Amphiscolops hamus Ono and Kajihara, 2025

Ono, Rintaro & Kajihara, Hiroshi, 2025, Description of a new species of Amphiscolops (Xenacoelomorpha: Convolutidae), Journal of Natural History 59 (21 - 24), pp. 1611-1624 : 1615-1622

publication ID	https://doi.org/10.1080/00222933.2025.2489502
persistent identifier	https://treatment.plazi.org/id/03819522-F11B-FFF8-C82E-FE86CDF88B05
treatment provided by	Plazi
scientific name	Amphiscolops hamus Ono and Kajihara
status	sp. nov.

Treatment

Amphiscolops hamus Ono and Kajihara View in CoL sp. n.

( Figures 1–4 View Figure 1 View Figure 2 View Figure 3 View Figure 4 )

urn:lsid:zoobank.org:act:A72A17E6-9E8E-4365-B9B0-D2C3388BC8BB

Material examined

Nine serial section series were examined . Holotype: JAPAN, Okinawa Bisezaki (26.7106°N, 127.8795°E) and Seragaki (26.5066°N, 1278626°E) GoogleMaps . ICHUM 8933 View Materials (10 slides), complete set of serial cross sections . Paratypes: ICHUM 8934 View Materials (7 slides) and ICHUM 8935 View Materials (5 slides), two complete sets of serial sagittal sections; ICHUM 8936 View Materials (6 slides) and ICHUM 8937 View Materials (7 slides), two incomplete sets of serial cross sections; ICHUM 8938 View Materials (2 slides), incomplete set of sagittal sections; all from the type locality . Other materials: two living specimens in squeezed preparations from Nagahama , collected on 10 ( ICHUM 8936 View Materials ) and 11 May 2024; three sets of sections (two sagittal and one cross, each consisting of 3 slides) from Seragaki, collected on 10 May 2024 .

Diagnosis

Amphiscolops with zooxanthellae, dorsal white spots due to gaps of zooxanthella distribution, and trilobed posterior end. Statocyst and eyes present anteriorly. A mid-ventral pre-oral body (either an amorphous tissue or a cell mass) present. Frontal organ extending from anterior pit to some distance in front of mouth. Ciliated pit on each posterior lateral edge. Female gonopore absent. More than 20 bursal nozzles present. Seminal bursa present, with fibrous wall. Multiple vacuoles containing allosperm distributed within seminal bursa, with some vacuoles fused together. Ovaries and follicular testes paired. Male gonopore terminal, leading to male antrum. Male antrum swollen at entrance of seminal vesicle, connected to ciliated penis. Curved penis invaginated into seminal vesicle.

Description

Living mature specimens up to 4.2 mm long. Anterior end indented, forming pit that houses frontal organ ( Figure 1 View Figure 1 (a–c)). Posterior end trilobed, with two long lateral lobes and single short medial lobe ( Figures 1 View Figure 1 (a–c), 2(a), 3(a)). In life, specimens slightly curling their lateral sides ventrally ( Figure 1 View Figure 1 (b)); occasionally swimming into water column by ciliary movement, with lateral sides unrolled ( Figure 1 View Figure 1 (c)). Body colour light brown due to zooxanthellae ( Figure 1 View Figure 1 (a–c)). White spots due to gaps of zooxanthella distribution scattered on dorsal side ( Figure 1 View Figure 1 (a, c)), with single yellowish spot on anterior ventral side ( Figure 1 View Figure 1 (b)). Statocyst spheroidal in shape, located at anterior end ( Figure 1 View Figure 1 (d)). Single pair of reddish eyespots present at level of statocyst ( Figure 1 View Figure 1 (d)).

Lateral sides of body curled ventrally when fixed ( Figure 2 View Figure 2 ); degree of curling becoming stronger posteriorly. When laterally curled, body in dorsal view appearing tapered posteriorly ( Figures 2 View Figure 2 , 3 View Figure 3 (a)).

Epidermis fully ciliated. Zooxanthellae dispersed throughout peripheral parenchyma. Mucous gland cells densely distributed on dorsal and lateral sides ( Figure 4 View Figure 4 (a)). Frontal organ extending from anterior pit to some distance in front of mouth ( Figures 3 View Figure 3 (b), (4a)). Nervous arch surrounding statocyst ( Figure 3 View Figure 3 (a, b)). Nerve cords untraceable in sections. Mouth located on ventral surface, from U30 to U 42 in holotype ( Figures 2 View Figure 2 (a), 3(a, b), 4(b)). Ciliated pit on each lateral edge, extending from level of anterior end of seminal vesicle to terminal, from U71 to U 97 in holotype ( Figures 2 View Figure 2 (a, d), 3(a)).

