Chaetonotus (Primochaetus) veronicae Kanneby 2013

Chaetonotus (Primochaetus) veronicae Kånneby, 2013

(Figs. 1, 2; Table 1; Kånneby 2013: Fig. 22)

Type locality. Small stream, Abisko Tourist Station, Abisko, Lapland (N 68º 21’ 23’’; E 18º 47’ 59’’), July 3, 2010.

Other localities. Snasahögarna, Jämtland (N 63º 12’ 39’’; E 12º 18’ 19’’), July 7, 2008; Kungsleden, Abisko, Lapland (N 68º 21’ 49’’; E 18º 46’ 64’’), July 3, 2010.

Type material. Photographs of one specimen, available at the Swedish Museum of Natural History, Stockholm, Sweden. Accession number: Holotype, SMNH Type-8446. Photographs of two specimens, available at the Swedish Museum of Natural History, Stockholm, Sweden. Accession number: Paratypes, SMNH Type-8447 and 8448.

Other material. Photographs of one specimen.

Etymology. This species is named in honour of Dr. Veronica Lundgren.

Diagnosis. Small to medium-sized Chaetonotus, 123–135 µm in total body length. Body width 25–30 μm, 18–24 μm, 28–38 μm and 15– 18 μm at head, neck, trunk and base of furca, respectively. Head five-lobed with two pairs of sensory ciliary tufts. Cephalion and pleurae present. Furca straight, 18–20 μm in length, with thick, rather stumpy, rigid adhesive tubes. Dorsal body surface covered by round to suboval overlapping scales, anterior parts of which tend to be fused with the body surface. Scales are distributed in 17–20 dorsal columns with 27–30 scales in each. Dorsal scales bear rather thin, almost hair-like, deeply bifurcated spines. The proximal part of each spine, gives a dotted appearance of the dorsal surface at specific optical sections; the distal bifurcation comes at an angle, varying between close to 90 degrees and 150 degrees. Each spine of the bifurcation is simple, e.g. not dentate. Ventral interciliary area with small, round to suboval keeled scales, which fuse with the body surface towards the animal’s anterior end. Ventral terminal scales keeled and suboval in shape and overlapping. Seven to eight simple ventral spines can be seen through the caudal incision. Ventral ciliation in two separate longitudinal bands. Mouth subterminal. Pharynx 34–35 μm in length, widens slightly towards the posterior end.

Description. Small to medium-sized Chaetonotus, 123–135 µm in total body length (holotype 125 µm in total body length). Head clearly five-lobed with two pairs of cephalic sensory ciliary tufts. Anterior pair with approximately five cilia, 5–12 μm in length; posterior pair with approximately five cilia of which 2–3 are longer, 14–30 μm in length. Cephalion, 12–14 µm in width. Hypostomium present just behind mouth, developed as a transverse somewhat shieldshaped sub-rectangular plate. Anterior dorsal sensory bristles not observed; posterior dorsal sensory bristles emerging from weakly developed rounded double-keeled scales at U79.

Body width of fairly squeezed specimens: 25–30 µm at head (U11–13), 18–24 µm at neck (U26–30), 28–38 µm at trunk (U56–65), and 15–18 µm (U85–88). Head delimited from trunk by a neck constriction; the trunk reaches its greatest width at about halfway to two thirds down the length of the body. Furca straight, 18–20 µm in length, with rather short, stumpy, rigid adhesive tubes, 8–10 µm in length.

