Phrikoceros sagamianus ( Kato, 1937 ), 2025

Phrikoceros sagamianus ( Kato, 1937) comb. n.

(New Japanese name: Kanoko-nisetsuno-hiramushi)

( Figures 3 View Figure 3 , 4 View Figure 4 )

Pseudoceros sagamianus Kato, 1937: 362–363 View in CoL , pl. 22, figs. 9–11, text-figs. 21, 22; Kato 1939: 76–77, text-figs 15, 16; Marcus 1950: 88 (list); Kato 1944: 300 (list); Faubel 1984: 209 (list); Prudhoe 1985: 196, 228 (list).

Material examined

ICHUM 8977 View Materials , sagittal sections of the reproductive organs (9 slides) along with the remaining unsectioned body (preserved in 70% ethanol), collected from underwater cages used for ascidian cultivation in Misaki , Kanagawa, Japan (35.157383° N, 139.611917°E), on 10 June 2022 GoogleMaps . ICHUM 8978 View Materials , sagittal sections of the reproductive organs (10 slides) along with the remaining unsectioned body (preserved in 70% ethanol), collection data same as above GoogleMaps . ICHUM 8979 View Materials , whole mount specimen, collection data same as above GoogleMaps . CMNH-ZC 209 , preserved in 70% ethanol, collected in intertidal rockly reef at Okino-shima Islet , Tateyama, Chiba, Japan (34.990833°N, 139.822783°E), on 19 March 2014 GoogleMaps .

Diagnosis

Pair of pseudotentacles small, ear-like, held erect; pale brown to light olive green background, conspicuously darker along midline with numerous white maculae of various sizes and shapes scattered over surface; cerebral-eyespot cluster horseshoe shaped; pseudotentacular eyespots present in dorsal and ventral sides; pharynx plicated with simple folds; pair of sperm ducts or spermiducal vesicles opening separately posterolateral to seminal vesicle; ejaculatory duct and prostatic duct entering base of penis papilla independently.

Description

Body elongated oval, 27.8–30.2 mm in length and 12.4–13.1 mm at its widest point in living state ( Figure 3 View Figure 3 (A,B)). Pair of pseudotentacles small, ear-like, held erect ( Figure 3 View Figure 3 (A,C)). Dorsal surface smooth, pale brown to light olive green background, conspicuously darker along median line with numerous white maculae of various sizes and shapes scattered all over surface; white maculae forming transversal lines along body periphery. Body margin fringed by narrow black line ( Figure 3 View Figure 3 (A)). Tips of pseudotenctacles white without black line ( Figure 3 View Figure 3 (A)). Ventral surface translucent, with narrow black line ( Figure 3 View Figure 3 (B)). Cerebral-eyespot cluster horseshoe shaped, 0.35 mm in length, composed of about 55 eyespots, distributed in translucent area ( Figure 3 View Figure 3 (D)). Frontal and marginal eyespots absent. Pseudotentacular eyespots numerous, sparsely distributed on both dorsal and ventral sides, extending from base to tip of pseudotentacle at about 0.67 mm length and 0.22 mm width along margin ( Figure 3 View Figure 3 (C)). Mouth, male gonopore, and female gonopore, and sucker present along midline on ventral side. Pharynx, 3.8–4.9 mm in length, plicated with simple folds, located at anterior 1/3 of body ( Figure 3 View Figure 3 (B)). Mouth situated centrally in pharynx. Male gonopore, 220 µm in diameter, located immediately behind posterior end of pharyngeal pouch ( Figure 3 View Figure 3 (B)). Male copulatory apparatus composed of large seminal vesicle, prostatic vesicle, and armed penis papilla ( Figure 4 View Figure 4 (A)). Pair of spermiducal vesicles running anteriorly, opening separately and posterolateral to seminal vesicle ( Figure 4 View Figure 4 (A)). Seminal vesicle elliptical (long axis 738 µm, short axis 299 µm), coated with 29 µm thick muscular layer ( Figure 4 View Figure 4 (A–C)). Ejaculatory duct not coiled, entering penis papilla. Prostatic vesicle elongated oval, lying horizontally at base of penis papilla, lined with smooth granular epithelium, covered by muscular wall ( Figure 4 View Figure 4 (C,D)). Prostatic duct entering penis papilla at its base independent of ejaculatory duct ( Figure 4 View Figure 4 (A,C)). Penis papilla, 510 µm in length, armed with tubular, straight penial stylet (246 µm in length, 43 µm in width), enclosed in penis pouch, protruding into male atrium ( Figure 4 View Figure 4 (A,B)). Male atrium opening to exterior via male gonopore ( Figure 4 View Figure 4 (A–C)). Female gonopore, 230 µm in diameter, situated at 0.86 mm posterior to male gonopore. Female copulatory apparatus composed of cement pouch surrounded by numerous cement glands, vagina, and uteri. Vagina leading to female atrium across cement pouch ( Figure 4 View Figure 4 (E)). Female atrium opening to exterior via female gonopore. Cement glands well developed and concentrated around cement pouch ( Figure 4 View Figure 4 (E)). Sucker 0.44–0.49 mm in diameter, situated slightly anterior to middle of body (2.66 mm posterior to female gonopore) ( Figures 3 View Figure 3 (B), 4(F)).

