Material examined: Three individuals studied. One specimen collected in Playa Langosta, sagittally sectioned and designated as holotype. 44 slides: MNCN 4.01 /2576 to MNCN 4.01 /2619. Two more specimens collected in Playa Carbón sagittally sectioned and stained with Azan.
Type locality: Playa Langosta 10 ° 17’32.2 “N 85 ° 51’12.7” W, Las Baulas National Park of Guanacaste, Costa Rica (Fig. 8A), May 17-24, 2018.
Habitat: Intertidal zone of Playa Langosta, under stones within a rock pool 20 cm deep.
Etymology: The name of the species, Prostheceraeus fitae n.sp., is dedicated to Josefa Novelle, mother of the first author, also called “Fita”.
Description: External features (Fig. 8B,C): Elongated body shape with straight lateral margins and rounded posterior end. Few, short marginal waves. Background colour ivory yellowish with black elongated stains. The body margin with a pigmented band formed by a thin purple border followed by a lilac stripe that gradually darkens into a dim violet band. The marginal colouration surrounds the entire body, including pseudotentacles, pointed and with a dark violet tone. A thin orange-lilac line is drawn from behind the tentacles to the back region, in the middle dorsal region. A white patch with numerous cerebral eyes is located between the two tentacles. Ventrally whitish or colourless. Pharynx, tubular in living organisms, acquires bell shaped outline in fixed individuals. Reproductive system is located in the middle body region. Male and female gonopore separated.
Reproductive System (Fig. 8D,E): The male copulatory organ comprises in a seminal and prostatic vesicle, a folded penis papilla and a naked ejaculatory duct. The spermiducal bulb opens into the oval seminal vesicle. The seminal vesicle is surrounded by a well-developed muscle layer. The oval-shaped prostatic vesicle is anterior-dorsally oriented as shown by the prostatic duct, although the vesicular region is bent towards the posterior body end, due to the contractions during the fixation. (Fig. 8E). The spermatic duct empties into the distal region of the prostatic vesicle. Distally the characteristic penis papilla of the genus housed the short, naked ejaculatory duct without stylet (Fig. 8D). Male atrium ciliate.
The female reproductive system consists of a vagina interna (Fig. 8B) in which the oviduct empties and the vesicle externa lined with a thick and ciliated epithelium. Shell and cement-gland open into the vagina externa, but no. Uterine vesicles not observed surely due to the state of sexual maturity of the studied animal. The female atrium is broad, marked by a well-developed epithelium.
Remarks: Due to its elongated oval shape, smooth dorsal surface with longitudinal stripes on the back, pointed tentacles, cerebral eye clusters in two elongate rows. In addition to de frontal branch of the gut and the anterior-dorsally oriented prostatic vesicle, this species is placed within the genus Prostheceraeus.
Based on the colouration pattern, mainly the longitudinal stripes, Prostheceraeus fitae n.sp. can be compared to five species of the genus: Prostheceraeus fuscolineatus Dixit, Raghunathan & Chandra, 2017; Prostheceraeus vittatus (Montagu, 1815); Prostheceraeus pseudolimax Lang, 1884; Prostheceraeus giesbrechti Lang, 1884 and Prostheceraeus roseus Lang, 1884. The violet, mauve and lilac colour of P. fitae n.sp. clearly differs from the white background colour and black stripes of P. vittatus, pink with white stripes of P. roseus, the orange stripes of P. fuscolineatus, the yellow central stripe of P. giesbrechti and the two lateral black stripes and central yellow of P. fuscolineatus.
The internal anatomy of the reproductive system shows clear similarities, but P. fitae n.sp. presents a much more delicate stylet than the rest of the species and a long, narrow male atrium.
The genus Prostheceraeus is well represented in the North Atlantic and the Mediterranean; on the contrary, only two species are known from the Pacific Ocean and adjacent Seas. Prostheceraeus panamensis Woodworth, 1894 from Panama is a species described on fixed material and mostly damaged; therefore, the information on colouration or body shape is ambiguous. It also lacks internal description, so a comparison is not possible. The other known species is P. fuscolineatus from the Indian Ocean, which clearly differs from P. fitae n.sp. by colouration and the shape of the dorsal longitudinal stripes, as discussed above.