Rissoella morrocoyensis, Caballer, Manuel, Ortea, Jesus & Narciso, Samuel, 2011

Caballer, Manuel, Ortea, Jesus & Narciso, Samuel, 2011, Description of two new species of Rissoella Gray, 1847 (Mollusca, Gastropoda, Heterobranchia) from Venezuela, with a key to the Caribbean species known for the genus, ZooKeys 115, pp. 1-18 : 2-7

Rissoella morrocoyensis sp. n. Figs 1 –59–1013– 17Tabs 124

Description.

Shell very small (Tab. 2), smooth, translucent and fragile (Fig. 13); protoconch about half a whorl after the nucleus (Fig. 14); teleoconch of two and a half whorls to two and three quarters, convex profile, well marked suture; aperture semicircular, columella almost straight; umbilicus small, very narrow and deep, partially closed by the expansion of the columellar edge, with a preumbilical cord (Figs 9-10, 15), average height of the last whorl, 85% of shell length; average height of the aperture, 60% of shell length (Tab. 2); shell length/width ratio = 1.32; spiral angle = 60°; umbilical angle = 21°-28°.

Operculum semicircular, translucent amber, rather opaque in the center, 453 µm long by 255 µm wide in a shell of 0.92 mm length; inner side with a triangular projection with spearhead-shaped apex.

Head dark brown to black; black eyes set in a circular area of translucent white colour; oral lobes wider and shorter than the cephalic tentacles; both translucent, dark brown or black, completely or just at the base (Figs 1-2); foot slightly bilobed, with a white hyaline sole; dorsal part of the foot, white or with an irregular dark drawing (Fig. 3); flanks of the animal the same colour as the head (Fig. 4); mantle white, with several black blotches around the hypobranchial gland; hypobranchial gland translucent white, with white spots, irregular black blotches and a pale yellow band marking its boomerang-shaped contour with no transversal bands (Fig. 5); visceral mass in the first whorls, dark brown to black.

Odontophoral cartilages rectangular, with polygonal uncini of 10-16 µm long, regularly imbricated (Fig. 16); radular formula of a specimen 1.02 mm shell length, 15 × 1.1.R.1.1; rachidian tooth wide, with bilobed apex forming two smooth cusps (Fig. 17), slightly shorter than marginal tooth; lateral tooth thorn-shaped, 34.2 µm long, with the apex hooked and smooth, imbricated with the opposite tooth above the rachidian; marginal tooth triangular, 26.1 µm long, with sharp apex.

Types.

Holotype, 1.06 mm × 0.82 mm, collected alive (June 10, 2010), preserved dry and deposited in SOM-IVIC (IVICCMT005). Paratypes 1-2, 1.25 mm × 0.86 mm and 1 mm × 0.69 mm, preserved in etanol 96%, SOM-IVIC (IVICCMT006). Paratypes 3-4, 1.16 mm × 0.80 mm and 1.06 mm × 0.82 mm, preserved dry, SOM-IVIC (IVICCMT007). Paratypes 5-7, 0.90 mm × 0.68 mm; 0.86 mm × 0.78 mm and 1.10 mm × 0.80 mm, preserved dry, FUDENA (CFPM0001).

Further material.

1.02 mm × 0.86 mm, collected (March 24, 2010) in Boca Grande, PNM (10°51'01.71"N, 68°14'16.48"W), used to obtain the radula, therefore the specimen was destroyed.

Type locality.

Boca Grande, National Park Morrocoy, Venezuela (10°51'28.85"N, 68°13'17.04"W), at the base of the leaves of Thalassia testudinum , 1 m depth.

Etymology.

morrocoyensis, latinization of morrocoy, place name of National Park Morrocoy, Venezuela, where the type locality is located.

Remarks.

According to the classification given by Ortea and Espinosa (2004), Rissoella morrocoyensis sp. n. would cluster within the Caribbean species group with a preumbilical cord, which include: Rissoella zaidae Ortea & Espinosa, 2004 (Fig. 27), Rissoella florae Ortea & Espinosa, 2004 (Fig. 28), Rissoella elsae Ortea & Espinosa, 2004 (Fig. 29) and Rissoella taniae Ortea & Espinosa, 2004 (Fig. 30). Rissoella morrocoyensis sp. n. has a preumbilical cord thicker than these four species, all of which have their type locality on the shores of Cuba. Additionally, the body colouration of Rissoella florae , Rissoella elsae and Rissoella taniae is very different as well as the shape and proportions of their shells, which are larger.

The shell of Rissoella zaidae is similar in size to that of Rissoella morrocoyensis sp. n., but the animal has a different colour and the oral palps equal the cephalic tentacles, while in Rissoella morrocoyensis sp. n., they are shorter. Additionally, Rissoella zaidae has transverse yellow bands in the hypobranchial gland.

Rissoella contrerasi Rolán & Hernandez, 2004, from Africa, has the design on the hypobranchial gland similar to Rissoella morrocoyensis sp. n., but it distinguishes by the curved inner edge of the columella, by the oral lobes being longer than the cephalic tentacles, and by it’s different body colour (black), which is displayed even on the sole of the foot and on the hypobranchial gland. In addition, the whorls of the shell and the aperture are different and it has a wider umbilicus.