Type locality and distribution

The species was initially described from ‘Bentensita’ (likely equivalent to Bentenshima), Misaki, Kanagawa, Japan ( Kato 1937) . Subsequent records have documented its presence in Onagawa, Miyagi, approximately 400 km from the type locality ( Kato 1939), and the present study reports its occurrence in Tateyama, Boso Peninsula, located across the mouth of Tokyo Bay.

Habitat

In Onagawa, previous studies reported the species from submerged slates ( Kato 1939). The present study observed numerous specimens inhabiting underwater cages used for ascidian aquaculture.

Remarks

Kato (1937) first described this species as Pseudoceros sagamianus based on a single specimen collected from Misaki, Kanagawa, Japan. The original description included a detailed account of the morphology and a black-and-white photograph of the live colouration ( Kato 1937). Later, Kato (1939) reported an additional specimen of Ps. sagamianus from Konorihama, Onagawa Bay, Miyagi, Japan ( Kato 1939). Unfortunately, during WWII, polyclad material studied by Kojiro Kato (1906–1981) was destroyed by the air raid on Tokyo in 1945 ( Okuno et al. 2012). Consequently, the topotypic specimens examined herein are crucial for understanding the taxonomic status of Ps. sagamianus . Our specimens were assigned to Ps. sagamianus based on the following morphological characters: (1) a dorsal surface with white maculae of varying sizes and shapes distributed over a brown background, (2) a narrow black fringe bordering the body periphery, and (3) separate openings of the paired sperm ducts into the seminal vesicle. As noted by Kato (1939), Ps. sagamianus can be distinguished from the morphologically similar Ps. exoptatus Kato, 1938b distributed in Seto, Wakayama, Japan, by the absence of a common sperm duct in Ps. sagamianus (fig. 22 in Kato 1937; fig. 16 in Kato 1938b).

The current taxonomic status of Phrikoceros Newman and Cannon, 1996b is unclear, with some suggesting its synonymy with Tytthosoceros Newman and Cannon, 1996a (see Bahia 2022). Both genera were previously distinguished from other pseudocerotid genera by the presence of (1) a slightly folded pharynx, (2) a single male copulatory organ, and (3) a single female copulatory organ ( Newman and Cannon 1996a, 1996b). However, the differentiation based solely on pseudotentacle shape (double-folded or complex in Phrikoceros vs pointed or ear-like in Tytthosoceros ; Bahia 2022) is unreliable due to the potential to misinterpret or overlook this feature. Consequently, Litvaitis et al. (2019) synonymised Tytthosoceros with Phrikoceros based on the phylogenetic placement of Tytthosoceros mopsus ( Marcus, 1952) which had been misidentified as Phrikoceros mopsus (see Bahia 2022). In contrast, Bahia (2022) argued for the reinstatement of Tytthosoceros as a valid genus, highlighting the misidentification of T. mopsus . Resolving the taxonomic confusion between these genera requires further investigation, involving more molecular data especially from the type species of Phrikoceros , as suggested by Bahia (2022). Therefore, we adopt a conservative approach, considering both genera as Phrikoceros sensu lato (s.l.) and provisionally including all Tytthosoceros species herein.

