Chaetonotus (Primochaetus) annae 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 : 479-481

Chaetonotus (Primochaetus) annae Schwank, 1990

( Fig. 9 )

Type locality. Among Lemna spp. in ponds, Galt Creek, Puslinch district, Ontario, Canada. Type material. Drawing of one specimen. Lectotype, SMNH Type-8555 deposited at the Swedish Museum of Natural History, Stockholm, Sweden.

Other material. 4 specimens that are no longer extant.

Etymology. This species is dedicated to my friend Anne Möhle.

Diagnosis. Medium-sized species, 140–160 µm in total body length. Head five-lobed with two pairs of short, scanty ciliary tufts. Furca forcipate, 19–20 µm long, with reduced proximal part and pointed adhesive tubes. Dorsal surface covered by 4-6 columns of anteriorly rounded and posteriorly indented smooth scales with straight simple spines. Lateral spines on average two to three times as long as the dorsal spines. Posteriormost dorsal and lateral spines, up to 39–40 µm in length, positioned in the posterior trunk region. Posterior to these spines the dorsal trunk is covered by 7 columns of smaller keeled scales. Ventral interciliary area covered by rectangular transverse scale plates with jagged anterior edges. Posteriormost interciliary area covered by small, rounded, triangular, keeled scales with very short simple spines. Pharynx 29–30 µm in length.

Description. Medium-sized species with a stocky tenpin-shaped body, 140–160 µm in total body length. Head five-lobed with scarcely cuticularized cephalion, 13 µm in width, and pleuria. Two pairs of short and scanty sensory ciliary tufts; posterior pair with longer cilia than anterior pair.

Body width 23–28 µm at the head (U08), approximately 20 µm at the neck (U19), 26–30 µm at the trunk (U59) and 15–16 µm at the base of the furca (U83). Head delimited from trunk by neck constriction that gradually widens into the trunk, which reaches its greatest width approximately 2/3 of the way down the length of the body. Furca forcipate, 19–20 µm in length, elongate with pointed adhesive tubes. Proximal parts of furcal branches are naked and reduced. Two pairs of dorsal sensory bristles, anterior pair anchored at U17. Dorsal surface covered by 4–6 columns with 19–21 anteriorly rounded and posteriorly indented smooth scales with straight simple spines. Length of scales and spines increases towards the posterior end. On the head, scales are widely spaced and are a quarter or less of the size of the largest scales of the trunk region. Scales of the trunk region measure 8–12 µm in length and 6–9 µm in width. The dorsal spines are 5–6 µm long at the neck and 15–20 µm long in the trunk region. The lateral spines are on average three to four times as long as the dorsal spines. The posteriormost 4 long, up to 39–40 µm in length, dorsal and lateral simple spines, are situated at U 69 in the posterior trunk region. Posterior to these spines the dorsal trunk is covered by seven columns of smaller keeled scales in 5 rows. Most of them do not carry spines but five more or less median spines and a pair of short parafurcal spines are situated at the posteriormost end of the dorsal trunk. These spines are 8–12 µm long and much shorter than the furcal branches.

Ventrolateral areas covered by 2–3 columns of keeled scales at either side. Ventral interciliary area, from neck region to posterior trunk region, covered by 26–30 rectangular transverse scale plates, 4–5 µm x 14–15 µm, with jagged anterior edges. Posteriormost interciliary area covered by 7–8 columns each with 7–8 small, rounded, triangular, keeled scales with very short simple spines. Ventral ciliation in two separate longitudinal bands.

Mouth very small, 3–4 µm in diameter. Pharynx approximately 29–30 µm in length with anterior and posterior muscular bulbs. PhIJ at U23.

Taxonomic remarks. Originally designated as Chaetonotus (Zonochaeta) annae by Schwank (1990) we decided to classify this species within Chaetonotus (Primochaetus) instead. This because all the morphologically closely related species formerly classified as C. (Zonochaeta) are now found within C. ( Primochaetus ). Moreover, the morphological characters of our species agree better with the diagnosis of C. ( Primochaetus ), which had not been published in 1990, than with that of C. (Zonochaeta).

C. (P.) annae can easily be separated from the bulk of Chaetonotus species because of its forcipate furca and its peculiar arrangement of scales in the interciliary area. There are three species with a forcipate furca and a similar arrangement of the interciliary scales: Chaetonotus (P.) macrolepidotus macrolepidotus Greuter, 1917 , Chaetonotus (P.) macrolepidotus ophiogaster Remane, 1927 and Chaetonotus (P.) soberanus Grosso & Drahg, 1983 . C. (P.) macrolepidotus macrolepidotus can be separated from C. (P.) annae because the former has an interciliary field that is completely covered by small spined scales and a dorsal posteriormost surface that is mostly naked. C. (P.) macrolepidotus ophiogaster has much shorter, if any, dorsal and lateral spines and the posterior ventral interciliary area is covered by apparently larger, oval, smooth spineless scales. C. (P.) annae appears to be morphologically most closely related to C. (P.) soberanus . However, C. (P.) soberanus differs by virtue of its possession of the following characteristics: (i) Grosso & Drahg (1983) do not describe keeled scales ventrally; (ii) there is only one pair of longer spines at the base of the furca; (iii) all the scales of C. (P.) soberanus are strongly overlapping; (iv) ventrolateral scale rows seem to be absent in C. (P.) soberanus ; (v) the locomotory cilia bands are situated more laterally and they do not reach the mouth region; and (vi) fewer ventral plates (only 11–12) that reach only from the posterior neck region to the middle of the trunk region.

Another character worth mentioning is that a third, wide longitudinal band of cilia was observed on the ventral side, medially in the head and neck region of one specimen. Whether this is due to a deformation or if it was some kind of artifact or possibly even a true observation could not be determined. However, it is the first time that a feature like this is reported for any gastrotrich.

Figures

FIGURE 9. Schematic drawing of Chaetonotus (Primochaetus) annae. A) Dorsal view. B) Ventral view. C) Head scale. D) Scale of mid-trunk region. E – F) Dorsal scales of posteriormost trunk region.