Plehnia cascaisensis, Pérez-García & Gouveia & Calado & Noreña & Cervera, 2024

Pérez-García, Patricia, Gouveia, Filipa, Calado, Gonçalo, Noreña, Carolina & Cervera, Juan Lucas, 2024, Acotylea (Platyhelminthes, Polycladida) from the southern and western Iberian Peninsula, with the description of five new species, Zoosystematics and Evolution 100 (4), pp. 1487-1513 : 1487-1513

Plehnia cascaisensis sp. nov.

Figs 6 , 7

Holotype.

• MNHNC MB 16-000107 , Station 3 , 29 May 2015 at 28 m depth, 9 mm long fixed, sagittal sectioned into 8 slides.

Additional material.

• MNHNC MB 16-000108 , Station 3 , 26 May 2015, at 9.1 m depth, 4 mm long, fixed, and sagittally sectioned into 3 slides . • MNHNC MB 16-000109 , Station 3 , 28 May 2015 at 24 m depth, 6 mm long fixed . • MNHNC MB 16-000110 , Station 3 , 28 May 2015 at 24 m depth, 6 mm long fixed .

Diagnosis.

Plehniidae with a yellowish-brown background colour and multiple brown spots scattered on the dorsal surface; pale yellow ventral body; well-separated gonopores; penis papilla characterised by a cap-shaped connective tissue; wide external vagina; and very long internal vagina.

Description.

Plehniidae with fleshy and elongated body shape, rounded anteriorly and posteriorly, with high thickness. Length of voucher specimens varies between 4 and 6 mm fixed (average of 6.25 ± 2.06 mm). Ground colouration yellowish-brown with numerous brown spots scattered across the dorsal surface (see Fig. 6 A, B ). Ventral side pale yellow. Nuchal tentacles present (Fig. 6 D ). Tentacular eyes arranged in two clusters, located just under the tentacles. Small cerebral eyes in two clusters, extending from the tentacle to the anterior margin (Fig. 6 C ). Frontal eyes scattered at the anterior body margin. Ruffled pharynx positioned between the second and third regions. Genital pores separated and posterior to the pharyngeal pocket. Ovaries dorsal to the testes.

Reproductive system.

Male reproductive complex directed backwards, composed of two spermiducal vesicles, a free prostatic vesicle, and an unarmed penis papilla (Fig. 6 E ). Vas deferens run ventrally under the vagina and turn backwards once reached the pharynx. Vas deferens expand and distally form spermiducal vesicles. Spermiducal vesicles reach the prostatic vesicle separately and medially (Fig. 7 C ). Prostatic vesicle large and pear-shaped (0.27 mm high and 0.32 mm wide) with a smooth glandular lining and strong muscular wall (0.12 mm thick). Numerous prostatic glands. Distally, a short penis papilla (0.1 mm long) is housed in a shallow male atrium. Penis papilla surrounded by cap-shaped connective tissue.

Female reproductive systems were large and completely developed in the studied animals (Figs 6 F , 7 A – C ). Wide external vagina (0.08 mm), but not bulbous (Fig. 6 F ). Narrow, tube-like internal vagina ending in Lang’s vesicle. Lang’s vesicle is small (Fig. 7 B ) and oval-shaped (0.07 mm wide). The oviducts open in the distal region of the internal vagina.

Type locality.

Cascais, Portugal.

Etymology.

The specific name is dedicated to Cascais, the area where the holotype was collected.

Remarks.

The presence of a separate opening of the vas deferens into the neck of the prostatic vesicle, the smooth glandular lining, the absence of a seminal vesicle, the long female vagina, and the presence of a well-developed Lang’s vesicle allow the inclusion of the specimens in the genus Plehnia . Plehnia comprises four recognised species: Plehnia arctica ( Plehn, 1896) , P. ellipsoides (Girard in Stimpson, 1854), P. caeca Hyman, 1953 , and P. ovatus Kato, 1937 .

