Baicalellia forcipifera, Steenkiste, Niels Van, Volonterio, Odile, Schockaert, Ernest & Artois, Tom, 2008

Steenkiste, Niels Van, Volonterio, Odile, Schockaert, Ernest & Artois, Tom, 2008, Marine Rhabdocoela (Platyhelminthes, Rhabditophora) from Uruguay, with the description of eight new species and two new genera, Zootaxa 1914, pp. 1-33 : 17-20

Baicalellia forcipifera n.sp.

( Figs. 7 , 8 A)

Locality. La Coronilla, Departamento de Rocha, Uruguay (33°54’18.50”S, 53°30’39.30”W). Beach and mouth of the canal near hotel Parque Océanico: sand covered by a thin green layer of organic material and sand with organic material near a small pool in open contact with the ocean (01/08/2004): type locality.

Material. Observations on a live animal. Three whole mounts, one of which designated holotype ( SMNH 7499), another one designated paratype (HU no. 410). Nine serially-sectioned specimens, of which seven designated paratypes (HU no. 411–417). Unfortunately one of the better whole mounts, originally intended to be designated as the holotype and shown in fig. 7B (lower figure), was lost. Another whole mount was chosen as the holotype.

Etymology. The species name refers to the pincers-shaped stylet. Forceps (Lat.): pincers. Ferre (Lat.): to carry.

Description. The body length of the animal varies between 0.5–0.7 mm. The internal organisation does not differ from the other species of the taxon Baicalellia Nasonov, 1930 (see Nasonov 1930, 1932; Luther 1962; Ax 1995).

The anteriorly-situated pharynx doliiformis lacks tentacle-like structures. The copulatory bulb is surrounded by a thick, inner, spirally-running to circular muscle layer and outer longitudinal muscles. The prostate vesicle is also surrounded by longitudinal muscles. Distal to the copulatory bulb, a pincers-shaped stylet is present. It consists of a common base from which two separate, curved arms depart. The longest of these arms is 44–49 µm long and hollow. It is filled with the prostate secretion that proximally enters the stylet. The shortest arm is 20–27 µm long and seems to be an accessory outgrowth of the longer arm's wall. From the base to the distal apex of the longest arm, the stylet measures 54–59 μm. The base of the stylet has a proximal rim and is connected to the copulatory bulb.

The male atrium fits tightly around the stylet. Distally it opens into the common genital atrium. The bursa copulatrix is a sack-shaped protrusion of the common genital atrium and is filled with spermatozoa. It runs dorsally from the male atrium and has a very broad connection with the common atrium. In consequence, differentiation between the bursa copulatrix and the common atrium is not always clear. The bursa copulatrix and the male genital atrium are lined with a relatively high, anucleated epithelium and surrounded by a weaklydeveloped circular muscle layer. The gonopore is at about 75% and surrounded by a sphincter.

The female duct enters the common genital atrium through its caudal wall. At the place of entry, a strong sphincter is present. This female duct is relatively short and broad, lined with a high, nucleated epithelium and surrounded by a strongly-developed circular muscle layer. The lumen is filled with spermatozoa. Proximally, both oviducts and a bundle of eosinophilic glands enter the female duct. The oviducts are very short and lined with a membranous epithelium. Ventrally from the oviducts, a seminal bursa opens into the female duct through a sphincter. The seminal bursa is spherical to club-shaped and has a fairly long bursal stalk. It is lined with a membranous epithelium. The bursa contains sperm, which are thinner and less darkly-stained than the sperm found elsewhere in the animal. A uterus is absent.

Discussion. This species can easily be placed within the taxon Baicalellia Nasonov, 1930 , as it shows diagnostic characters of this taxon: paired gonads, with the testes rostrally connected to each other, and ovaries fused with vitellaria to form two ovovittelaria.

Almost all of the 17 species of Baicalellia have a tubiform to funnel-shaped prostate stylet, which is straight or curved. Apart from Baicalellia forcipifera n.sp., only two other species have a stylet with a lateral outgrowth: B. canadensis Ax & Armonies, 1987 and B. anchoragensis Ax & Armonies, 1990 . In B. anchoragensis the stylet is short and broad, and the outgrowth has a little spine of its own. In B. canadensis the stylet is somewhat longer and less sturdy, and the outgrowth is a simple, relatively short spine. In B. forcipifera n.sp. both the stylet and the outgrowth are long and very slender, giving it its typical pincer-like shape.

Nasonov (1930) describes the seminal bursa as a seminal receptacle and a phagocytic organ that serves to resorb redundant sperm. Ax (1954, 2008), Luther (1918, 1921) and Marcus (1946) use the name seminal bursa, while Ax & Armonies (1990) keep to syncytial tissue. According to Nasonov (1930) the strongly-thickened, syncytial epithelial wall of this organ contains vacuoles with sperm in various degrees of disintegration.

The existence of such vacuoles is confirmed by Ax (1954), Ax & Armonies (1990), Luther (1921), Marcus (1946) and Joffe & Selivanova (1988). In Baicalellia forcipifera n.sp. only a very thin epithelium without vacuoles was observed. However, the lumen of the seminal bursa contains disintegrating sperm. The base of this organ is surrounded by a sphincter. This has also been observed in nearly all species except for B. canadensis , B. evelinae Marcus, 1946 and B. groenlandica Ax, 1995 . The opening and position of this organ in the genital system vary from species to species, as discussed by Ax & Armonies (1990) for B. posieti Nasonov, 1930 , B. sewardensis , B. anchoragensis and B. brevituba ( Luther, 1921) Nasonov, 1930 . In the other species, the seminal bursa is always situated between, ventrally or dorsally from the ovovitellaria. In B. canadensis , a seminal bursa (indicated as seminal receptacle by Ax & Armonies 1987) was only observed as an appendage of the copulatory bursa. This makes the placement of this species within the genus Baicalellia uncertain (see Ax & Armonies 1987).

In Baicalellia forcipifera n.sp. the female duct is clearly separated from the genital atrium by a stronglydeveloped sphincter. This is a somewhat unusual situation in Baicalellia , where the female duct is mostly very short, even considered part of the common genital atrium. Only Ax & Armonies (1987, 1990) use the term “female duct” as such, but in B. forcipifera n.sp. it is unusually long, even compared with that of B. canadensis , where it is also clearly separated from the common genital atrium by a strong sphincter. This feature, together with the construction of the seminal bursa and the form of the stylet, clearly shows that the material from Uruguay belongs to a new species.

Diagnosis. Baicalellia forcipifera n.sp.: species of Baicalellia with a ± 54–59 μ m -long, slender stylet, carrying a ± 20–27 μ m -long, slender spine. Stylet and outgrowth together have the shape of pincers. Female duct long, clearly delimited, with a nucleated epithelium and surrounded by a strongly-developed, circular muscle layer. Seminal bursa without vacuoles and containing disintegrated sperm.

Figures

FIGURE 7. Baicalellia forcipifera n. sp. (A) Reconstruction of the anterior body part from the right side. (B) Stylet (upper from the holotype). (C) Reconstruction of the atrial organs from the left side.

FIGURE 8. Baicalellia forcipifera n. sp. (A) Habitus of a live animal. Polliculus cochlearis n. g. n. sp. (B) General organisation of an animal based on serial sections. (C) Stylet and accessory stylet (from the paratype). (D) Reconstruction of the atrial organs from the right side (arrow indicates intracapsular seminal vesicle).