Setopus lemnicola ( 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 : 466-468

Setopus lemnicola ( Schwank, 1990)

( Fig. 3 )

Syn: Dasydytes (Setodytes) lemnicola Schwank, 1990

Type locality. Among Lemna spp. and Wolffia spp., Puslinch district, Ontario, Canada.

Type material. Drawing of one specimen. Lectotype, SMNH Type-8549 deposited at the Swedish Museum of Natural History, Stockholm, Sweden.

Other material. 3 specimens that are no longer extant.

Etymology. The species name is derived from the preferred habitat among Lemna spp.

Diagnosis. Small frail species, 110–115 µm in total body length. Strong neck constriction. Cephalion reduced and totally grown together with the cuticle. Cilia on head arranged in 6 semicircular bands. Caudal end with two lobes, sometimes squared or angular. Two short very fine spines situated on the inside of each lobe. Two pairs of posterior dentate spines of unequal length and 5 pairs of lateral dentate spines in trunk region.

Description. Small, very frail animals that gently float through the Lemna spp. plantlets. Body tripartite, 110–115 µm in total body length, with a strong neck constriction. The cephalion is rounded, 7 µm in diameter, strongly reduced or almost lacking and totally grown together with the cuticle. The cilia on the head are arranged in 6 semicircular bands. One pair of dorsal sensory bristles situated in posterior trunk region at U90. Cuticle and ventral interciliary area devoid of scales.

Body width 30 µm at the head (U18), 18–19 µm at the neck (U28) and 37–40 µm at the trunk (U57). The caudal end is divided into two lobes, which can sometimes be almost squared or angular. Two very short fine spines are situated on the inner side of each caudal lobe. There are also two outer dentate spines of unequal length. The longer one, situated on the left side, measures 48–49 µm while the shorter one measures approximately 10 µm. Cuticle apparently naked except for 5 pairs of long, strongly curved, dentate spines that are situated laterally on each side of the trunk region, starting from the neck and ending approximately 20 µm from the posterior end. Their length decreases from 60–65 µm at the neck and 40 µm in the middle of the trunk to 25 µm at the trunk end.

The ventral interciliary area is naked. The exact number of ventral locomotory ciliary tufts could not be observed.

Mouth terminal, 4–5 µm in diameter. Pharynx cylindrical, 30 µm in length and increased slightly in width towards the pharyngeal intestinal junction (PhIJ) at U34.

Taxonomic remarks. S. lemnicola can be separated from the species within the genus that possess equally long posterior spines, e.g. Setopus abarbitus ( Visvesvara, 1964) , Setopus bisetosus ( Thompson, 1891) Setopus dubius ( Voigt, 1909) , Setopus iunctus Greuter, 1917 , Setopus primus Grünspan, 1908 . The remaining species with unequal posterior spines are Setopus aequatorialis Kisielewski, 1991 , Setopus chatticus ( Schwank, 1990) and Setopus tongiorgii ( Balsamo, 1983) . All three species can be differentiated from S. lemnicola because they have a cuticle more or less covered by scales and a higher number of usually different spines.

Figures

FIGURE 3. Schematic drawing of Setopus lemnicola. A) Dorsal view. B) Distal end of a macrospine.