Paraustrorhynchus elixus Karling & Schockaert 1977
Creators: name: Diez, Yander L.
name: Hernández, Claudia Sanjuan
name: Reygel, Patrick
name: Roosen, Paulien
name: Artois, Tom
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Platyhelminthes
Rhabditophora
Rhabdocoela
Polycystididae
Paraustrorhynchus
Paraustrorhynchus elixus

description: Paraustrorhynchus elixus (Marcus, 1954) Karling & Schockaert, 1977
(Fig. 3 A–B)
Material and distribution in Cuba. Four specimens studied alive and whole mounted (UH VIII.3.11–VIII.3.14): two from Siboney (February 7, 2016 & April 4, 2017), one in very fine sand, 0.6 m depth, and the other on Ulva lactuca, 0.3 m deep; one specimen from Chivirico, on Halimeda sp. partially covered by organic matter, 0.6 m deep; one specimen from Las Sardinas, on Dictyota menstrualis partially covered by organic matter, 0.5 m depth. Salinity 32–35 ‰.
Remarks. According to Marcus (1954) the male sclerotized structures of P. elixus consist of three separate pieces: a small stylet connected to the prostate vesicle ("estilete da vesicula granulorum), a plate-like stylet associated with the ejaculatory duct ("estilete da vesicula seminais"), and an S-shaped plate ("flabelo"), to which a hook is connected ("flagelo"). Karling & Schockaert (1977), studying the original material of Marcus (1954), concluded that the plate-like stylet and the S-shaped plate are connected to each other, forming one complex structure. This view was followed by Artois & Tessens (2008), who consider the entire structure a prostate stylet type III, considering the small stylet connected to the prostate vesicle type II as a prostate stylet type II.
However, the new material collected in Cuba clearly shows that the complex accessory copulatory organ (the "prostate stylet type III") indeed consists of two separate structures: the stalk and the S-shaped plate with a hook. Both structures are only connected proximally by a muscular bulb, as was mentioned by Marcus (1954). In several specimens both structures lie on top of each other, making it extremely difficult to observe that they are separated. In live animals, however, as well as in more lightly-squeezed whole mounts, it is obvious that both structures are indeed separated. At the moment it is impossible to determine what are the exact homologies of the two parts, i.e. which of the two parts, if any, corresponds to the prostate stylet type III of other species of Polycystidinae. Because of its close association with the ejaculatory duct, it could be deduced that the "stalk" actually is the prostate stylet type III and that the S-shaped plate with a hook is either a newly-formed structure, or is a part that in the course of evolution has come loose from the stalk. This, of course, is all highly speculative. The only other species within Polycystidinae that has three stylets is Triaustrorhynchus armatus Willems, Schockaert & Artois, 2006, which has a prostate stylet type II, a prostate stylet type III and an accessory stylet type III (Willems et al. 2006), but the accessory stylet in this species is in a much more distal position in the male atrium and is associated with a glandular vesicle of its own. Therefore, it is doubtful that T. armatus and P. elixus are very closely related.
The above suggests that P. elixus does not belong to the same genus as the other species of Paraustrorhynchus, which currently includes three other species that typically have two stylets only. However, we prefer not to create a separate, new genus for P. elixus before molecular data affirms this decision.
As to morphology, the prostate stylet type II in the specimens from Cuba is as described by Schockaert & Karling (1977) and Artois & Tessens (2008) for the specimens of other areas. It is 31–37 µm long (x̄ = 35 µm; n = 4) (Fig. 2 A–B: ps2), which fits the range recorded up to now (see Artois & Tessens 2008): 27 µm (specimen from New Caledonia) – 33 µm (specimen from Brazil). As mentioned above, the "prostate stylet type III" actually consists of two separated parts: a long, somewhat slender "stalk" and an S-shaped plate with a hook. In the specimens from Cuba the stalk is 76–84 µm long (x̄ = 80 µm; n = 4) (Fig. 3 A–B: sk), the S-shaped plate is 63–78 µm long (x̄ = 70 µm; n = 4) (Fig. 3 A–B: spl), and the hook is 13–23 µm long (x̄ = 18 µm; n = 4) (Fig. 3 A–B: h). These measurements correspond to the measurements given by Artois & Tessens (2008) for the other populations of this species.
description: Published as part of Diez, Yander L., Hernández, Claudia Sanjuan, Reygel, Patrick, Roosen, Paulien & Artois, Tom, 2018, First record of Polycystididae (Platyhelminthes, Kalyptorhynchia) from Cuba, with the description of a new genus and five new species, and remarks and the description of one new species from Panama, pp. 107-125 in Zootaxa 4514 (1) on page 115, DOI: 10.11646/zootaxa.4514.1.9, http://zenodo.org/record/2605858