Tissue of unknown function, herein referred to as pre-oral body, located mid-ventrally in front of mouth; pre-oral body being either amorphous tissue (n = 5) or cell mass (n = 4) ( Table 2). Pre-oral amorphous tissue 90–470 µm in length (n = 5), 80–320 µm in width (n = 4), eosinophilic ( Figure 4 View Figure 4 (d)) or acidophilic ( Figure 4 View Figure 4 (e, f)), containing multiple nuclei ( Figure 4 View Figure 4 (d, f)). Pre-oral cell mass 140–530 µm in length (n = 4), 340–510 µm in width (n = 3), expanding from U17 to U 32 in holotype, seemingly completely encompassed by digestive parenchyma, anteriorly abutting posterior extension of frontal glands, ventrally abutting ovaries, and lacking any connection with epidermis ( Figures 2 View Figure 2 (b), 3(a, b), 4(a, b)); each cell being 6–7 µm in diameter and having neutrophilic cytoplasm, with eosinophilic nucleus of about 4–5 µm diameter and containing cyanophilic granules ( Figure 4 View Figure 4 (c)). Position of pre-oral cell mass corresponding with external yellowish spot. In one specimen (ICHUM 8934), two fragments of cell masses with same histological appearance found isolated from main pre-oral cell mass, embedded in digestive syncytium partially protruded from mouth ( Figure 4 View Figure 4 (b)).

Ovaries paired, joining each other anterior to seminal bursa, and extending ventrally to testes from level of anterior-most pre-oral cell mass to bursa, from U16 to U 59 in holotype ( Figures 2 View Figure 2 (c), 3(a), 4(i)). Anterior to bursa, where oocytes are considered to be inseminated, mature eggs migrate to dorsal side ( Figure 4 View Figure 4 (i)). Female gonopore absent. Seminal bursa without definite boundary, extending from digestive parenchyma to seminal vesicle ( Figures 3 View Figure 3 (a, b), 4(g)). Bursa-like tissue covering anterior part of seminal vesicle ( Figure 4 View Figure 4 (j)). Bursa with fibrous wall present in ICHUM 8935–8937 and two specimens from Seragaki (field ID 0510–7 and 0510–8), all considered to be immature ( Figure 4 View Figure 4 (h)). Multiple vacuoles containing allosperm, distributed in seminal bursa ( Figures 2 View Figure 2 (c), 3(a, b), 4(g)). Vacuoles varying in shape and size, some fused with each other ( Figures 2 View Figure 2 (c), 3(a, b), 4(g)). Bursal nozzles more than 20 in number, usually attached with vacuoles, sometimes separated from latter ( Figures 1 View Figure 1 (e), 3(a, b)). Length of nozzles varying from 58 to 154 µm in living specimens.

Testes paired, follicular, dorsolateral to ovaries, extending from U13 to U 87 in holotype ( Figures 2 View Figure 2 (b, c, 3(a), 4(i))). Seminal vesicle extending from U77 to U 88 in holotype ( Figures 3 View Figure 3 (a, b), 4(j)); lacking sperm in ICHUM 8934, 8935 and one Seragaki specimen (field ID 0510–7) ( Figure 4 View Figure 4 (k); Table 2); muscular wall of seminal vesicle thicker in these specimens ( Figure 4 View Figure 4 (j, k)). Male gonopore terminal, opening to tubular male antrum extending to U 87 in holotype ( Figures 3 View Figure 3 (a, b, 4(k))). Longitudinal muscle basal to circular in antrum ( Figure 4 View Figure 4 (k)). Antrum swollen at entrance of vesicle, connected with ciliated penis ( Figures 3 View Figure 3 (a, b), 4(k)). Arrangement of musculature switched at swollen part; circular muscle basal to longitudinal in penis ( Figure 4 View Figure 4 (k)). Penis invaginated into vesicle, curved counterclockwise with tip pointing backwards ( Figure 3 View Figure 3 (a, b)); orientation of curvature varying individually. Curved penes observed in all examined specimens except ICHUM 8938, which lacked sexual organs. Swollen connections and tubular antra observed in all examined specimens except ICHUM 8936 and 8937, in which applicable sections were lost, and 8938.

Etymology

The new specific name is a noun in apposition (in the nominative case), derived from the Latin hamus (meaning ‘hook’). The specific name refers to the curved penis, one of the diagnostic features of the new species.

Type locality and distribution

Intertidal zone, Nagahama, Okinawa, Japan (26.6240°N, 128.1842°E) GoogleMaps . Specimens identifiable as A . hamus sp. n. were also collected from Bisezaki (26.7106°N, 127.8795°E) and Seragaki, Okinawa, Japan (26.5066°N, 1278626°E) GoogleMaps .

Genetic distances

The intraspecific genetic distance of A. hamus sp. n. was 0.687% in uncorrected p-distance and 0.691% in K2P ( Table 3). Among the three other species included in this analysis, the genetic distances between A. oni and A. hamus sp. n. were the smallest (23.5–23.7% in uncorrected p-distance, 28.4–28.7% in K2P), followed by those between A. bermudensis and A. hamus sp. n. (25.9% in uncorrected p-distance, 32.0% in K2P). The largest genetic distances were observed between H. blumi and A. hamus sp. n. (26.1% in uncorrected p-distance, 32.3% in K2P).

