Heterolepidoderma trapezoidum, Kånneby, Tobias, 2011

Kånneby, Tobias, 2011, New species and new records of freshwater Chaetonotida (Gastrotricha) from Sweden, Zootaxa 3115, pp. 29-55 : 42-44

Heterolepidoderma trapezoidum n. sp.

( Figures 10 , 11 )

Type locality. Small stream, Mount Snasahögarna, Jämtland, Sweden (N 63º 12’ 39’’; E 12º 18’ 19’’).

Type material. Photographs of one specimen, available at the Swedish Museum of Natural History, Stockholm, Sweden. Accession number: Holotype, SMNH Type-8139.

Etymology. This species is named after its peculiar head-shape which from the dorsal side resembles an isosceles trapezoid.

Diagnosis. Body length 133 µm, body width 31 µm, 20 µm, 38 µm and 9 µm at head, neck, trunk and base of furca respectively. Head with well developed hypopleuria and well delimited from trunk by a constricted neck. Posterior trunk region greatly reduced in width. Furca straight, 14 µm in length, with adhesive tubes pointing slightly outward at their distal ends. Anterior sensory bristles absent, posterior sensory bristles present. Dorsal body surface covered by suboval to round scales, with poorly developed edges, distributed in 16–17 columns with 22–24 scales in each. Dorsal scales absent above anterior part of pharynx, scales on either side of this area are hemi-elliptical and arranged diagonally relative to the longitudinal body axis. Ventral interciliary area naked except in the posterior part where thin, smooth, round, reduced and overlapping scales are present. Pharynx without swellings, 37 µm in length.

Description. Heterolepidoderma trapezoidum n. sp. is a mid-sized species, 133 µm in total body length. Head appears three-lobed but is weakly five-lobed, shaped as an isosceles trapezoid. Cephalion 14 µm in width, epipleuria barely visible while hypopleuria are large and well developed. Hypostomium present as a smooth kidneyshaped plate just behind the mouth. Two pairs of cephalic ciliary tufts present. The cilia of the posterior tufts are approximately 20 µm in length and those of the anterior tufts 5–8 µm in length. Ocellar granules absent.

Body width 31 µm at the head (U14), 20 µm at the neck (U26), 38 µm at the trunk (U60) and 9 µm at the base of the furca (U81–87). Head well delimited from trunk by a constricted neck, which gradually widens into the trunk. The posterior trunk region is drastically reduced in width from 38 µm at U60 to 9 µm at U81, into a tapered furca base. The furca is more or less straight, 14 µm in length. Adhesive tubes 11 µm in length, almost parallel, pointing slightly outward at their distal ends. Anterior sensory bristles apparently absent, posterior sensory bristles anchored by papillae at U82.

Dorsal body surface covered by suboval to round keeled partially overlapping scales distributed in 16–17 columns with 22–24 scales in each column. The edges of the scales are poorly developed and may be partly fused with the cuticle. The dorsal part of the head above the pharynx and down half the length of the pharynx naked. The number of scales per column was counted from the posterior end of this naked area. The scales situated laterally on either side of the naked portion are hemi-elliptical, half as wide as long and diagonally arranged relative to the longitudinal body axis. The median dorsal scale column is straight while the columns on either side tend to diverge, progressively approaching parallelism with the lateral body outline. Just anterior of the caudal cutting there are two medial keeled scales. The base of the furca also bears two pairs of dorsal keeled scales diagonally arranged relative to the longitudinal body axis, the outer pair being half the size of the inner pair. Two pairs of larger keeled dorsolateral scales are also present on the furca. Ventrolaterally in the anterior portion of the body, the keels form short stout spines. These spines become progressively fused with the scales towards the posterior end. The posterior part of the ventral interciliary area with apparently weakly developed, rounded, smooth, reduced, partially overlapping scales. Ventral ciliation in two separate longitudinal bands not merging on the head.

Mouth terminal, 4 µm in diameter. Cylindrical pharynx, 37 µm in length, widens slightly towards the pharyngeal intestinal junction which is situated at U30. Intestine straight, anus at U80.

The observed specimen was an adult in parthenogenetic phase.

Taxonomic remarks. The new species can be affiliated with either Heterolepidoderma or Chaetonotus . In Chaetonotus , C. silvaticus ( Varga, 1963) , has a similar body shape to Heterolepidoderma trapezoidum n. sp., with a constricted neck and a tapering caudal section. However, C. silvaticus has a much greater number of dorsal scales, distributed in 28–32 columns with 50–60 scales in each compared to 16 columns with 22–24 scales in each for H. trapezoidum n. sp. Moreover, the adhesive tubes of C. silvaticus are significantly longer and of different shape than in H. trapezoidum n. sp. The new species has short ventrolateral spines that progressively fuse with the scales towards the posterior end. The dorsal scales are keeled without spines and in absence of further evidence (e. g. molecular data) this species is provisionally considered a member of Heterolepidoderma . H. trapezoidum n. sp. can be separated from all hitherto described species of the genus on the basis of its elongated furcal base and the presence of a dorsal apparently naked region above the pharynx. Of the described species within the genus H. trapezoidum n. sp. seems closest to H. famaillensis Grosso & Drahg, 1991 and H. longicaudatum Kisielewski, 1979 . H. famaillensis has a somewhat similar head-shape with small epipleuria and a kidney-shaped hypostomium. H. longicaudatum has similar trunk scales and a naked fairly elongated furcal base.

Figures

FIGURE 10. Heterolepidoderma trapezoidum n. sp. A) Habitus. B) Dorsal view of scales and anterior part of furcal base. C) Ventral view of head and neck region showing ventral ciliation and the smooth kidney-shaped hypostomium. Scale bars: A, 20 µm; B and C, 10 µm.

FIGURE 11. Schematic drawing of Heterolepidoderma trapezoidum n. sp. Dorsal view to the left and ventral view to the right. Scale bar: 10 µm.