Tetranchyroderma aethesbregmum, Lee & Chang, 2012

Lee, Jimin & Chang, Cheon Young, 2012, Three new gastrotrich species of the genus Tetranchyroderma (Macrodasyida: Thaumastodermatidae) from Korea, Zootaxa 3368 (1), pp. 245-255 : 247-249

Tetranchyroderma aethesbregmum sp. nov.

( Figs 2 , 5A–C )

Type material. Holotype (DBG1701) and 11 paratypes (DBG1702–1709, NIBRIV0000245094, 0000245095) mounted in glycerin on H - S slides, 7 Oct. 2001, leg. J. Lee and C.Y. Chang.

Type locality. Hwansun , Jeju Island, Korea, 33°14'26"N 126°19'50"E, 4–5 m deep .

Etymology. The specific name, aethesbregmum , is taken from the Greek aethes, meaning “strange, unusual” and bregma “front part of head” which refers to the peculiar shape of the head with an extremely undulating oral hood and a median trapezoidal lobe flanking three pairs of papillae.

Diagnosis. A small Tetranchyroderma with an adult length to 384 µm; pharynx length to 96 µm; with peculiar shape of head bearing a median trapezoidal lobe of oral hood and three pairs of papillae; cephalic tentacles absent; dorsal cuticular armature with pentancres only; eight ventrolateral cirratum-type tubes per side behind foremost TbVL to posterior intestinal region; four TbA per side; 15–16 TbVL per side, leaf-shaped, foremost tube located in anterior pharyngeal region and others in intestinal region; a pair of TbV situated in front of anus; paired pedicles trifid with two distal tubes and a dorsal cirrate tube, three lateral tubes on either side of each pedicle, and a pair of medial tubes between the pedicles; testis reaching to middle of trunk; copulatory organ elongated, vesicle-shaped.

Description of holotype. Body ( Fig. 2A, B ) slender, Lt 353 µm; a little constricted just behind PhJIn, and gradually broadened to middle of intestinal region, then gently narrowing towards posterior end; caudal pedicle short and naked; widths of oral opening, neck, PhJIn, trunk, and caudal base 46/29/29/39/17 µm at U3/U18/U26/ U50/U87, respectively.

Head ( Figs. 2C, D , 5A, B ) with extremely undulating oral hood dorsally, comprising a thin, broad, trapezoidal lobe bearing a median sensillum and paired long papillae each ornamented with membranous extension medially; a small, conical papilla arising from concavity of dorsal oral hood, flanking an elongated cylindrical papilla, which emerges from inside of the oral hood; oral opening flaring and undulating dorsolaterally, laterally and ventrally; sensory hairs scattered around oral opening; cephalic tentacles absent; about 10–12 hairs (9–14 µm long) aligned dorsolaterally and laterally throughout nearly whole body length (U8–U96).

Epidermal glands generally circular or ovoid, containing granules; seven to eight glands per side, along lateral side from posterior pharyngeal region to posterior intestinal region at U20, foremost one longest (9X19 µm in diameter), others mixed in size (6X7–7X17 µm); elliptical gland openings (stomata) with cuticularized lips (ca. 5–6 µm in diameter) dorsal surface of each gland.

Cuticular armature with pentancres only ( Figs. 2E , 5C ), spread over dorsal, lateral, and ventrolateral surfaces, except for front part of oral hood and caudum; arranged in 10–11 columns in mid-trunk region at U49, each column with 35–39 pentancres; pentancres 17 µm in diameter in most parts of the body, but smaller on oral hood (6 µm in diameter) and caudal base (5 µm in diameter); tines of pentancres are all similar in length.

Ventrolateral cirratum-tubes slender, minute granules, eight per side, aligned alongside TbVL; foremost (9 µm long) located just behind the first TbVL in anterior pharyngeal region at U13, others (9–12 µm long) more or less evenly spaced at intestinal region (U32–U81), asymmetrically arranged.

Adhesive tubes: four TbA per side, comprising a medial tube (7 µm long) at U3 and three ventrolateral tubes (6−8 µm long), lateralmost TbA pointing backward at U5. TbD(L) absent. TbVL 15–16 per side; foremost tube (9 µm long) slender and short, not foliaceaous, located in anterior pharyngeal region at U10; others (11–20 µm long) leaf-shaped, stumpy, much larger than foremost tube, aligned side by side in intestinal region from U35 to U83. A pair of TbV situated ahead of anus at U85, foliaceaous, 12–14 µm long. TbP seven per side, forming a feeble, trifid pedicle with two distal tubes (7 µm long) and a dorsal cirrate tube (5 µm long), accompanied with a medial foliaceaous tube (7 µm long) and three lateral foliaceaous tubes (6–13 µm long) at U92–U95.

Ventral locomotor ciliation arranged in a single column from behind oral opening to caudal base at U5.

Digestive tract: mouth terminal to 35 µm wide; pharynx 83 µm long (measured from ventral border of oral opening to PhJIn), pharyngeal pores at U22; intestine broad and narrowing toward posterior end; anus at U92.