As shown in Table 3 , Plehnia arctica and P. cascaisensis sp. nov. share only the position of the pharynx. However, many characteristics of P. arctica remain unclear due to its insufficient description. Notably, there are differences in size; P. arctica is larger and lacks nuchal tentacles, whereas P. cascaisensis sp. nov. is smaller and possesses tentacles.

On the other hand, the new species exhibits certain similarities with P. ellipsoides (see Table 3 ), including the presence of tentacular, cerebral, and marginal eyespots, as well as similar background colouration. Substantial differences emerge, such as the absence of tentacles in P. ellipsoides and the position of the spermiducal vesicles’ opening into the prostatic vesicle, which is distally positioned in P. ellipsoides and medially positioned in P. cascaisensis sp. nov. Additionally, differences in the female reproductive system exist; P. ellipsoides has a strong external vagina and an elongated Lang’s vesicle, whereas the new species from Cascais has a wide external vagina and a small, rounded Lang’s vesicle.

P. cascaisensis sp. nov. and P. ovatus are the only Plehnia species that share the presence of tentacles and the position of the opening of the spermiducal vesicles into the prostatic vesicle (see Table 3 ). Nonetheless, P. ovatus is smaller and possesses an oval body shape, resembling that of Stylochidae species (see fig. 4 in Kato 1937). In contrast, P. cascaisensis sp. nov. is elongated and narrower (Fig. 6 A ). The dorsal pigmentation also differs, with P. ovatus being darkish brown and P. cascaisensis sp. nov. being yellowish brown, displaying a spotted pattern. In P. ovatus , the prostatic vesicle is elongated, and the penis papilla opens into a large male atrium lined with ciliated cells. In P. cascaisensis , the vesicles are rounder and wider, and the penis papilla is contained within a shallow male atrium. The Lang’s vesicle in the Japanese species is larger than that in the Iberian species.

P. caeca and P. cascaisensis differ externally and within the reproductive system, mainly in the female system. P. caeca lacks eyes and tentacles and has an elongated and less muscular prostatic vesicle. P. caeca also lacks a Lang’s vesicle, and the oviducts empty into the proximal region of the internal vagina. In contrast, P. cascaisensis has a well-developed Lang vesicle and oviducts that empty into the distal region of the internal vagina.

All of these differences, as well as the unique characteristics of P. cascaisensis sp. nov., such as its spotted dorsal pattern, wide external vagina, and long internal vagina, led us to consider the specimens from Cascais as a distinct species.

Biology.

collected in a rocky area covered by a reef of Sabellaria Lamarck 1818 (Annelida, Polychaeta).

Distribution.

only known from Cascais, Portugal.

Figures

Figure 6. Plehnia cascaisensis sp. nov. Histological sections are postero-anteriorly oriented: A. Dorsal view of a living specimen (holotype, MNHNC MB 16-000107); B. Dorsal view of a living specimen (additional material, MB 16-000108); C. Detail of the eyes (additional material); D. Histological section of the tentacle (holotype); E. Histological section of the male copulatory organs (holotype); F. Histological section of the male and female organs (holotype). Abbreviations: cg, cemental glands; el, epithelial lining; ev, external vagina; fp, female pore; iv, internal vagina; ma, male atrium; mp, male pore; mw, muscular wall; ov, oviduct; ph, pharynx; pp, penis papilla; pv, prostatic vesicle; t, tentacle; vg, vesicular glands.

Figure 7. Plehnia cascaisensis sp. nov. (holotype, MNHNC MB 16-000107). Histological sections are postero-anteriorly oriented: A. Histological section of the long internal vagina (holotype); B. Histological section of the end of the internal vagina and Lang’s vesicle (holotype); C. Sagittal reconstruction of the reproductive system (holotype). Abbreviations: cg, cemental glands; ed, ejaculatory duct; el, epithelial lining; ev, external vagina; fp, female pore; i, intestine; iv, internal vagina; lv, Lang’s vesicle; ma, male atrium; mp, male pore; mw, muscular wall; ov, oviduct; ph, pharynx; pp, penis papilla; pv, prostatic vesicle; sb, spermiducal bulbs; spv, spermiducal vesicle.