, a dorsal prominence with a bilobed anterior margin and no hood-like extension (type 3; cf. Fig. 15 G-I), (2) pereopodal coxal plates that are expressed as pointed processes on the lateroventral margins of the pereon and (3) minute protuberances at or around the posterolateral corners of the fifth incubatory plates, representing the vestigial sixth pair of pereopods. Both sexes of all three species ( Ho. fusiformis , Ho. polyandrus , Ho. richardi ) fall within comparatively similar size ranges, with females also having virtually identical morphometric values for the ventral cephalic ridge (length:maximum width), fifth incubatory plate (length relative to body length) and pleon (proportional length). Females of Ho. fusiformis differ from those of Ho. richardi in the bilobed anterior margin of the ventral cephalic ridge, the absence of posteriorly directed appendices originating from the cephalic sternal plate, the presence of paired papilliform protuberances posterior to the fifth pair of oostegites (possibly representing vestigial pereopods 7), and the pleon typically with a minute, terminal, median process. Males of Ho. fusiformis can readily be differentiated by the dorsal fusion of pereonites 5–7 (vs. 2–6 completely discrete) and the complete fusion of the pleon to pereonite 7 (vs. only fused dorsally). Females of Ho. polyandrus differ from those of Ho. richardi in the straight anterior margin of the ventral cephalic ridge, the reduced number of extensions on the posterior margin of the fifth incubatory plates (4–5 long spinular vs. 11–13 shorter setular extensions), the obsolete condition of the fifth coxal plates, and the shape of the pleon which terminates in a pointed end. Males of both species are more difficult to distinguish since virtually no information is available for Ho. richardi ( Rustad, 1935) . According to Schultz’s (1978) description of the male of Ho. polyandrus , pereonites 2–7 are completely free and the pleon is discrete whereas it is fused dorsally to pereonite 7 in Ho. richardi .

Figures

Figure 15. Schematic showing general morphology of cephalon in Ho.citriformis (A-C) (Type 1), Ho.quadratahumerale (D-F) (Type 2) and Ho. acanthephyrae (G-I) (Type 3) in ventral (A, D, G), dorsal (B, E, H) and lateral (C, F, I) aspect. CP1–2 = coxal plates 1–2, DP = dorsal prominence, HE = hood-like extension, L = length of ventral cephalic ridge, VCR = ventral cephalic ridge (shaded), W = width of ventral cephalic ridge.