Stylochus pilidium ( Goette, 1881 )

Marquina, Daniel, Osca, David, Rodríguez, Jorge, Fernández-Despiau, Estrella & Noreña, Carolina, 2014, State of knowledge of the Acotylea (Polycladida, Platyhelminthes) from the Mediterranean coasts of Spain: new records and new species, Zootaxa 3780 (1), pp. 108-134 : 128-130

Stylochus pilidium ( Goette, 1881)

Figs. 1 (9), 13

Synonyms: Planaria neapolitana Delle Chiaje, 1841 in Goette, 1878; Stylochopsis pilidium Goette, 1881 .

Material examined. One mature specimen, collected in Los Urrutias (Mar Menor, Murcia). From Caulerpa prolifera found two metres deep; Fig. 1 (9).

Description. Fixed specimen about 10 mm long and 7 mm wide. Colour lost during conservation. Well developed nuchal tentacles, with numerous internal eyes. Numerous cerebral eyes behind the tentacles, arranged in two longitudinal clusters. Large marginal eyes arranged around the first third of the body edge. Few frontal eyes. The muscular pharynx is located in the middle third of the body. Mouth opening in the centre ( Fig. 13 A).

Gonopores separated and located in the last third of the body. Testes ventral. Male copulatory apparatus with simple seminal vesicle, which has weak muscular walls. Spermiducal bulbs developed. Prostatic vesicle elongated (225 µm in length and 75 µm in width) and well developed, dorsal to the seminal vesicle, with thick muscular envelope. The prostatic duct joins the ejaculatory duct at the base of the prostatic vesicle ( Figs. 13 B, 13E). The ejaculatory duct runs straight to the tip of a short conical penis papilla housed in a ciliated atrium.

Ovaries dorsal. Female system lacks Lang’s vesicle ( Figs. 13 C, 13E). The oviducts enter separately. Proximal tract of female duct, the vagina interna or internal vagina, extending forwards, first anterior-dorsally and then anterior-ventrally. The last tract, which is known as external vagina or vagina externa, runs straight down, with strong muscular walls, wider than the proximal tract. Narrow, forward-oriented diverticulum at the top of the external vagina ( Fig. 13 D), not described for any species of the genus Stylochus .

Discussion. Given that descriptions of Stylochus pillidium are rare, we aim to provide a detailed description of this species.

Goette (1881) first designed this species as Planaria neapolitana , but later identified it as Stylochopsis pilidium . In his original description, Stylochopsis pilidium was only compared to Stylochus neapolitanus . Stylochus and Stylochopsis are both cited by other authors ( Lang 1884, Meixner 1907), however, these identifications were made based only on external morphology. Novell (2003) was the first to describe its internal anatomy.

Stylochus pilidium is clearly similar to S. neapolitanus as both species share the following characters: the junction of the ejaculatory and prostatic duct, the shape and size of the penis papilla and the outline of the seminal vesicle and of the female system. Nonetheless, although in previous descriptions ( Lang 1884; Meixner 1907), S. pilidium and S. neapolitanus are described as quite similar or even equal, some characters are different in both species: S. pilidium shows separated gonopores, which in S. neapolitanus are common, and the distance from the female gonopore to the posterior end is greater and the prostatic vesicle is longer in S. pilidium than in S. neapolitanus .

Distribution. West Mediterranean Sea: Naples ( Lang 1884); Sitges, Catalonia ( Novell 2003); Mar Menor, Murcia (this paper).

Figures

FIGURE 1. Distribution map of examined specimens. General views of A the eastern coast of the Iberian Peninsula, B Mar Menor (Murcia). Numbers: Notoplanella inarmata (1), Notoplanella estelae sp. nov. (2), Trigonoporus cephalophtalmus (3), Leptoplana mediterranea (4), Planocera graffi (5), Imogine mediterranea (6), Imogine stellae sp. nov. (7), Stylochus neapolitanus (8), Stylochus pilidium (9).

FIGURE 13. Stylochus pilidium (Goette, 1881). A Sagittal reconstruction of the whole animal anterior to the left); B – D histological sections, B male copulatory complex, C, D female reproductive system; E sagittal reconstruction of male and female genital systems (anterior to the left). Abbreviations: b, brain; ce, cerebral eyes; cg, cement glands; div, diverticulum; ed, ejaculatory duct; fe, frontal eyes; fp, female pore; i, intestine; m, mouth; me, marginal eyes; mp, male pore; o, ovary; ov, oviduct; ph, pharynx, pp, penis papilla; pv, prostatic vesicle; sv, seminal vesicle; t, tentacle; ts, testes; te, tentacular eyes; v, vagina; vd, vas deferens. Scale bars: A 1.5 mm, B, C, D, E 200 µm.