Holophryxus alaskensis Richardson 1905

HOLOPHRYXUS ALASKENSIS RICHARDSON, 1905 A

Synonyms: Holophryxus californiensis Richardson, 1908; Hypodajus georgiensis Nierstrasz & Brender à Brandis, 1931.

Original description: Richardson (1905a): 221–117, figs 8–10 (♀).

Additional descriptions: Richardson (1905b —as Ho. alascensis): 576–577, figs 639–641 (♀). Richardson (1908 — Ho. californiensis): 692–694, figs 4, 5 (♀ and ♂). Nierstrasz & Brender à Brandis (1931—as Hyp. georgiensis): 212, 213, figs 106–110 (♀ and ♂). Butler (1964): 973–975, figs 1–3 (adult and immature ♀♀, and ♂). Coyle & Mueller (1981): 1439–1441, figs 1, 2 (epicaridium, microniscus, cryptoniscus and juvenile female). Boyko & Williams (2021b): 112–114, fig. 1 (♀ and ♂).

Hosts: Pasiphaea pacifica (P l e o c y e m a t a, Pasiphaeidae); Paraeuchaeta elongata (Esterly, 1913) (intermediate host).

Distribution: North Pacific Ocean (Table 2).

Differential diagnosis: Holophryxus. Body length 6 mm (♂; N = 1) (Richardson, 1908); 10.5 mm (♀; N = 1), about 2 mm (♂; N = 1) (Nierstrasz & Brender à Brandis, 1931); 4.3–5.5 mm (immature ♀♀; N = 2); 5.5– 12.0 mm (adult ♀♀; N = 18), 1.9–6.0 mm (♂♂; N = 2) (Butler, 1964); 0.2–0.3 mm (epicaridium), 1.2 mm (cryptoniscus) (Coyle & Mueller, 1981); 13.6 mm (♀; N = 1), 2.0 mm (♂; N = 1) (Boyko & Williams 2021b). Colour in life maize (♀), in alcohol and formalin yellowish white (both sexes) (Butler, 1964); after preservation uniformly light yellow (♀) (Richardson, 1905a); canary yellow in life (♀) (Richardson, 1908).

FEMALE: Body oblong-ovate, about 1.8–2.3 times as long as maximum width. Cephalon only slightly demarcated from pereon in dorsal aspect by constriction bilaterally and transverse surface fold dorsally; dorsal prominence with emarginate (bilobed) anterior margin; ventral cephalic ridge partly discernible in dorsal aspect, slightly arched, about 0.35 times as long as maximum width, anterior margin slightly sinuous, without paired pit organs; posterolateral corners of cephalic ridge with rounded processes, representing coxal plates of incorporated first pereonite. Maxilliped present, bilobate and irregulary shaped. Median sternal plate presumably with paired posteriorly directed appendices. Pereon with five pairs of incubatory plates; oostegite 1 slightly bilobate, medial projection on proximal lobe unconfirmed but probably present; oostegite 5 unilobate, covering 62–63% of ventral side of body, posterior margin with 10–11 setular extensions. Coxal plates of pereopods 2–5 expressed as rounded processes on lateroventral margins of pereon, separated by transverse folds; those of pereopod 5 weakly developed. Posterior part of pereon with pair of ventrolateral papillae near outer distal corners of fifth incubatory plates. Pleon clearly demarcated from pereon; about 17% of total body length; broadly conical.

MALE: Body elongate, about 2.4–2.6 times as long as maximum width, dorsoventrally flattened. Cephalon completely fused to first pereonite, forming cephalothorax; original segmentation marked by dorsal groove and weak lobate extensions at posterolateral corners; distinctly wider than long, clearly narrower than pereon; anterior margin slightly rounded or almost straight. Antennule with lobate base but no indication of flagellum. Antenna extending to at least posterior margin of first (incorporated) pereonite; segmentation unconfirmed. Pereonites 2–6 completely separated, pereonite 7 narrowest, almost completely fused dorsally with pleon; lateral margins of pereonites 2–7 rounded. Pleon oblong-conical, markedly narrower than pereonite 7, representing 30–38% of body length; posterior margin rounded.

