Holophryxus acanthephyrae, STEPHENSEN, 1912

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 : 636-637

HOLOPHRYXUS ACANTHEPHYRAE STEPHENSEN, 1912

Synonym: Isophryxus concavus Schultz, 1977 .

Original description: Stephensen (1912 —as H. Acanthephyrae ): 112–117, figs 13 (top), 15–21 (♀, ♂ and epicaridium).

Additional descriptions: Jones & Smaldon (1986): 306–312, figs 1–6 (♀ and ♂). Schultz (1977 —as I. concavus ): 93–96, figs 17, 18A-H, O, P (♀ and ♂). Wasmer (1988): 21–23 (♀ and ♂). Alves-Júnior et al.

(2019): 14 (♀). Boyko & Williams (2021b): 114–116, fig. 2A-C (♀ and ♂).

Hosts: Acanthephyra acanthitelsonis , A. pelagica (type host) and A. purpurea (Pleocyemata, Acanthephyridae ).

Distribution: North Atlantic Ocean, south-western Atlantic Ocean, South Pacific Ocean, Southern Ocean ( Table 2 ).

Differential diagnosis: Holophryxus . Body length 22 mm (♀; N = 1), 0.2 mm (epicaridium) ( Stephensen, 1912); 12.18–13.94 mm (♀♀; N = 2), 18.28–22.68 mm (ovigerous ♀♀; N = 2), 3.33 mm (♂; N = 1) ( Jones & Smaldon, 1986); 8.5–21.6 mm (♀♀; N = 7), 2.6–3.4 mm (♂♂; N = 3) ( Schultz, 1977); 22.7 mm (ovigerous ♀; N = 1), 2.3 mm (♂; N = 1) ( Wasmer, 1988); 20.33 mm (♀; N = 1) ( Alves-Júnior et al., 2019); 13.7 mm (♀; N = 1), 2.0 mm (♂; N = 1) ( Boyko & Williams 2021b). Colour in alcohol reddish brown spotted with yellow ( Stephensen, 1912) or light yellow ( Wasmer, 1988) in ♀; yellowish white in ♂ ( Wasmer, 1988).

FEMALE: Body oblong-ovate, about 1.9–2.2 times as long as maximum width. Cephalon only slightly demarcated from pereon in dorsal aspect by weak constriction bilaterally and transverse surface fold dorsally; dorsal prominence with emarginate (bilobed) anterior margin; part of ventral cephalic ridge clearly discernible in dorsal aspect ( Fig. 15H, I ), slightly arched, about 0.3 times as long as maximum width, anterior margin slightly sinuous, with pair of distinctive, slit-like pit organs; posterolateral corners of cephalic ridge with rounded 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 bilobate, with medial projection on proximal lobe; oostegite 5 unilobate, covering 57–63% of ventral side of body, posterior margin with 12–14 spinular extensions. Coxal plates of pereopods 2–5 expressed, represented by rounded processes on lateroventral margins of pereon, separated by transverse folds; those of pereopod 5 weakly developed. Posterior part of pereon without ventrolateral papillae. Pleon clearly demarcated from pereon; about 12–14% of total body length; subtriangular, tapering abruptly in posterior half to rounded extremity; dorsal surface deeply excavated either side of midline near border with pereon.

MALE: Body elongate but more compact than in most other species, about 2.5 times as long as maximum width, dorsoventrally flattened. Cephalon completely fused to first pereonite, forming the cephalothorax; original segmentation marked by dorsal groove and weak lobate extensions at posterolateral corners; distinctly wider than long, clearly narrower than pereon; anterior margin rounded. Antennule bilobate, outer lobe with non-articulated flagellum. Antenna 2-segmented; with broad basal part and elongate, posteriorly directed, slender part; not extending to posterior margin of first pereonite. Pereonites 2–6 completely separated, pereonite 7 almost completely fused dorsally with pleon; lateral margins of pereonites 2–6 rounded, those of seventh conical or rounded. Pleon conical, markedly narrower than pereonite 7, representing 31–40% of body length; posterior margin rounded.

Variability: Recorded in the number (12–14) of spinular extensions around the posterior margin of the fifth incubatory plate in females ( Stephensen, 1912; Schultz, 1977; Jones & Smaldon, 1986; Wasmer, 1988; Alves-Júnior et al., 2019), and in the shape of the lateral margins of the seventh pereonite in males ( Jones & Smaldon, 1986; Wasmer, 1988).

Remarks: By far the most detailed account of the adults is that by Wasmer (1988) who pointed out some pertinent errors in previous descriptions. However, his statement (not accompanied by any figures) that the male antennule is 3-segmented and the antenna possibly also displays the same segmentation is almost certainly incorrect (see above). Schultz’s (1977 —as I. concavus ) description of the cephalic appendages is grossly inadequate and difficult to interpret. The author states that the antennules and antennae are absent in the female and ‘… some mouth parts [are] distinct, but most not immediately apparent as mouth parts’. It is indeed unclear which appendages Schultz’s figures (his fig. 18C-G) refer to. His description of the male antennule and antennae is similarly ambiguous, the former being described as consisting of three conical segments with a single aesthetasc on the distal one, and the latter as 4-segmented, composed of a broad irregularly-shaped peduncular segment, supporting two subequal, narrow segments, and a small apical segment tipped with several setae.

Holophryxus acanthephyrae is the only representative of the genus that is distributed in both northern and southern hemispheres. Within the cephalic type 3 group it is most closely related to Ho. alaskensis but differs from it in a number of significant characters such as the presence of pit organs on the ventral cephalic ridge, and the shape and dorsal sculpturing of the pleon in the female (see above). The epicaridium stage was described by Stephensen (1912: figs 19–21) based on larvae (0.2 mm) recovered from the marsupial cavity of the female; however, the morphology of the pereopods indicate that it was only partially developed ( Coyle & Mueller, 1981).

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.