section at level of the pre-oral amorphous tissue, stained with HE ( ICHUM 8936). (e) Sagittal section through the pre-oral amorphous tissue, stained with Mallory’s trichrome ( ICHUM 8935). (f) Magnification of the pre-oral amorphous tissue, stained with Mallory’s trichrome ( ICHUM 8935). (g) Cross section at level of the seminal bursa, stained with HE ( ICHUM 8933); black arrowheads indicate the boundary of seminal bursa. (h) Cross section at level of the seminal bursa, stained with HE ( ICHUM 8936); black arrowheads indicate the boundary of seminal bursa. (i) Sagittal section through the ovary in front of the seminal bursa, stained with HE ( ICHUM 8934). (j) Cross section at level of the seminal vesicle, stained with HE ( ICHUM 8933); black arrowhead indicates bursa-like tissue. (k) Sagittal section through the male copulatory organ, with the tip slightly inverted, stained with Mallory’s trichrome ( ICHUM 8935); black arrows indicate longitudinal muscle fibres and black arrowheads circular muscle fibres, and white arrow the slightly inverted tip of penis. Scale bars: a, b, d–h = 100 µm; c = 10 µm. Abbreviations: at: amorphous tissue; ds: digestive syncytium; fo: frontal organ; m: mouth; ma: male antrum; mg: mucous gland; o: ovary; p: penis; pcm: pre-oral cell mass; sm: sperm mass; sv: seminal vesicle; t: testis.

Remarks

One hypothesis is that the pre-oral amorphous tissue is a precursor of the pre-oral cell mass. The latter may represent a cluster of oogonia or primary oocytes, and be equivalent to what has been referred to as Eilager (meaning ‘egg storage’), from which eggs were thought to originate ( Graff 1904 –1908; Costello and Costello 1938). However, this hypothesis cannot explain why the cell mass was not found in ICHUM 8935 and 0510–8, which have egg strands extending posteriorly to the mouth ( Table 2). Another hypothesis is that the cell mass is involved in digestion. This may explain why isolated fragments of cell masses were found in the digestive parenchyma protruding from the mouth ( Figure 4 View Figure 4 (b)). On the other hand, these fragments may have resulted from deformation during fixation. A less likely explanation is that the cell mass represents food or parasites (such as orthonectids). Whether or not this pre-oral cell mass is homologous to the sponge cell mass in Waminoa Winsor, 1990 remains an open question. The histological appearance of the individual cells in A. hamus resembles that of the latter, characterised by neutrophilic cytoplasm (Ono, pers. obs.). However, the sponge cell mass in Waminoa is located just behind the brain and is separate from the egg strands ( Winsor 1990; Ogunlana et al. 2005; Cooper et al. 2015), suggesting that it is not involved in oogenesis. Future studies with transmission electron microscopy should clarify the function of the pre-oral bodies in A. hamus .

Among other species in the genus, the new species resembles A. bermudensis and A. trifurcatus in having a trilobed posterior end, zooxanthellae, frontal glands, a statocyst, multiple bursal nozzles, and a single male gonopore. Amphiscolops hamus sp. n. is most similar to A. trifurcatus in having colouration due to zooxanthellae with white spots owing to gaps of zooxanthella distribution, multiple bursal nozzles with vacuoles containing allosperm, a ciliated penis invaginated into the seminal vesicle, and ciliated pits. However, the two species exhibit notable differences: the body length is up to 4.2 mm in A. hamus but 0.6–1 mm in A. trifurcatus , eyes are present in A. hamus but absent in A. trifurcatus ; testes are paired in A. hamus but unpaired in A. trifurcatus ; and the male copulatory organ consists of (i) a significantly curved penis, (ii) a swollen antrum at the connection to the seminal vesicle, and (iii) a tubular antrum in A. hamus , in contrast to (i) a slightly curved penis and (ii) a relatively short antrum in A. trifurcatus .

As mentioned above, three named Amphiscolops species have been reported from Asia ( A. japonicus , A. potocani and A. oni ), but A. hamus can be distinguished from all of them. Amphiscolops hamus possesses only zooxanthellae, whereas A. japonicus lacks symbionts, and A. potocani and A. oni possess both zoochlorellae and zooxanthellae. Amphiscolops hamus , A. japonicus and A. potocani each possess a statocyst, but adult A. oni lacks one. Amphiscolops hamus and A. japonicus possess eyes, but eyes are absent in A. potocani and adult A. oni . Amphiscolops hamus , A. potocani and A. oni possess a trilobed posterior end, whereas A. japonicus possesses a bilobed one. Amphiscolops hamus lacks a female gonopore, whereas A. japonicus , A. potocani and A. oni possess a single female gonopore. Amphiscolops hamus possesses more than 20 bursal nozzles, whereas A. japonicus possesses four, A. potocani possesses two, and A. oni possesses about nine. Amphiscolops hamus , A. japonicus and A. oni possess a single seminal bursa, whereas A. potocani possesses two. Amphiscolops hamus possesses the male copulatory organ consisting of the three parts, as described above, with the penis invaginated into the seminal vesicle, whereas A. japonicus possesses a ciliated tubular penis leading to the paired spermatic ducts and the relatively short antrum without the seminal vesicle, A. potocani possesses a ciliated, glandular, and tubular penis invaginated into tissue plug, and A. oni possesses a ciliated and tubular penis.