Prosthiostomum cf. parvicelis, , Hyman, 1939

Ramos-Sánchez, Mariela, Carrasco-Rodríguez, Dahani S., García-Madrigal, María Del Socorro & Bastida-Zavala, J. Rolando, 2021, Marine flatworms (Platyhelminthes: Polycladida) found in empty barnacle shells including a new species, from southern Mexican Pacific, Zootaxa 4965 (2), pp. 301-320 : 311-315

Prosthiostomum cf. parvicelis

( Figures 6A–F , 7A–F , 8A–C )

Type locality of nominal species: Sullivan Bay, Santiago Island, Galápagos Archipelago ( Hyman 1939b, 1953b).

Material examined. Two specimens. One specimen as whole mount: UMAR PLAT-042 , 1 spec. (La Tijera Beach, inside the empty shell of Megabalanus peninsularis , in rocky intertidal, March 7, 2017, Coll. DSCR). One specimen in histology sections: UMAR PLAT-043 A-C, 1 spec. in sections of reproductive structures in four slides (San Agustinillo Beach, inside the empty shell of M. peninsularis , in rocky intertidal, March 11, 2017, Coll. MRS) .

Description of external features

Color. The background color is light brown, with a coloration pattern composed of dark brown to dark green spots distributed around the body, dorsal mid-region without pigmentation. Three longitudinal lines, two of which are brown to dark green conglomerate patches distributed from the anterior to the posterior region of the body; separated by a thin line without pigmentation ( Figs 6A–C ). Light brown brain region ( Fig. 6C ).

Form: Elongated, rounded anterior region and posterior with slightly pointed end; 11.1– 20 mm long and 4.16–5 mm wide ( Figs 6A, D–F , 8A ).

Pseudotentacles. Absent.

Eyes. Marginal eyes distributed in the first third of the body, extending along the first third of the pharynx; between 24–27 cerebral eyes distributed in two clusters and located on the cerebral region ( Figs 6C , 8A ).

Brain. Slightly bilobed, measuring 0.25 mm in length and 0.25 mm in width; lacks globuli cell masses of the brain.

Digestive system. Cylindrical pharynx ( Figs 6E , 8A ), located in the first third of the body, measuring 3.2 mm long and 0.7 mm wide, is positioned 1 mm from the anterior margin of the body. The mouth is located in the anterior region of the pharynx, posterior to the brain. The main intestine ( Fig. 7A ) is oriented towards the posterior region of the body.

Gonopores and ventral sucker. Separate gonopores ( Figs 6E , 8A ) located in the central region of the body. The male gonopore is positioned 4.6 mm from the anterior margin of the body, 0.55 mm from the end pharynx, at a distance of 0.5 mm from the female gonopore and 2 mm from the sucker.

Sucker circular to oval ( Figs 6E , 8A ), located posterior to the female gonopore at a distance of 1.6 mm, at 2.55 mm from the pharynx, and at 7.15 mm from the anterior margin of the body; measuring 0.55 mm long and 0.5 mm wide.

Description of internal features

Male reproductive system. With prominent oval-elongate seminal vesicle ( Figs 7E–F , 8B ), measuring 0.45 mm long and 0.35 mm wide; located posterior to the male gonopore and accessory vesicles ( Figs 7E , 8B ); with an ejaculatory duct ( Figs 7A–C , 8C ) running through in the middle of the accessory vesicles; the duct ends in the penis papilla, armed with a stylet.

The male reproductive system with paired spermiducal ducts that connect in the anterior region of the seminal vesicle. Spermiducal bulbs located between the accessory vesicles and seminal vesicle and distributing towards the posterior region of the body and ventral to the intestine. Paired circular accessory vesicles ( Figs 7A–C, E–F , 8B–C ) located on both sides of the male gonopore, measuring 0.2 mm in length and width, and located antero-dorsal to the seminal vesicle ( Figs 7E , 8B ).

Glandular epithelium ( Fig. 7B ) in the anterior region of the penis papilla and on both sides of it. The penis papilla ( Fig. 7C ) is highly muscularized, hook-shaped, ends in a tip and is armed with a sclerotized, tubular, obtuse and curved stylet ( Figs 7D , 8B–C ), measuring 0.2 mm long, 0.07 mm wide in the basal region and 0.03 in width in the apical region.

Male atrium ( Figs 7A–C , 8C ) short, 0.4 mm long and 0.05 mm wide; in an oblique position to the male gonopore.

Female reproductive system. With a short vagina ( Figs 7A , 8C ), 0.08 mm long and 0.2 mm wide, oriented towards the anterior region of the body. The cement glands are distributed around the vagina; with a cement pouch located around the walls of the vagina and the female atrium. The female atrium is short and narrow, measuring 0.1 mm long by 0.05 mm wide. The uterus is oriented towards the anterior region of the body.

Habitat. This species is found inside and around empty shells of the barnacle Megabalanus peninsularis ( Fig. 6G ), which encrust the surf zone of the rocky coast.

Distribution. Tropical Eastern Pacific. Central coast of Oaxaca: La Tijera and San Agustinillo beaches.

Taxonomic remarks. The nomenclature of the structures used in the diagnosis of the species was made based on the proposal of Hyman (1953a); the present study considered a “seminal vesicle” as a tubular, oval, or spherical, muscularized body of fibrous epithelium ( Hyman 1953a; Faubel 1983); a “prostatic vesicle” as an oval, circular or cylindrical structure which is composed of a muscular wall and a granular epithelial lining, that can form compartments ( Faubel 1983); and an “accessory vesicle” as an oval, circular body of muscularized epithelium composed of circular fibers ( Lang 1884; Laidlaw 1902).

