Pristina osborni ( Walton, 1906 )

Schmelz, Rüdiger M., Jocque, Merlijn & Collado, Rut, 2015, Microdrile Oligochaeta in bromeliad pools of a Honduran cloud forest, Zootaxa 3947 (4), pp. 508-526 : 519-520

Pristina osborni ( Walton, 1906)

Naidium osborni Walton, 1906: 703 –705, fig. 12.

Naidium minutum Stephenson, 1914: 327 –329, figs 4, 5.

Pristina minuta (Stephenson) . Sperber 1948: 222 –223.

Pristina osborni (Walton) . Brinkhurst 1971: 395, fig. 7.22N – P. Righi & Hamoui 2002: 144 –145. Timm 2009: 70 –71. Pristinella osborni (Walton) . Brinkhurst 1985: 472.

Material examined. One juvenile specimen, not contracted, unstained whole mount, IG 327183-33 [15.512632 -88.28871], 1545 m asl, M. Jocque 06/08/2013.

Description. Length 0.7 mm, diameter max. 0.1 mm. 12 segments; specimen is an anterior zooid, posterior part missing. Dorsal and ventral chaetae from II, 1 hair and 1 needle per bundle dorsally, ventrally 3–4 bifids. Hairs smooth, max. length 120 Μm, c. 60 Μm in II, increasing in size from II to VI; needles bifid with very short equally long teeth diverging at wide angle. Ventrals with teeth equally long, length 24 Μm in II, 32 Μm in VII. Prostomium with slight inner protuberances, inner epithelium almost smooth; pharyngeal glands in III, IV, V and VI, distinct masses attached to anterior faces of respective septa; small in III (diameter c. 10 Μm), large and of equal size in IV– VI (diameter 30–40 Μm), shape roughly triangular, with one corner directing forwards. Nephridia not seen. Coelomocytes spherical, diameter 5–6 Μm, nucleated, faintly vesicular. Stomach present in anterior half of VIII, onion-shaped, beginning with abrupt enlargement directly behind 7/8, epithelium with stomachal canals, stomachal lumen with elongate cilia directed backward. Behind stomach intestine narrowed, convoluted, widened abruptly again in 1/ 2 IX into intestine proper. Midgut pars tumida circumferal in XI.

Remarks. Our specimen agrees with the original desciption of P. osborni in body size, chaetal pattern and presence of a stomach in VIII, but it differs in needle length (50 Μm in P. osborni ). It also agrees closely with the detailed original description of P. minuta ( Stephenson, 1914) . The latter is a junior synonym of P. osborni since Brinkhurst (1971); see also Righi & Hamoui (2002), Timm (2009), and Van Haaren & Soors (2013). The original description of the intestinal tract, usually neglected in species description or redescriptions of Pristina , is a neat match to the structures as found in our specimen: "The septal [=pharyngeal] glands are rather variable; they are apparently always present in segments iv and v, with the addition usually of a smaller pair in segment iii, or in segment vi, or (in one specimen) in both iii and vi [the condition seen in our specimen]. The stomach is in viii, of a pyriform shape with the broad end directed forwards; it has a somewhat streaked appearance, due apparently to the chloragogen granules being arranged more or less in rows. The alimentary tube is still narrow for some distance behind the stomach, and in contracted specimens forms here a small loop directed backwards; this is not quite straightened out even when the animal extends itself. The intestine begins in segment ix" ( Stephenson 1914: 328).

P. osborni is a cosmopolitan species; there is one Central American record from montane rainforest moss in Costa Rica at 2900 m asl ( Harman 1982), and one record from bromeliads in French Guiana ( Céréghino et al. 2011). "This limnetic species is also known from soil in Brazil " ( Righi & Hamoui 2002); see this paper for further records in South America. Not all synonymies may be correct. For example, Righi's Amazonian P. minuta ( Righi 1973b) , synonymized with P. osborni in Righi & Hamoui (2002), has a stomach in VI and is therefore probably a different species.

P. osborni should be considered as an aggregate of several species. Large variation is allowed in chaetal length, stomach location and segmental position of the budding zone ( Timm 2009). It has been shown in other species of Pristina that specimens with almost identical chaetal pattern may turn out to belong to different species, once the reproductive organs are known ( Erséus & Grimm 1998; Collado & Schmelz 2002). Therefore Collado & Schmelz (2002) questioned identifications and synonymizations in P. osborni based on chaetal characters alone. Since reproductive organs are rarely seen in Pristina , a genus that reproduces mainly by paratomy, other internal structures such as the intestinal tract may help to clarify taxonomic questions in this group. Sexual organs of P. osborni have been described by Erséus and Grimm (1998) but the single specimen from a freshwater creek in Northern Australia has no sudden stomachal dilatation, which makes the identification questionable.