Variability: Richardson’s (1905a) type specimens are markedly stouter (1.4 times as long as maximum width) and less symmetrical than the females examined by Nierstrasz & Brender à Brandis (1931) and Butler (1964). The latter attributed the variation in body shape to different stages in maturation, with the stout body form being associated with the attainment of sexual maturity. Although this may explain the significant difference in length/width ratio, we believe excessive pressure during mounting also contributed to the discrepancy [as revealed by all three habitus views in Richardson’s (1905a) description]. Butler (1964) observed slight asymmetry in female body shape in some individuals.

Remarks: There is a gross similarity in female maxillipedal morphology between Richardson’s (1905a: fig. 9) illustration and Rustad’s (1935: figs 9, 10) detailed account of the appendage in Ho. richardi, except that in the former the posteriorly directed appendix arising from the underlying sternal plate appears to be broken off. As pointed out by Coyle & Mueller (1981: 1440), Butler (1964: 973, fig. 3B) misinterpreted part of the antenna as the maxilliped, an observational error also made previously by Nierstrasz & Brender à Brandis (1931: fig. 109). There appears to be some confusion regarding the outline of the first incubatory plate. Butler (1964: 973) describes the oostegite as seemingly ‘… divided into 4 lobes, 2 lobes visible …’ and like Nierstrasz & Brender à Brandis (1931: fig. 109) illustrates it with a transverse suture line. The incubatory plate is here reinterpreted as slightly bilobate (as in Ho. citriformis) with the transverse ‘suture’ representing the posterior border of the underlying second oostegite (IP2) (compare with Fig. 4A). It is conceivable that Richardson’s (1905a: fig. 10) illustration of the ‘terminal segment of first lamella of incubatory plate’ is based on a similar misinterpretation [but see Rustad (1935: 25) for a dissenting opinion]. The male antenna is elongate and, depending on the authority, extends to the posterior margin of the first (Butler, 1964) or second pereonite (Richardson, 1908). Although the segmentation of the antenna requires confirmation, Richardson’s (1908) claim that it consists of seven segments is almost certainly incorrect.

Females of Ho. alaskensis display a cephalon that lacks a hood-like extension and has a slightly arched ventral cephalic ridge, part of which is discernible in dorsal aspect. This cephalic type 3 (Fig. 15 G-I) is also exhibited by Ho. acanthephyrae, Ho. fusiformis, Ho. polyandrus and Ho. richardi. In Ho. acanthephyrae and Ho. alaskensis the coxal plates of pereopods 1–5 are expressed as rounded processes on the lateroventral margins of the pereon whereas in the other three species the processes are pointed. Females of the former species can be distinguished from Ho. alaskensis by (1) presence of paired slit-like pit organs on the ventral cephalic ridge (vs. absent); (2) posterior part of pereon without ventrolateral papillae (vs. with); (3) pleon subtriangular, tapering abruptly in posterior half to rounded extremity (vs. broadly concial); and (4) dorsal surface of pleon deeply excavated either side of midline near border with pereon (vs. no surface sculpturing). Males of Ho. acanthephyrae are usually more compact, and unlike Ho. alaskensis, possess a non-articulated flagellum on the outer lobe of the antennule (vs. no indication of a flagellum) and exhibit antennae that do not extend to the posterior margin of the first pereonite.

Geiger (1969: 108) reported a juvenile between the eighth pair of pereopods of Mysis polaris Holmquist, 1959 (Mysidae) from the central Arctic Ocean (84°24’N, 169°02’E) which he identified as Ho. alaskensis. Boyko & Williams (2021b) dismissed this record, stating that no species of Holophryxus are known from the marsupium of mysid hosts and that it probably belongs to a different dajid genus.