The specimens described here were identified as Prosthiostomum cf. parvicelis , based on a ventral sucker posterior to the female gonopore, a penis papilla armed with a stylet, free accessory vesicles, and marginal eyes distributed only in the anterior region of the body ( Hyman 1939b; 1953b; Faubel 1984). The separation between Prosthiostomum cf. parvicelis and the nominal species is difficult, mainly because of lack of information in the original description and re-description by Hyman (1939b; 1953b). The genus Prosthiostomum had not been recorded previously in the Mexican Pacific.

Prosthiostomum cf. parvicelis was compared with the original description ( Hyman 1939b) and the redescription ( Hyman, 1953b) of the nominal species, P. parvicelis , from Sullivan Bay, Galápagos Archipelago ( Hyman 1939b), with the description of the type species of the genus, P. siphunculus (Delle Chiaje, 1822) , from the Gulf of Naples, Mediterranean Sea, considered a widely distributed species ( Laidlaw 1902; Noreña et al. 2014), and with the description of two other species recorded in the Eastern Pacific region: P. latocelis Hyman, 1953 , from California; and P. multicelis Hyman, 1953 , from San Gabriel Bay, Baja California Sur.

In the original description and redescription of P. parvicelis, Hyman (1939b ; 1953b) characterized the male reproductive system of her specimens with structures everted and the reproductive system in a lateral position. Furthermore, she did not describe the female apparatus. A revision of type and topotype material of P. parvicelis , together with the redescription of the following characters of the hermaphroditic apparatus in sagittal section would be desirable: the position of the accessory vesicles with respect to the seminal vesicle; the position and distribution of the ejaculatory duct; terminal morphology of the stylet and the orientation of the vagina, as well as a description of the color pattern of the species and contrast it with the morphological characteristics present in Prosthiostomum cf. parvicelis .

The pattern of coloration and the orientation of the vagina are main characters that allowed us to differentiate Prosthiostomum cf. parvicelis from P. siphunculus , P. latocelis , and P. multicelis . Prosthiostomum cf. parvicelis has a light brown color with dark brown to dark green spots distributed around the body, followed by the dorsal region without pigmentation. It presents three longitudinal lines, two of which are brown to dark green conglomerate patches distributed from the anterior to the posterior regions of the body, separated by a thin line without pigmentation. The vagina oriented is towards the anterior region of the body. Prosthiostomum siphunculus which has “dorsal coloration beige to yellow, without spots or bands, the ventral side smooth and pale” ( Noreña et al. 2014: 14; Figs 6A–B ); the “pharyngeal and whitish cerebral region with a faint pigmentation of black color” ( Lang 1884: 594–601; Pl 5, Fig. 3 ), and a vagina oriented towards the posterior region of the body ( Lang 1884: pl 30, Fig. 20; Noreña et al. 2014:15; Figs 6C–D ). Prosthiostomum latocelis has a yellow dorsal surface ( Hyman 1953a; 382–384; Figs 155–157), and a vagina oriented towards the posterior region of the body; and P. multicelis has a usually pale background, with a tan overcast and slightly yellowed margin ( Brusca 1980: 73).

Figures

FIGURE 3. Stylochus mistus sp. nov. A–G (paratype UMAR-PLAT 41). A–G. specimen stained in Mayer´s carmalum and cleared in methyl salicylate; B–G. details of A, in whole mount; B–C. anterior region of the body, D. brain; E. pharynx; F–G. male and female reproductive system. Structures: b= brain, ce= cerebral eyes, fe= frontal eyes, fg= female gonopore, m= mouth, me= marginal eyes, mg= male gonopore, nt= nuchal tentacle, ovi= oviducts p= pharynx, sv= seminal vesicle, spv= spermiducal vesicle, te= tentacular eyes. Scaling bar of the figure A= 1 mm.

FIGURE 6. Prosthiostomum cf. parvicelis. A–C. (UMAR PLAT-043) specimen in vivo; D–F. (UMAR PLAT-042); D–E. specimen preserved in ethanol; F. specimen stained in Mayer´s carmalum and cleared in methyl salicylate; G. Megabalanus peninsularis. Structures: ce= cerebral eyes, fg= female gonopore, mg= male gonopore, me= marginal eyes, p= pharynx, s= sucker. Scaling bar of the figure A–B= 8 mm, D–F= 1 mm, G= 1 cm.

FIGURE 7. Prosthiostomum cf. parvicelis. A–D, F. (UMAR PLAT-043), E (UMAR PLAT-042). A–D. male and female reproductive system, in sagittal histological sections; D.stylet and F. accessory vesicles andseminal vesicle in sagittal histological section; E. male reproductive system, in whole mount. Structures: ac= accessory vesicle, acd= accessory vesicle duct, cg= cement gland, ed= ejaculatory duct, gp= glandular epithelium, ma= male atrium, mg= male gonopore, mi= main intestine, ovi= oviducts, pp= penis papilla, st= stylet, sv= seminal vesicle, va= vagina.

FIGURE 8. Prosthiostomum cf. parvicelis. A–B. (UMAR PLAT-042); C (UMAR PLAT-043); A. diagrammatic representation of complete specimen; B. diagrammatic representation of the male and female reproductive systems, in whole mount; C. diagrammatic representation of the male and female reproductive systems, in sagittal section. Structures: ac= accessory vesicle, acd= accessory vesicle duct, ce= cerebral eyes, cg= cement gland, ed= ejaculatory duct, fg= female gonopore, ma= male atrium, mg= male gonopore, me= marginal eyes, p= pharynx, s= sucker, st= stylet, sv= seminal vesicle, ovi= oviducts, va= vagina.