Ichthydium skandicum, Kånneby, Tobias, Todaro, Antonio & Jondelius, Ulf, 2009

Kånneby, Tobias, Todaro, Antonio & Jondelius, Ulf, 2009, One new species and records of Ichthydium Ehrenberg, 1830 (Gastrotricha: Chaetonotida) from Sweden with a key to the genus, Zootaxa 2278, pp. 26-46 : 27-30

Ichthydium skandicum n. sp.

( Figures 1 , 2 ; Table 1 )

Type material: Holotype mounted in glycerin with 10 % borax neutralized formalin. Department of Invertebrate Zoology, Swedish Museum of Natural History, Stockholm, Sweden. Accession no. 7753.

Other material: Six specimens which were measured and photographed but are no longer extant. Several additional specimens in 95% Ethanol for genetic studies.

Type locality: A small pool in a bog close to highway E14 between Åre and Storlien, Jämtland, Sweden, July 2008 (63º21.248’N; 12º35.403’E).

Other localities: Rockpool on Skaftö among Sphagnum sp., Bohuslän, Sweden, July 2009 (58º14.837’N; 11º26.269’E); Artificial pond on Skaftö among Sphagnum sp., Bohuslän, Sweden, July 2009 (58º14.634’N; 11º27.220’E); Rockpool on Blåbergsholmen among Sphagnum sp., Bohuslän, Sweden, July 2009 (58º14.976’N; 11º26.383’E).

Etymology: The species name refers to the Scandinavian mountain range “Skanderna” where the species was first found.

Diagnosis: Mid-sized Ichthydium species. Head with three lobes and one pair of ciliary tufts. Furca long and tong-shaped, each branch with very short base. Two pairs of thick sensory bristles, the posterior pair anchored to bi-keeled subtriangular scales. Dorsal body surface naked except for one pair of subtriangular bikeeled scales anchoring the posterior sensory bristles and four pairs of keeled subrectangular scales at the base of the furca. Pharynx large, 30–35 µm in length with posterior swelling; pharyngeal-intestinal junction (PhIJ) at U28. Locomotory cilia arranged in two rows in the anterior body region divided into four pairs of tufts in the trunk region. Ventral interciliary area naked except for two subrectangular terminal scales. Specimens observed were in parthenogenetic phase.

Description: Mid-sized Ichthydium species, 132–142 µm in total body length. Three-lobed head bearing one pair of cephalic ciliary tufts at U6. The longest cilium in each tuft is 20 µm in length. No cephalic plates were detected. Width of head/neck/trunk/furcal base are as follows: 23–32/20–28/28–42/12–14 µm at U13/ U28/ U60/ U82. Furca tong-shaped, 27–30 µm in length of which the adhesive tubes contribute 20–25 µm. A pair of bi-keeled subtriangular scales anchoring the posterior sensory bristles is present at U75. Another pair of sensory bristles arises from the neck region at U20. The sensory bristles are thick, the anterior pair (ASB) measures 10–12 µm and the posterior pair (PSB) 20 µm in length. Moreover, four pairs of dorsal subelliptical, keeled scales are present at the posterior end, two pairs situated dorsolaterally at U76 (6 x 1.5 µm) and U80 (6 x 1.5 µm) respectively and two pairs dorsally at U78 (4.5– 6 x 1–2 µm) and U81 (4.5– 5 x 1–2 µm) respectively. Apart from these five pairs of scales the dorsal body cuticle appears naked.

Ventral ciliation consists of locomotor cilia, 20–25 µm in length, arranged in two rows. In the trunk region the two rows are divided into four pairs of tufts. The anterior-most pair of tufts is situated in the anterior trunk region. The posterior-most pair of tufts is separated by a greater distance from the others of the same row. Interciliary area naked except for two rectangular keeled terminal scales, 8–9 µm in length and 4–4.5 µm in width. Mouth subterminal, 6–7 µm in diameter. Pharynx, 30–35 µm in length, widening towards the pharyngeal intestinal junction at U28. Intestine straight, tapering towards posterior end, anus ventral at U74.

The observed specimens were in parthenogenetic phase. More or less mature eggs, situated on either side or dorsal to the intestine, were seen in all specimens.

