Stylochoplana sisalensis, Cuadrado & Hernández-Gonzalez & Noreña & Simões, 2024

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

Stylochoplana sisalensis sp. nov.

Fig. 19

Material examined.

Holotype: Campeche coast, Mexico • 1; 9 slides; Cayos sumergidos del Oeste; 20.4 ° N, 92.2 ° W; 0 m; 11 Sep. 2017; A. Hernández leg.; CRPPY - 0013 . Paratypes: Campeche coast, Mexico • 1; Cayo Arcas; 20.2 ° N, 92.0 ° W; 6.2 m; 20 Aug. 2018; A. Hernández leg.; CRPPY - 0110 • 1; Cayo Arcas; 20.2 ° N, 92.0 ° W; 3.3 m; 20 Aug. 2018; A. Hernández leg.; CRPPY - 0111 .

Distribution.

Found in submerged West Keys of Reef Triángulos and Cayo Arcas, Campeche coast, Mexico.

Description.

Body shape oval with rounded anterior and posterior end, 10 mm long and 5 mm wide. Whitish translucent colour with a pale brown tonality due to the gut contents. A network of independent intestinal branches, not anastomosing, extends to the body’s margin (Fig. 19 A ). Presence of two compact clusters tentacular eyes (12–17 eyes per cluster), two scattered clusters of cerebral eyes (15–20 eyes per cluster, distributed within 0.03 mm in front of the tentacular eyes) and some marginal eyes in the frontal region (Fig. 19 B ). Ruffled pharynx located in the second 1 / 3 of the body. Reproductive system. Testes dorsal and ovaries ventral. Seminal vesicle well developed, elongated and wide surrounded by thin muscular walls. The ejaculatory duct runs upward, backward, and then curves downward before widening as it enters the penis papilla. Lining of walls of this internal dilation forms epithelial glandular prostate tissue which functions as a prostatic vesicle. The glandular prostate epithelium stores the secretion from the extra-vesicular glands (Fig. 19 B, D ). Conical, naked penis papilla (without a stylet) covered by a non-ciliated, flat epithelium. It projects into a deep male atrium, with a tall ciliated epithelium (Fig. 19 C ). The female copulatory organ was barely developed in the only specimen observed, so it could not be described in detail.

Etymology.

The name sisalensis is dedicated to the town where the research centre is located, the UNAM campus in Sisal, Yucatán province, Mexico.

Remarks.

Currently, the genus Stylochoplana comprises 25 valid species worldwide. This genus is one of the most species-rich within the order Polycladida and has been divided into different informal groups by several authors ( Bock 1913; Marcus and Marcus 1968) since it was described by Stimpson (1857). Stylochoplana sisalensis is included in group B of Bock (1913) or B 1 of Marcus and Marcus (1968), characterised by tentacles absent, unarmed papilla peneal, absence of penial pocket, and Lang’s vesicle present. This group includes the following species:

S. chilensis ( Schmarda, 1859) : with epithelial-glandular prostate tissue ( Stummer-Traunfels 1933).

S. chloranota (Boone, 1929) : with interpolated prostatic vesicle ( Hyman 1953).

S. graffi (Laidlaw, 1906) : with interpolated prostatic vesicle ( Bock 1913).

S. longipenis Hyman, 1953 : with interpolated prostatic vesicle.

S. minuta Hyman, 1959 : with epithelial-glandular prostate tissue, but forms a receptacle or container in the proximal region of the papilla peneal.

S. nadiae (Melouk, 1941) : without data.

S. suosensis Kato, 1943 : with epithelial-glandular prostate tissue. The female apparatus is not known, and so it is unknown whether the species belongs to B 1 (with Lang’s vesicle) or B 2 (without Lang’s vesicle).

S. utunomii Kato, 1943 : with epithelial-glandular prostate tissue.

S. walsergia Marcus & Marcus, 1968 (no. 12): with epithelial-glandular prostate tissue.

Stylochoplana sisalensis sp. nov. presents the greatest similarity with S. walsergia from Brazil, S. chilensis from Chile, S. utunomii from Japan, and S. minuta from the Palau Islands ( Micronesia). These species are all characterised by the presence of a well-developed and elongated seminal vesicle, as well as an ejaculatory duct that widens and is covered by a prostatic glandular epithelium. All other species in this group present an isolated, more or less elongated and interpolated prostatic vesicle.

On the other hand, the species of Marcus’ Group BI present a very similar female copulatory apparatus directed towards the anterior region and, at the level of the internal vagina, then curving towards the posterior region. In the middle of the female duct, the oviduct opens and the internal epithelium thickens to form Lang’s duct that ends in the rounded Lang’s vesicle.

Stylochoplana sisalensis differs from S. walsergia by the location of the prostate tissue and the shape of the penis papilla. In S. walsergia , the prostatic dilation is included entirely in the penis papilla and surrounded by the male atrium, while in S. sisalensis the penis papilla encloses only 1 / 2 of the prostatic tissue and the common male duct. This characteristic is shared by S. chilensis , but not with S. utunomii in which the prostate tissue is practically outside the penis papilla, a short protrusion within the male atrium. As in S. suosensis , we lack data on the female apparatus, but we assume that its arrangement is like that of the entire Marcus group B of Stylochoplana .

Figures

Figure 19. Stylochoplana sisalensis sp. nov. A dorsal view B sagittal section of a male copulatory organ without a prostatic vesicle, only showing a penis papilla C anterior end with cerebral and marginal eyes D sagittal reconstruction of the reproductive system.