Ichthydium malleum Schwank, 1990

Schwank, Peter & Kånneby, Tobias, 2014, Contribution to the freshwater gastrotrich fauna of wetland areas of southwestern Ontario (Canada) with redescriptions of seven species and a check-list for North America, Zootaxa 3811 (4), pp. 463-490 : 473

Ichthydium malleum Schwank, 1990

( Fig. 6)

Type locality. Among Lemna spp. and Wolffia spp. and on muddy substrata in ponds, Galt Creek, Puslinch district, Ontario, Canada.

Type material. Drawing of one specimen. Lectotype, SMNH Type-8552 deposited at the Swedish Museum of Natural History, Stockholm, Sweden.

Material: 2 specimens that are no longer extant.

Etymology. Species name derived from the mallet-shaped head.

Diagnosis. Small species, 120 µm in total body length. Head five-lobed with conspicuously widened hammerlike posterior lobes. Two pairs of short sensory ciliary tufts, anterior pair anchored by papillae. One pair of cilia present on the bulges of the posterior head lobes. Body completely devoid of scales, hyaline in appearance, with irregular lateral folds. Pharynx 30 µm in length.

Description. A small very delicate and frail species, 120 µm in total body length. Head five-lobed with the posterior lobes conspicuously widened. Their shape is angular like a hammer. Cuticular head plates absent. Two pairs of sensory ciliary tufts. Anterior pair anchored by papillae and substantially shorter than posterior pair. One pair of extra cilia also present on the anterior dorsal side of the bulges of the posterior head lobes. Posterior pair of dorsal sensory bristles inserted at U82.

Body width 20–22 µm at the head (U09), 14–16 µm at the neck (U19), 18–20 µm at the trunk (U62) and 11–12 µm at the base of the furca (U89). Furca short and straight, 13–15 µm in length, with adhesive tubes constituting 9–10 µm of the total furca length. The cuticle is very thin, hyaline, and naked. It is mostly folded irregularly.

Ventral surface naked except for ventral locomotory cilia.

Mouth subterminal, 3–4 µm in diameter. Pharynx, 30 µm in length, narrow, cylindrical, widens only very slightly towards the PhIJ at U30.

Taxonomic remarks. The diagnostic protuberant bulging head lobes of I. malleum differentiate this species from other similar naked species, e.g. I. podura , I. maximum , and Ichthydium pellucidum Preobrajenskaja, 1926 . Other species with a naked pellucid cuticle are Ichthydium rostrum Roszczak, 1969 and Ichthydium galeatum Konsuloff, 1921 , but these species have very well developed cuticular head plates. I. malleum seems morphologically closely related to the following species: Ichthydium auritum Brunson, 1950 , which has two earlike flaps on the head, no dorsal bristles, and angular adhesive tubes; Ichthydium macrocapitatum Sudzuki, 1971 and Ichthydium macropharyngistum Brunson, 1949 , both of which have 4 earlike flaps.