Phrikoceros sagamianus Tsuyuki & Kohtsuka & Okuno 2025, comb. n.
Creators: name: Tsuyuki, Aoi
name: Kohtsuka, Hisanori
name: Okuno, Junji
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Platyhelminthes
Polycladida
Pseudocerotidae
Phrikoceros
Phrikoceros sagamianus

description: Phrikoceros sagamianus (Kato, 1937) comb. n.
(New Japanese name: Kanoko-nisetsuno-hiramushi)
(Figures 3, 4)
Pseudoceros sagamianus Kato, 1937: 362–363, 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, 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. ICHUM 8978, sagittal sections of the reproductive organs (10 slides) along with the remaining unsectioned body (preserved in 70% ethanol), collection data same as above. ICHUM 8979, whole mount specimen, collection data same as above. 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.
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 (A,B)). Pair of pseudotentacles small, ear-like, held erect (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 (A)). Tips of pseudotenctacles white without black line (Figure 3 (A)). Ventral surface translucent, with narrow black line (Figure 3 (B)). Cerebral-eyespot cluster horseshoe shaped, 0.35 mm in length, composed of about 55 eyespots, distributed in translucent area (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 (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 (B)). Mouth situated centrally in pharynx. Male gonopore, 220 µm in diameter, located immediately behind posterior end of pharyngeal pouch (Figure 3 (B)). Male copulatory apparatus composed of large seminal vesicle, prostatic vesicle, and armed penis papilla (Figure 4 (A)). Pair of spermiducal vesicles running anteriorly, opening separately and posterolateral to seminal vesicle (Figure 4 (A)). Seminal vesicle elliptical (long axis 738 µm, short axis 299 µm), coated with 29 µm thick muscular layer (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 (C,D)). Prostatic duct entering penis papilla at its base independent of ejaculatory duct (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 (A,B)). Male atrium opening to exterior via male gonopore (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 (E)). Female atrium opening to exterior via female gonopore. Cement glands well developed and concentrated around cement pouch (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 (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 (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).
description: Published as part of Tsuyuki, Aoi, Kohtsuka, Hisanori & Okuno, Junji, 2025, Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea), pp. 303-330 in Journal of Natural History 59 (5 - 8) on pages 318-322, DOI: 10.1080/00222933.2024.2439032, http://zenodo.org/record/14765230