Holophryxus fusiformis SHIINO 1937

HOLOPHRYXUS FUSIFORMIS SHIINO, 1937

Original description: Shiino (1937): 188–191, figs 1, 2 (♀ and ♂).

Additional descriptions: Huang et al. (2018): 44–50, figs 1–5 (♀ and ♂). Shimomura et al. (2020): 46, 47, fig. 1.

Hosts: Prehensilosergia prehensilis (type host), L u c e n s o s e r g i a l u c e n s, S e r g i a l a m i n a t a (a l l Dendrobranchiata, Sergestidae).

Distribution: Taiwan, Japan.

Differential diagnosis: Holophryxus. Body length 9.7 mm (♀), 1.6 mm (♂) (Shiino, 1937); 7.00– 7.50 mm (non-ovigerous ♀♀; N = 2), 6.8–10.12 mm (ovigerous ♀♀; N = 14), 1.24–1.65 mm (♂♂; N = 10) (Huang et al., 2018); 9.0 mm (ovigerous ♀; N = 1) (Shimomura et al., 2020). Colour yellowish white in ♀ and white in ♂, in ethanol (Shiino, 1937); predominantly beige and yellow in ♀, pleon pale green; white in ♂ (Huang et al., 2018); orange in ♀ (Shimomura et al., 2020).

FEMALE: Body spindle shaped, about 2.2–2.3 times as long as maximum width in Taiwanese specimens, 1.95 times in Japanese specimens. Cephalon only slightly demarcated from pereon in dorsal aspect by weak constrictions bilaterally and transverse surface fold dorsally; dorsal prominence with emarginate (bilobed) anterior margin; ventral cephalic ridge partly discernible in dorsal aspect, slightly arched, about 0.4 times as long as wide, anterior margin with medial incision producing slightly bilobed appearance, without paired pit organs; posterolateral corners of cephalic ridge with pointed processes, representing coxal plates of incorporated first pereonite. Maxilliped present, bilobate, inner margin deeply concave. Median sternal plate without paired posteriorly directed appendices. Pereon with five pairs of incubatory plates; oostegite 1 bilobate, with medial projection on proximal lobe; oostegite 5 unilobate, covering 64% of ventral side of body, posterior margin with 10–12 minute, spinular extensions. Coxal plates of pereopods 2–5 expressed, represented by pointed processes on lateroventral margins of pereon, separated by transverse folds. Posterior part of pereon with proximal (1) and posterior (1–3) groups of paired ventrolateral papillae close to posterior margin of fifth incubatory plate. Pleon variable in shape but not clearly demarcated from pereon; about 15% of total body length; posterior margin rounded, typically with tiny, terminal, median, spinous process.

MALE: Body elongate, about three times as long as maximum width, dorsoventrally flattened. Cephalon completely fused to first pereonite, forming cephalothorax with weak lobate extensions at posterolateral corners; distinctly wider than long, as wide as pereon; anterior margin rounded. Antennule trilobate, outer lobe with bi-articulated flagellum. Antenna probably 2-segmented; with broad basal part and elongate, posteriorly directed flagellate part; not extending to posterior margin of first pereonite. Pereonites 2–4 completely separated, pereonites 5–7 fused dorsally; lateral margins rounded. Pleon completely fused to pereonite 7, forming elongate cone, markedly narrower than pereonite 7, representing 40% of body length; posterior margin rounded or with extension; anal region with few setiform elements.

Variability: Recorded in females in the number of ventrolateral papillae of both the anterior (sometimes absent) and posterior pair (1–3), the contour of the posterior margin of the pleon (occasionally without terminal process) and colour pattern [cf. Huang et al. (2018: 50) for details]. The caudal margin of the male pleon occasionally forms an irregularly-shaped (Huang et al., 2018: fig. 3E) or angular [‘quadrangular plate’ sensu Shiino (1937: fig. 1H)] extension.

Remarks: Huang et al. (2018) discussed differences between Shiino’s (1937) original description, based on type material from Japan, and their own specimens from Taiwan, including (1) host utilization, (2) the presence of a quadrangular plate at the end of the pleon in the Japanese males (absent in the Taiwanese ones, but see variability above), and (3) the number of oostegites (four vs. five). Although Huang et al. (2018) state that a re-examination of the type material would be required to resolve the discrepancy in oostegite number, reinterpretation of Shiino’s (1937: fig. 1C) illustration indicates that it was the second incubatory plate that was most likely overlooked.

Both Japanese and Taiwanese populations of Ho. fusiformis display two paired sets of ventrolateral papilliform processes on the pereon in adult females, one just anterior to the fifth pair of incubatory plates and one posterior to them (Shiino, 1937; Huang et al., 2018). The processes conceivably represent the vestigial sixth and seventh pairs of pereopods, respectively, the latter of which are usually only expressed in subadult females and disappear later in maturation (Rustad, 1935; but see Ho. septapodus).

Holophryxus fusiformis is morphologically closest to Ho. richardi (see above for differences with this species) and Ho. polyandrus. Although Schultz’s (1978) description is deficient in many aspects, his illustrations of the female reveal a few diagnostic characters that separate it from Ho. fusiformis, including the ventral cephalic ridge with a straight anterior margin (vs. bilobed), the presence of only 4–5 long spinular extensions around the posterior margin of the fifth oostegites (vs. 10–12 shorter setular ones), the obsolete coxal plates of the fifth pair of pereopods (vs. well developed pointed process), the absence of the posterior papilliform protuberances representing the vestigial seventh pair of pereopods (vs. represented by three papillae on either side), and the shape of the pleon which tapers significantly towards a pointed posterior end (vs. rounded with a minute median process). According to Schultz (1978), the male of Ho. polyandrus has a discrete pleon and pereonites 2–7 are likewise free, showing no evidence of fusion, which contrasts with the condition in Ho. fusiformis where pereonites 5–7 are fused dorsally and the pleon is completely fused to pereonite 7 (Shiino, 1937; Huang et al., 2018). Observations of a live female specimen showed that Ho. fusiformis was similar in colour to that of the host shrimp, which was interpreted as a form of camouflage helping the parasite to avoid being detected by predators in the bathypelagic layer (Shimomura et al., 2020). This supposition seems unlikely since visual predators that live in the darkness below the sunlit surface waters (below the epipelagic or photic zone) are unlikely to detect prey items unless they are bioluminescent.