Acirrostylus poncedeleoni

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 : 4-8

Acirrostylus poncedeleoni n.g. n.sp.

( Figs. 1–2 )

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. Two whole mounts, one of which designated holotype ( SMNH 7495), and two serially-sectioned specimens, one of which designated paratype (HU no. 400).

Etymology. The genus name refers to the lack of a cirrus and the presence of a stylet. The species is dedicated to Prof. Dr. Rodrigo Ponce de León (Montevideo, Uruguay).

Description. The animal is about 0.5 mm long and has two eyes. The syncytial epidermis is strongly ciliated and ± 2–3 μm thick in the rostral and caudal part of the animal. In the central part it is ± 1–2 μm thick. Cilia are ± 2 μm long. Small round to oval nuclei with a diameter half of the epidermis height occur throughout the epithelium. Round rhabdites of a similar size to the nuclei are present in the apical part of the epidermis. They are most numerous on the dorsal side of the animal. The basal membrane is very thin. Circular and longitudinal muscle layers are successively present under the very thin basal membrane.

The proboscis is about 1/6 to 1/7 of the body length long (± 70–80 μm). The proboscis glands run anteriorly through the muscular bulb to form a girdle of eight coarse-grained eosinophilic glandular ampullae. In between these ampullae, small muscles run from the cone to the peripheral wall of the proboscis. The convex cone is completely filled with a darkly-staining substance, possibly a glandular secretion, although its origin could not be traced. The proboscis bulb is provided with well-developed longitudinal muscles, surrounded by circular muscles. The latter do not surround the glandular girdle. The epithelium of the proboscis sheath lacks cilia. Anteriorly it is membranous and anucleated. Here the sheath is surrounded by small basophilic glands, but whether they empty into the lumen of the proboscis sheath or more frontally is not fully clear. The epithelium of the sheath becomes higher and nucleated near the proboscis bulb, where it is surrounded by circular muscles. These continue for a short distance around the anterior part of the glandular girdle. The different sets of motional muscles are not fully visible in the serial sections. Parts of the protractors run around the proboscis bulb. Relatively thick muscle fibers, which probably function as dilators, insert at the transition zone between the sheath and the glandular girdle. Fixators insert on the bulbar septum just below the glandular girdle and adhere on the epidermal basal membrane at the same level where the integument retractors insert. The latter were observed dorsally as well as ventrally and run posteriorly to the body wall. Posterior to the proboscis bulb, remnants of the proboscis retractors were visible. Their insertion place could not, however, be verified.

The pharynx rosulatus is situated in the first body half, posterior to the proboscis, brain and eyes. In the serial sections, the pharynx is somewhat shifted rostrally with regard to the position that was observed in the live animal. The mouth is situated at approximately 35%. The pharynx contains eosinophilic and basophilic glands; the exact location of their opening into the pharynx lumen could not be determined. The pharynx bulb is surrounded by a circular muscle layer. These muscles are more developed around the distal part of the bulb. The tube-shaped lumen is lined with a membranous epithelium surrounded by outer circular and inner longitudinal muscles. The membranous epithelium of this lumen is probably ciliated, but this could not be confirmed. The prepharyngeal cavity is also lined with a membranous epithelium and is only surrounded by a circular muscle layer. Glands of Minot empty into the oesophagus. A bundle of eosinophilic and basophilic glands is situated caudally from the brain.

The paired testes are located at both ventrolateral sides of the animal. In the serial sections they are situated at approximately 50% of the body length. However, in the live animal they were observed more caudally. A vas deferens leaves from each testis, distally broadening to form a seminal vesicle. Just before entering the copulatory bulb, the vasa deferentia join to form the ejaculatory duct. This ejaculatory duct runs centrally through the prostate vesicle (conjuncta-type copulatory organ; terminology of Karling 1956a). The prostate vesicle contains fine- and coarse-grained eosinophilic prostate glands. Only the fine-grained glands are extracapsular. The prostate vesicle is lined with a membranous, anucleated epithelium and surrounded by an inner circular and an outer longitudinal muscle layer. The outer longitudinal layer continues around the male atrium. Distally the prostate glands and the ejaculatory duct enter the stylet, which is a single-walled, slightly curved tube, 62 μm long. Its proximal end is funnel-shaped, and somewhat asymmetrical. For approximately 2/3 of its length, the stylet forms two lateral, wing-like protrusions, which give the impression of forming a sheath around the stylet. The distal blunt end of the stylet shows a round opening.

The ovary is unpaired and lies in the middle of the body at the dorsal side. The vitellarium runs rostrally to the proboscis and is caudally connected with the ovary to form an ovovitellarium. In the live animal, two lateral branches of this vitellarium were observed. A long oviduct connects the ovary with the female duct. This oviduct is lined with a membranous, anucleated epithelium and not surrounded by muscles. The female duct is strongly widened and surrounded by a longitudinal muscle layer. It is lined with a rather high, degenerated epithelium. Where the female duct enters the common genital atrium, it is surrounded by a weak circular muscle layer.