Nine species are currently recognised within Phrikoceros s.l.: Phrikoceros baibaiye Newman and Cannon 1996b , Phrikoceros diadaleos Newman and Cannon, 1996b , Phrikoceros fritillus Newman and Cannon, 1996b , Phrikoceros galacticus Newman and Cannon, 1996b , Phrikoceros katoi Newman and Cannon, 1996b , Phrikoceros lizardensis ( Newman and Cannon, 1996a) , and Phrikoceros nocturnus ( Newman and Cannon, 1996a) in the Indo-Pacific ( Newman and Cannon 1996a, 1996b; Bolaños et al. 2016). Additionally, Phrikoceros inca ( Baeza et al., 1997) is found on the Pacific coast of South America ( Baeza et al. 1997; Bahia et al. 2012, 2014; Araya and Aliaga 2016; Bahia and Schrödl 2018; Reyes et al. 2020), and Ph. mopsus is found on the Atlantic coast of South America ( Marcus 1952; Quiroga et al. 2004; Bulnes et al. 2011). In Japan, Ph. baibaiye , Ph. diadaleos , and Ph. fritillus , along with three additional unidentified species, have been reported solely from Okinawa based on photographic records ( Ono 2015).

We propose transferring Ps. sagamianus to Phrikoceros s.l. based on several morphological characters: the pharynx (plicated with shallow, simple folds in our specimens; Figure 3 View Figure 3 (B)), body margin characteristics, and pseudotentacle morphology. Remarkably, the plicated pharynx of our specimens differs from the ‘butterfly-shaped’ pharynx with complex pharyngeal lobes of Pseudoceros species (see Newman and Cannon 1994). However, limitations exist in definitively confirming this distinction due to the absence of type material and insufficient detail in the original descriptions by Kato (1937, 1939). The morphology of this species, particularly the deeply ruffled body margin of this species (unlike the smooth margins in Pseudoceros ) and the erect pseudotentacles (in contrast to the ‘folded flaps’ described for Ps. sagamianus by Kato 1939, p. 76), aligns better with Phrikoceros sensu lato (s.l.) than with Pseudoceros ( Newman and Cannon, 1994, 1996b). The species also corresponds to the diagnosis of Tytthosoceros in having (1) the ear-like shape of the pseudotentacles and (2) dorsal pseudotentacular eyespots not in four clusters, which are supposed to distinguish Tytthosoceros from Phrikoceros . Therefore, depending on the confirmation of the taxonomic separation between Phrikoceros and Tytthosoceros , a future transfer to Tytthosoceros might be warranted.

Phrikoceros sagamianus comb. n. exhibits a colour pattern similar to Ph. mopsus , originally described from the Atlantic coast of Brazil ( Marcus 1952; Quiroga et al. 2004; Bulnes et al. 2011; Bahia et al. 2012, 2014). The two species share a reddish-brown background with white blotches and a narrow black rim on the dorsal surface. However, Ph. sagamianus comb. n. can be distinguished by having white maculae forming transversal lines along the body periphery based on our specimens, contrasting with the absence of those maculae in Ph. mopsus ( Bulnes et al. 2011; Bahia et al. 2012, 2014). In addition to the colour pattern, the species share internal reproductive features: (1) separate openings of the paired sperm ducts (or spermiducal vesicles) into the seminal vesicle, and (2) independent entry of the ejaculatory duct and the prostatic duct into the base of the penis papilla ( Marcus 1952; Bulnes et al. 2011).