Taxonomic remarks: Ichthydium is currently divided into three subgenera (but see Balsamo et al. 2009): Euichthydium Schwank, 1990 predominantly species with a naked cuticle, Forficulichthys Schwank, 1990 , characterized by species bearing tong-shaped furca with tapering tips and the monotypic Pseudichthydium Rudescu, 1967 which contains I. balatonicum Varga, 1950 , characterized by a large comb-like keel on its dorsal body surface. It should be emphasized that many species are still not affiliated to any of the three above subgenera (see Appendix 1). It should also be noted that the subgenus Euichthydium is invalid; because it contains the type of the genus Ichthydium , hence it must bear the same name as the genus (see ICZN Article 44.1).

Ichthydium skandicum n. sp., by virtue of its tong-shaped furca, most closely resembles species within the subgenus Forficulichthys . According to Schwank (1990) the latter taxon contains four species, I. cephalobares Brunson, 1949 , I. forficula Remane, 1927 , I. forcipatum Voigt, 1901 and I. tanytrichum . The only species within this subgenus with both a tong-shaped furca and dorsal scales in the posterior trunk region is I. tanytrichum , with bi-keeled scales that carry the posterior sensory bristles in contrast to I. skandicum n. sp. which carries an additional four pairs of keeled dorsal scales.

Ichthydium contains an additional four species that carry scales or spines in the posterior trunk region I. crassum , I. diacanthum , I. dubium and I. squamigerum . I. crassum , I. diacanthum and I. dubium have spines but apparently lack scales which together with the different shape of the furca, differentiate them from I. skandicum n. sp. ( Daday, 1905; Schwank, 1990; Balsamo & Todaro, 1995). I. squamigerum has two pairs of keeled scales at the base of the furca compared to four pairs in I. skandicum n. sp. Moreover, at least another noticeable trait distinguishes these two species, that is the furca which is long (28–30 µm) and tong-shaped in I. skandicum n. sp. and short (13–18 µm) and straight in I. squamigerum (see Balsamo & Fregni, 1995).

Hence, I. skandicum n. sp. is morphologically closest to I. tanytrichum .

Associated gastrotrich fauna: Chaetonotus oculifer Kisielewski, 1981 and Ichthydium diacanthum .

Figures

FIGURE 1. A – C) Different representations of Ichthydium skandicum n. sp. Schematic drawing of A) dorsal view and B) ventral view. C) Detailed drawing of dorsal view. Scale bar: 20 µm.

FIGURE 2. A – C) Photographs of live specimens of Ichthydium skandicum n. sp. A) Habitus. B) Ventral view showing distribution of ciliary tufts in the trunk region. C) Dorsal view of posterior trunk region showing distribution of scales. Scale bars: A, B 20 µm; C, 10 µm.

Tables

TABLE 1. Morphometric data (µm) for Ichthydium skandicum n. sp. Specimen H, 1, 2 and 3 were found in Jämtland 2008 while specimens 4, 5 and 6 were found in Bohuslän 2009. L, Length; W, Width; H, Holotype; Na, Not available. Character Specimen

Total body L	H 135	1 137	2 132	3 135	4 140	5 140	6 142
Head W Neck W Body W	30 25 35	25 20 33	25 22 32	23 13 28	28 22 33	32 22 30	32 28 42*
Furca Base W Furca L	13 30	12 30	12 28	12 28	13 27	13 28	14 28
Pharynx L Mouth ø Sensory Bristle Scale L x W	34 7 4 x 4	31 6 6 x 5	30 6 Na	30 6 5 x 5	32 6 4 x 4	33 6 4 x 4	35 6 4 x 4
Anterior Sensory Bristle L Posterior Sensory Bristle L Furca Scales Dorsolateral L x W	~10 20 6 x 2	Na 20 Na	Na ~20 Na	~12 ~20 Na	13 20 Na	9 26 5 x 1	10 Na 6 x 1.5
Furca Scales Dorsal L x W Ventral Cilia L Ventral Terminal Scales L x W	6 x 2 Na 9 x 4	Na ~24 Na	Na ~25 Na	Na Na Na	4.5 x 1 20-25 9 x 4.5	4.5 x 1 24 8 x 4	5 x 1 20-25 Na
* Specimen with large mature egg.