Archimacrostomum sublitorale, Faubel & Warwick, 2005

Faubel, A. & Warwick, R. M., 2005, The marine flora and fauna of the Isles of Scilly: Free-living Plathelminthes (‘ Turbellaria’), Journal of Natural History 39 (1), pp. 1-45 : 14-16

Archimacrostomum sublitorale sp. nov.

( Figure 2 )

Material examined

HOLOTYPE: Site 7: one specimen, in squash preparation stored on CD-ROM: 2003.5.22.2; WM 2003.5.22.2.

Etymology

Species name derived from its sublittoral habitat.

Description

Length of body of living sexually mature specimen up to 2.54 mm, when squashed under a cover glass; maximum width in mid-body, area of oocyte ( Figure 2 ). Outline of body with characteristic macrostomid-like anterior end; posterior end rounded with several prominent adhesive glands ( Figure 2A ). Around margin few, very weak sensory hairs of varying length and stiffness present. In transmitted light body greyish translucent with yellowish brown intestine. Frontal glands present reaching anterior level of brain. Glandular ducts open at the frontal tip separately. Lateral to the pores of the glandular ducts, fields with dark granulations of unknown function. Very small eyes present, 270 M m from anterior margin of the body; distance between eyes 306 M m. Anterior margin of crescentic brain 207 M m distant from anterior end. Rhabdites with bundles of three to six rods distributed over the dorsal and ventral body surface. Digestive system with pharynx simplex pierced by extrapharyngeal glands. The intestine fills the median parts between testis and ovary and extends caudad over oocytes, ending up laterally at the level of the male system. Mouth behind brain, 390 M m distant from anterior end. Male and female pore 465 M m distant from each other.

Reproductiυe system ( Figure 2 ). The male system is typically macrostomid-like. It consists of ventro-lateral testes, vasa deferentia running caudad lateral of the intestine, a seminal vesicle, a reduced prostatic vesicle and a male stylet which projects into the male gonopore. Prostatic glands surround the proximal part of the stylet, the ampulla-like distal ducts of which are encased by the proximal enlargement of the stylet ( Figure 2C ) representing the reduced prostatic vesicle. An intravesicular duct joins the prostate part and the seminal vesicle. The measurements of the stylet (m. HT) depicted in Figure 2B are a.90 ( Ferguson 1940: Figure 27: angle a of flexure) and STw 545 M m and STL 5105 M m (Gehlen and Lochs 1990: Figure 2 ). The distal opening is subterminal on the convex side of the distal bent part.

The female system could be incomplete in that only the bilateral ovary and the female pore could be discerned. The female pore is surrounded by cement glands. A seminal bursa, not clearly discernible, probably lies immediately caudal close to the oocyte.

Discussion

Within the Macrostomidae the penis stylet has been recognized among others as a main diagnostic feature. Most of the stylets of the Macrostomidae are bent or flexed. The methods of measurement of the stylets, however, have not always been consistent, and in several cases the validity of a species is often not easy to decide although the stylet is known.

Based only on the outline of the stylet, Archimacrostomum sublitorale sp. nov. belongs to the Macrostomum hystricinum group ( Beklemishev 1951) having flexed stylets (see Rieger 1977: 212–213). However, that group proves not to be a phylogenetic unit. The M. hystricinum group represents an accumulation of different forms distributed in freshwater, brackish and marine water habitats. Type of the M. hystricinum group is M. hystricinum hystricinum Beklemishev, 1951 distributed in freshwater habitats of the palaearctic, near east, oriental and nearctic regions.

In 1954, Ferguson established the genus Archimacrostomum for all species having a reduced prostatic vesicle encased by the proximal enlargement of the stylet. Consequently, the species Macrostomum beaufortensis Ferguson, 1937 , M. hustedi Jones, 1944 , and M. pusillum Ax, 1951 , M. rubrocinctum Ax, 1951 , M. brasiliense Marcus, 1952 and M. peteraxi Mack-Fira, 1971 had to be transferred to Archimacrostomum Ferguson, 1954 .

Uncertainty of membership to Archimacrostomum remains for M. hystricinum marinum Rieger, 1977 because Rieger presents no information as to whether the prostatic vesicle is free or reduced. Because of the reduced prostatic vesicle, the new species Archimacrostomum sublitorale represents a member of this genus. All archimacrostomid species live in the marine environment.

Rieger (1977: Figure 3 ) illustrated stylets of different marine populations from Italy (Fiascherino, area near La Spezia, and Venice) and Croatia (Dubrovnik) with outlines comparable to the stylet of Archimacrostomum sublitorale . But because information was not given on the anatomy, in particular of the male system, of the individual populations, an affiliation is impossible to any species or genus.

In comparison with the species of the genus Archimacrostomum , A. sublitorale stands out through its general body magnitude and stylet measurements.

Figures

Figure 2. Archimacrostomum sublitorale sp. nov. (A) Dorsal view in squash preparation (dorsal gut in area of gonads omitted); (B) male sclerotized stylet; (C) male copulatory apparatus. Scale bars: 500 Mm (A); 50 Mm (B, C).

Figure 3. Bradynectes scilliensis sp. nov. (A) Dorsal view in squash preparation; (B) male copulatory apparatus; (C) male sclerotized stylet. Scale bars: 500 Mm (A); 50 Mm (B); 20 Mm (C).