The elongated bursa enters the common genital atrium. Dorsally, a sphincter divides the bursa into a small, thin-walled proximal part and a broad, muscular distal part. The latter is very darkly stained in the serial sections and apparently lined with a very high epithelium. Where the bursa enters the common genital atrium, it is surrounded by a circular muscle layer and lined with a high epithelium. The rest of the distal bursal part is surrounded by a longitudinal muscle layer. Proximally from the sphincter, two cuticularized spermatic ducts originate. These ducts run towards the ovary and the oviduct, but their exact proximal ends could not be ascertained. A possible connection between the ovary and the proximal part of the bursa could not be observed.

The common genital atrium is surrounded by a circular muscle layer and lined with a membranous, anucleated epithelium. Ventrocaudally the common genital atrium empties into the gonopore, which is surrounded by a sphincter. The caudal body region contains eosinophilic and basophilic caudal glands.

Discussion. The presence of a proboscis without hooks and muscular plates, but with a glandular girdle, places Acirrostylus poncedeleoni n.sp. clearly within the taxon Cicerinidae Meixner, 1928 . Traditionally, the family Cicerinidae is subdivided into three subfamilies: Cicerininae Meixner, 1928 , Nannorhynchidinae Evdonin, 1977 , and Xenocicerininae Evdonin, 1977 , the last containing a single genus ( Xenocicerina Karling, 1956 ). Morphological analyses of sperm ( Watson 1998) and proboscis ultrastructure ( De Vocht 1992; De Vocht & Schockaert 1999) strongly suggest a monophyletic Nannorhynchidinae . All representatives of Nannorhynchidinae show a reduction of the axonemata in the sperm, similar to the situation found in the Schizorhynchia, which makes them a possible sister group to the Schizorhynchia ( Watson 1998, 2001). Moreover, all species within Nannorhynchidinae have eyes with lenses, which is clearly not the case for the other species of Cicerinidae .

Most probably the taxon Cicerininae is not monophyletic, and even not related to the Nannorhynchidinae . The presence of sensory organs associated with the distal belt of the sheath epithelium suggests a close relationship of Cicerina remanei Meixner, 1928 with Psammorhynchus tubulipenis Meixner, 1938 ( Psammorhynchidae Karling, 1956 ) and Cytocystis clitellatus Karling, 1953 ( Cytocystididae Karling, 1964 ) ( De Vocht 1990; De Vocht & Schockaert 1999). These taxa possibly form a larger monophyletic group, together with species of Ethmorhynchus Meixner, 1928 , Ptyalorhynchus Meixner, 1938 , Paracicerina Meixner, 1928 , Xenocicerina Karling, 1956 , Placorhynchidae and Gnathorhynchidae , based on the presence of two sets of proboscis retractors ( De Vocht 1992). Ethmorhynchus , Ptyalorhynchus , Cicerina and Paracicerina differ from the other taxa within this group by the fact that they all have a nucleo-glandular girdle associated with the proboscis. Species of Zonorhynchus Karling, 1952 and Didiadema Brunet, 1965 have a nucleo-glandular girdle, but have only one set of proboscis retractors.

Acirrostylus poncedeleoni n.sp. does not have eyes with lenses, which places it outside the Nannorhynchidinae . It clearly differs from the typical species of “ Cicerininae ” by the lack of nuclei in the glandular girdle. A typical feature of A. poncedeleoni n.sp. is the presence of a stylet, a feature which it shares only with some species of Zonorhynchus , a taxon traditionally placed within the Cicerininae . However, species of Zonorhynchus also have an armed cirrus which is lacking in A. poncedeleoni n.sp. The most typical feature of Acirrostylus n.g., however, is the fact that there is only one ovovitellarium, a situation that is unique within the Cicerinidae . Moreover, a uterus is completely lacking, which is also very unusual within the Cicerinidae . Some species of Nannorhynchidinae have a well-developed uterus, whereas in the other Cicerininae and Xenocicerina , the uterus is weakly differentiated, often being no more than a small protrusion of the common genital atrium. All Cicerinidae , except for Toia and Zonorhynchus , have cuticularized spermatic ducts of variable length, which connect the bursa to the ovovitellaria. Although this connection could not be observed in the Uruguayan species, this is probably also the case for A. poncedeleoni n.sp.

From the discussion above it is clear that the species from Uruguay cannot be placed in any of the existing genera, and therefore a new genus is erected. The phylogenetic relationships between the different cicerinid taxa is far from clear, and should be established in a large phylogenetic study.

Diagnosis. Acirrostylus n.g.: small taxon of Cicerinidae with a girdle of eight separate eosinophilic glandular proboscis ampullae and two sets of proboscis retractors. Nuclei absent in the glandular girdle. Eyes without lenses. Pharynx in the anterior part of the body. Copulatory organ with stylet, without cirrus. Paired testes and seminal vesicles. Unpaired ovovitellarium. Large bursa composed of a distal and a proximal part separated by a sphincter. Two cuticularized spermatic ducts. Uterus absent. Type species: A. poncedeleoni n.sp.

Diagnosis. Acirrostylus poncedeleoni n.sp.: provisionally with the same diagnosis as the genus. The stylet is a 62 μ m long tube with wing-like, lateral protrusions.

Figures

FIGURE 1. Acirrostylus poncedeleoni n. g. n. sp. (A) Habitus of a live animal. (B) Stylet from the holotype. (C) Reconstruction of the entire animal from the right side.

FIGURE 2. Acirrostylus poncedeleoni n. g. n. sp. Reconstruction of the proboscis from the right side.