Dorsal body surface covered by round to suboval scales, the anterior part of each scale seem more or less fused with the body surface. The median columns of scales is more or less straight, while columns on either side of the median are curved, progressively approaching parallelism with the lateral body outline. Scales are distributed in 17–20 dorsal columns with 27–30 scales in each. The total number of scale columns is 30–35. Developed scales are 2–3 μm in length and 2–3 μm in width and appears slightly smaller anteriorly. Dorsal scales bear rather thin, almost hair-like, deeply bifurcated spines. The proximal part of each spine can be described as a peduncle, much like in Aspidiophorus (Voigt, 1903), originating from the posterior part of each scale, and gives a dotted appearance to the dorsal surface at specific optical sections. The “peduncle” measures 2–3 µm in length, and appears slightly thicker than the distal spine. The distal part of the spine is deeply bifurcated and measures 6–8 µm, and comes at an angle, varying between close to 90 degrees to approximately 150 degrees depending on the amount of pressure put on the animal, from the “peduncle”. The bifurcated spines are most easily viewed in the lateral areas. At the posterior end a pair of simple, slightly curved parafurcal spines, 14–16 μm in length. Dorsal scales and spines of the furcal region tend to be more reduced than the scales and spines of the other parts of the dorsal body surface. The scales of the ventrolateral areas becomes smaller towards the ciliary bands, spines are of a similar fashion to those of the dorsal side.

The anterior two thirds of the ventral interciliary area apparently naked. The posterior third is covered by round to oval small, keeled scales, 1–2 μm in length and 1–2 μm in width, and are distributed in approximately 8–10 columns. They become more and more fused with the body surface anteriorly, but can be followed to where the body reaches its greatest width (U63–65). In the posterior end a pair of keeled suboval strongly overlapping ventral terminal scales, 6–8 μm in length and 4–5 μm in width. Posterior to the ventral terminals scales 7–8 simple hair-like spines (their scales could not be observed; probably very small), 10–14 μm in length; they are confined within the caudal incision, the outer spines are curved towards the furcal branches, while the median spines are more or less straight or only slightly curved towards the furcal branches. Ventral ciliation in two separate longitudinal bands, with anterior cilia up to 15–20 μm in length.

Mouth subterminal, 4–6 μm in diameter. Pharynx widens slightly towards the posterior end, 34–35 μm in length. Pharyngeal intestinal junction at U30–34. Intestine straight with anus at U81–82. The specimens studied were all in parthenogenetic phase.

Taxonomic remarks. Chaetonotus (Primochaetus) veronicae was described in 2013 based on material from the Abisko area, northernmost Sweden (Kånneby 2013). However, data from an additional specimen, found already in 2008 in Jämtland, was not accounted for in the original description. Careful studies of this “new” specimen and the original specimens showed an important diagnostic character, that was overlooked in the original description, namely that the scales bear deeply bifurcated or bifid spines. In the original description this character was interpreted as a strong overlap of spines of different scales.

The most important diagnostic character for C. (P.) veronicae is the deeply bifurcated spines, which sets it apart from all other gastrotrichs. The proximal part of each spine seems peduncle-like, and at certain optical sections the dorsal “dotted” appearance is not unlike that of species of Aspidiophorus. However, given the fact that the distal spines are not developed as plates, as is the case for Aspidiophorus, the species is for now assigned to the genus Chaetonotus. Although two specimens were obtained for molecular studies efforts of sequencing the 18S rDNA gene has so far been fruitless.

Within Chaetonotus the species should be assigned to the subgenus Chaetonotus (Primochaetus) Kisielewski, 1997 for the following reasons: (i), the round scales; (ii), spines arise closer to the posterior edges of the scales; (iii), spines increase only slightly in length from anterior to posterior; (iv), all dorsal scales lack keels, and spines originates from a single point. Within Chaetonotus (Primochaetus) C. (P.) veronicae appears morphologically closest to Chaetonotus (Primochaetus) mutinensis Balsamo, 1977 with which it shares the thin hair-like spines and the rounded scales. The new species can be separated from C. (P.) mutinensis based on the following characters: (i) the new species have rather strongly curved deeply bifurcated spines compared to the more gently curved spines in C. (P.) mutinensis; (ii) the number of dorsal scale columns and scale rows which is greater in C. (P.) veronicae (17–20 vs. 9–12 and 27–30 vs. 22–25 respectively); (iii) the scales of the ventral interciliary field which are reduced in the new species compared to the developed scales of C. (P.) mutinensis.