Oneppus lacus Marcus 1954
Creators: name: Steenkiste, Niels Van
name: Volonterio, Odile
name: Schockaert, Ernest
name: Artois, Tom
Publisher: name: Zenodo
Year: 2008
Subjects: Biodiversity
Taxonomy
Animalia
Platyhelminthes
Rhabditophora
Rhabdocoela
Placorhynchidae
Oneppus
Oneppus lacus

description: Oneppus lacus Marcus, 1954
(Fig. 3)
New locality. Playa Ramírez, Departamento de Montevideo, Uruguay (34°54’58.55”S, 56°10’11.70”W). Beach with numerous holes of invertebrates, upper to lower eulittoral, relatively coarse sand with a large amount of fine fraction (12/08/04); same locality, mid-eulittoral, rather coarse sand from a sheltered area with a large amount of fine fraction (12/08/04); same locality, fine sand with large amounts of detritus from a tidal pool in the lower eulittoral (12/08/04).
Known distribution. Itanhaen, Brazil (Marcus 1954).
Material. One drawing of a live animal. Several whole mounts, some in bad condition, and three seriallysectioned specimens, all of rather poor quality. Original material from Oneppus lacus Marcus, 1954: SMNH 95815. Original material from Oneppus timius Marcus, 1952: SMNH 95805, SMNH 95806, SMNH 95807, SMNH 95808, SMNH 95809, SMNH 95810, SMNH 95811, SMNH 95812, SMNH 95813, SMNH 95914.
Additional remarks and discussion. The body length of the studied specimens ranges from 0.7 to 0.8 mm. Habitus and internal organisation correspond to those of Oneppus timius Marcus, 1952 and Oneppus lacus Marcus, 1954 (see Marcus 1952, 1954). However, O. timius is normally larger (1.2 mm) than O. lacus (0.6 mm).
The syncytial epidermis is ciliated and has a height of 1–2 μm. The cilia are as long as the epidermis is high, and the basal membrane is about ¼ of the epidermis height. As already mentioned by Marcus (1952) in the description of O. timius, the nuclei in the epithelium of the epidermis are most numerous at both body ends.
The proboscis and the pharynx of the studied individual also correspond completely to those of the species of Oneppus described by Marcus (1952, 1954).
The construction of the male system greatly resembles that of O. lacus as described by Marcus (1952, 1954). On the serial sections, the ejaculatory duct seems to be slightly sclerotized. However, this could not be confirmed in the whole mounts. Although Marcus (1952, 1954) does not mention it, this could also be observed in one of the original serial sections of O. timius, provided by the SMNH. The cirrus of the Uruguayan specimens is armed with hundreds of small spines. In some whole mounts and in one of the serial sections, a proximal crown of larger spines was clearly observed (arrow in fig. 3C). Distally, the cirrus is armed with very large, curved spines. According to the description given by Marcus (1954) this type of cirrus is also found in O. lacus, as opposed to O. timius, which has only one, proximal girdle of large spines. For O. lacus, only one slide of serial sections of rather poor quality is available, which makes verification of the original material and comparison with the Uruguayan material very difficult.
As far as could be ascertained from the sections of the Uruguayan specimens, the female system is constructed in the same way as in O. lacus and O. timius. It also seems to have a large separate seminal receptacle, completely filled with sperm, as in O. timius. Some uncertainties remain, especially concerning the ventral part of the female system, which was impossible to reconstruct from the serial sections.
Based on the relative length of the individuals and the construction of the cirrus, the Uruguayan specimens are placed in O. lacus. Future study should resolve the problems within this genus.
description: Published as part of 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, pp. 1-33 in Zootaxa 1914 on pages 8-10, DOI: 10.5281/zenodo.184571