Reproductive system ( Fig. 2B ): simultaneous hermaphrodite; single testis on right side, in dorsal view, short, reaching to U54, connected to copulatory organ via uncoiled vas deferens. A large ovum (28X51 µm) and a small oocyte present dorsally in mid-intestinal region at U51–U68. Seminal receptacle sac-like, located anterior to copulatory organ at U62–U74. Copulatory organ elongated, 69 µm long, vesicle-shaped, with anterior third narrowing, located at U71–U91.

Ecology. The species occurred in sublittoral fine basaltic sandy bottoms at 4–5 m near Hwasun Harbor, Jeju Island, often together with Tetranchyroderma multicirratum Lee & Chang, 2007 and Pseudostomella longifurca Lee & Chang, 2002 .

Measurements and Variability. Body lengths of 12 adult specimens mounted in glycerin range from 322–384 µm (mean 352 µm, standard deviation 23), maximum widths 40–52 µm (11U–15U).

The number and arrangement of adhesive tubes are a little variable: an additional ventrolateral TbA is shown in one side of only one specimen. TbVL in pharyngeal region is consistently single, but the number of TbVL in intestinal region is somewhat variable, ranging 13–16 per side. Number of TbP is rather consistent, except for lateral tubes, which range from two to four per side. Number of ventrolateral cirratum-type tubes in the intestinal region range from four to nine per side, asymmetrically arranged. Moreover, a specimen has an additional cirratum at the pharyngeal region in one side. A pair of TbV is consistently present .

Taxonomic affinities. The genus Tetranchyroderma currently comprises 75 valid species, of which four have a cuticular armature with triancres only, 29 with tetrancres only, 39 with pentancres only, and three with both tetrancres and pentancres ( Todaro 2002; Hummon & Todaro 2010).

Tetranchyroderma aethesbregmum sp. nov. shares the cuticular armature of pentancres only with 39 species, however, it is most characteristic and easily differentiated from them by a unique shape of the head comprising a median trapezoidal lobe, three pairs of papillae beside the lobe and extremely undulating border of oral opening.

Besides the characteristic shape of head, T. aethesbregmum sp. nov. is allied to 11 species in lacking both cephalic tentacles and TbD(L): T. polypodium Luporini, Magagnini & Tongiorgi, 1971 , T. pacificum Schmidt, 1974 , T. thysanophorum Hummon, Todaro & Tongiorgi, 1993 , T. insulare Balsamo, Fregni & Tongiorgi, 1994 , T. anomalopsum Hummon, Todaro, Balsamo & Tongiorgi, 1996 , T. kontosomum Hummon, Todaro, Balsamo & Tongiorgi, 1996 , T. tanymesatherum Hummon, Todaro, Balsamo & Tongiorgi, 1996 , T. norvegicum Clausen, 1996 , T. psilotopum Hummon, Todaro, Tongiorgi & Balsamo, 1998 , T. pentaspersus Nicholas & Todaro, 2006 , and T. oligopentancrum Hummon & Todaro, 2009 .

Among these 11 species, T. anomalopsum , T. psilotopum and T. oligopentancrum not fully covered with cuticular armature in contrast with the new species which is covered entirely. In T. thysanophorum , T. tanymesatherum and T. pentaspersus , the pentancres consist of a long central tine and four shorter peripheral ones, while pentancres with five equal tines are present in T. aethesbregmum sp. nov.

Moreover, T. polypodium , T. insulare , T. kontosomum and T. pacificum are distinguished from T. aethesbregmum sp. nov. in lacking cirratum-type tubes (versus eight pairs of cirrata in T. aethesbregmum sp. nov.).

Tetranchyroderma norvegicum is most similar to T. aethesbregmum sp. nov. in having cirratum-type tubes, TbV, and a trunk that is fully covered with pentancres of five equal tines. However, it differs from the new species by the arrangement of adhesive tubes: 7–8 TbA per side, 15 TbP per side, paired TbV consisting of seven tubes, and three pairs of dorsolateral cirrata.

Figures

FIGURE 2. Tetranchyroderma aethesbregmum sp. nov. A, habitus, dorsal view; B, habitus, ventral view; C, head, dorsal view; D, head excluding TbA, ventral view; E, pentancres. Scale bars = 50 µm.

FIGURE 5. A–C, Tetranchyroderma aethesbregmum sp. nov.: A, head, dorsal view, DIC micrograph; B, head, frontal view, SEM micrograph; C, pentancres, SEM micrograph. D–E, Tetranchyroderma tritentaculatum sp. nov., SEM micrographs: D, mid-intestinal region showing a pair of dorsal adhesive tubes; E, pentancres. F, Tetranchyroderma insolitum sp. nov., cuticular armature with tetrancres and triancres mixed, SEM micrograph. Scale bars = 10 µm (A, B, D), 5 µm (C, E, F).