Stylochus sixteni Marcus, 1947

Cuadrado, Daniel, Hernández-Gonzalez, Alejandro, Noreña, Carolina & Simões, Nuno, 2024, Polyclads (Platyhelminthes) in the southern Gulf of Mexico: unveiling biodiversity and descriptions of two new species, ZooKeys 1221, pp. 103-144 : 103-144

Stylochus sixteni Marcus, 1947

Fig. 24

Material examined.

Yucatan coast, Mexico • 1; Dzilam; 21.5 ° N, 88.9 ° W; 9.3 m; 8 May 2018; A. Hernández leg.; CRPPY - 0090 • 1; 12 slides; Dzilam; 21.5 ° N, 88.9 ° W; 13 m; 10 May 2018; A. Hernández leg.; CRPPY - 0099 .

Distribution.

The species was originally described in Cape Verde ( Bock 1931). New record for the Gulf of Mexico (Yucatan).

Description.

Elongated oval body shape, firm and fleshy consistency. In live specimens, body measures 20 mm in length, and 7 mm in width, whereas in fixed specimens, measures are reduced to 10 mm length, and 8 mm in width. Rounded nuchal tentacles visible. Pale beige colouration, translucent near the margins. Small white spots, denser along the midline and a white dotted line along the entire body margin (Fig. 24 A, D ). Two elongated clusters of cerebral eyes between nuchal tentacles. Densely packed tentacular eyes. Marginal eyes in the first 1 / 3 of the body, up to the level of the tentacles (Fig. 24 B ). Ruffled pharynx centrally located, oral opening at its centre. Reproductive system. Male and female copulatory apparatus posterior to pharynx, near second 1 / 2 of body. Male apparatus comprises a free prostatic vesicle and a small kidney-shaped seminal vesicle. Ejaculatory duct penetrates through an anteroposterior-oriented penis papilla into the deep atrium. Penis papilla, small, conical. Male atrium heart-shaped, deep (Fig. 24 C, E ). Female apparatus simple, anteroposteriorly oriented, slightly muscular vagina, and small ciliated female atrium (Fig. 24 C, E ).

Remarks.

To avoid confusion between Stylochus crassus Verrill, 1893 (from the coast of Maryland, USA) and S. crassus Bock, 1931 (from the coast of Cape Verde Island), Marcus (1947) renamed the specimens described by Bock (1931) as Stylochus sixteni . The specimen captured on the coast of Yucatan is most similar to the specimen described by Bock (1931). The major difference between the specimens described by Bock (1931) and Stylochus sixteni from the Gulf of Mexico is the distance of the reproductive system from the posterior end of the animal. The reproductive organs, in Bock’s description, are close to the body end, while in the Yucatan specimens, they are near to the posterior end, but not so close as in Bock’s. However, this may be due to the difference in size, since the specimens from Cape Verde measured 10 mm, while those from the Yucatan coast measured 20 mm (Fig. 24 E ).

Figures

Figure 24. Stylochus sixteni A live animal, photographed on a black background, tentacles B anterior end, marginal, cerebral and tentacular eyes, after fixation C sagittal section through the reproductive system (stained with AZAN) D specimen in situ E sagittal reconstruction of male and female copulatory organs.