Parapharyngiella involucrum Willems et al., 2005

Steenkiste, Niels Van, Tessens, Bart, Krznaric, Kathleen & Artois, Tom, 2011, Dalytyphloplanida (Platyhelminthes: Rhabdocoela) from Andalusia, Spain, with the description of four new species, Zootaxa 2791, pp. 1-29 : 2-4

Parapharyngiella involucrum Willems et al., 2005

( Figs. 1 A–1D)

New locality. Doñana National Park, Provincia de Huelva, Spain (36°52’40”N, 6°22’03”W). Salinas de San Rafael: small algae in shallow, brackish pools of abandoned salt pans (25/03/2008).

Known distribution. Western Indian Ocean ( Tanzania: Zanzibar) (Willems et al. 2005).

Material. Observations on a live animal that was subsequently whole-mounted. Type material from Proxenetes mackfirae Karling, 1978 ( SMNH, no. 42498) and Parapharyngiella involucrum Willems et al., 2005 (HU, nos. 281–283) for comparison.

Remarks. The animal is about 0.4–0.5 mm long. The pharynx is at 50–60% of the body length. The two testes lie anterolaterally from the pharynx. The genital system is situated in the caudal body half.

The stylet ( Fig. 1 C) is almost identical to that of the specimens from Zanzibar. It measures about 30 μm at its widest and is 60 μm long. It consists of a semicircular tube with a funnel-shaped base and a curved distal point (j); a curved, slender protrusion with a bluntly-rounded distal end (k); a mantle with a straight proximal rim and a distal hook encompassing the distal half of the semicircular tube (l).

The bursal canal ( Fig. 1 B) (bursal stalk in Willems et al. 2005) is sclerotized and about 52 μm long. It seems to connect the male or common genital atrium with the bursa. When entering the bursa it broadens to become slightly funnel-shaped. The bursal appendage ( Fig. 1 D) is not clearly visible in the whole mount. It is about 30 μm long and seems to consist of a pyriform to club-shaped proximal part, 10 μm long originating on the bursa, and a slender distal tube of 20 μm. Both parts appear to be separated by a sphincter. The tubular part seems provided with a very thick tissue, of which the exact origin could not be determined. It could not be seen whether it connects the bursa to the female duct as described by Willems et al. (2005) or not.

Discussion. When considering the different structures of the genital system visible in the live specimen and the whole mount, the Spanish specimen agrees with those from Zanzibar. Parapharyngiella involucrum is a monotypic taxon within the Trigonostomidae Graff, 1905 . The stylet, although slightly smaller and with a less slender distal hook than in the animals from Zanzibar (± 50 µm long), is almost identical to that of the Zanzibar specimens (see Willems et al. 2005). Both populations have a sclerotized bursal canal between the male or common genital atrium and the bursa, and a slender tubiform bursal appendage with a club-shaped base. In the Tanzanian specimens the latter tube connects the bursa with a sperm-containing vesicle on the female duct, but this could not be seen in the animal from Spain.

One difference concerns the position of the pharynx and the genital organs. While all reproductive structures of the East African specimens are located just behind the pharynx (which is situated in the first body half), male and female organs of the Andalusian specimen occupy a large fraction of the caudal body half behind the centrallylocated pharynx. Despite this difference and awaiting the availability of new material, we prefer to preliminarily assign the specimen from Spain to P. involucrum .

The stylet is also very similar to that of Proxenetes mackfirae from Bermuda (see Karling 1978). Although the stylet of this taxon is somewhat larger (70–100 μm), it also consists of a semicircular tube and a slender hooked mantel, a feature unique within Proxenetes Jensen, 1878 . Obvious differences distinguish P. mackfirae from Parapharyngiella involucrum . Compared with the protrusion of the stylet of P. i n v o l u c r u m (k in Fig. 1 C), the protrusion of P. mackfirae is longer, more pointed and connected with a cup-shaped membrane with its distal border cut out in a right angle. This membrane is lacking in P. involucrum . Minor differences are to be found in a longer bursal canal (± 80 μm) in P. m a c k f i r a e and the bursal appendage that seems to lack a thickened wall in the species from Bermuda. Although the construction of the stylet and the presence of a tubular bursal canal (internal vagina in Karling 1978) and a funnel-shaped, double-barrelled bursal appendage as described above, seem clear indications of its kinship with Parapharyngiella , we refrain from transferring P. mackfirae to this taxon because the female system of this species has not been described.

Figures

FIGURE 1. Parapharyngiella involucrum Willems et al., 2005 (A) Habitus from a live animal. (B) Bursal canal. (C) Stylet. (D) Bursal appendage.