Parastomonema papillosum, Kito & Aryuthaka, 2006

Kito, Kenji & Aryuthaka, Chittima, 2006, New mouthless nematode of the genus Parastomonema Kito, 1989 (Nematoda: Siphonolaimidae) from a mangrove forest on the coast of Thailand, and erection of the new subfamily Astomonematinae within the Siphonolaimidae, Zootaxa 1177, pp. 39-49 : 41-47

Parastomonema papillosum sp. nov.

( Fig. 2 , 3 )

Type specimens

Five males, 6 females and 1 juvenile. Holotype: male ( ZIHU 3137 ) . Paratypes: 4 males ( ZIHU 3135–3137 and KUMF T0007 ), 6 females ( ZIHU 3138–3142 and KUMF T0008 ), 1 juvenile ( ZIHU 3143 ). Collected on 11 January 2003 by T. Srisombat.

Type locality and habitat: Ban Khlong Khon, Samut Songkhram, Thailand; 13°19’7” N and 99°58’19” E. Muddy sediments in an Avicennia alba ­dominated mangrove forest near the mouth of the river Mae Klong in the Gulf of Thailand.

Measurements

Table 1.

Holotype: L = 1895 m, a = 79.0, b = –, c = 20.6

Males (n = 5, including holotype): L = 1454–1895 (1659 ± 165) m, a = 69.8–89.7 (79.6 ± 8.6), b = –, c = 17.0–24.6 (20.3 ± 2.8).

Females (n = 6): L = 1194 – 2996 (2153 ± 752) m, a = 36.2 – 117.9 (71.2 ± 28.8), b = –, c = –, V (%) = 41.3 – 87.2 (65.7 ± 20.0).

Juvenile (n = 1): L = 1169 m, a = 61.5, b = –, c = –.

Description

MALE (Holotype ZIHU 3137: range and/or avg ± sd in parentheses). Body 1.9 (1.5–1.9, 1.7 ± 0.17) mm long, slender, almost equal in diameter except for anterior and posterior region ( Fig. 2A ). Cuticle thin, weakly annulated from amphids level to tail end; 7–8 annules per 10 m at level posterior to amphids, 6–7 annules in middle of body and 10–11 annules in cloacal region. Short setae sparsely distributed in anterior and posterior body regions.

Head ( Fig. 2C ) rounded, equipped with 0+6+4 head sensilla; anterior sensilla papilliform ( Fig. 3B, H , cephalic papillae), minute and often inconspicuous, about 0.5 m long; posterior 4 setiform ( Fig. 3A,B , cephalic setae), very short, 2 (1.5–2.1) m long and located at level of 0.5 (0.4–0.6) hd length from anterior body end. Additional papilla with rounded tip faintly observed in ventral and dorsal position just posterior to cephalic setae. Amphids ( Fig. 3A, C ) circular, located at 1.1;1.1 (0.9–1.2, 1.0 ± 0.1) hd from anterior body end, 49;48 (43–55, 49.0 ± 3.7) % cbd; amphidial duct long, somewhat spiral­shaped in fovea (paratype ZIHU 3134 with aberrant left amphid, duct directly opening without circular aperture). A short cervical seta located in ventral and dorsal position at level of amphids and in lateral position posterior to amphids, about 2 m long; 2 (1–4) other shorter setae or papillae just posterior to amphid. Alimentary canal degenerated; mouth opening, buccal cavity and pharynx absent ( Fig. 2B, C ); vacuolated structures with a different size observed at anterior of this region ( Fig. 2C, G , 3B ). Rudimentary gut probably starting 70 (57–78, 65.8 ± 8.4) m or 7.2 (5.7–9.5, 7.2 ± 1.4) hd from anterior body end and containing small granules, large vacuoles and unknown micro­organisms with 2–5 m long by 1–2 m in diameter; rectum degenerated ( Fig. 2D ). Nerve ring ( Fig. 3A ) about 4.4 (3.4–5.1, 4.3 ± 0.7) hd from anterior body end; ducts and gland cells of amphids, nerve system, musculature and other structures present anterior to nerve ring; many peculiar cells, supposedly gland or nerve cells, located from level of nerve ring to anterior end of rudimentary gut. Excretory system not observed.

Testes paired, located left to the intestine ( Fig. 2A , 3E ); anterior testis outstretched and its distal end about 54% (54.3–56.6, 55.1 ± 1.3) of body length from cloaca, posterior one reflexed towards cloaca at 41% (35.3–41.3, 39.0 ± 2.5) and its distal end 38% (31.3–38.0, 33.9 ± 3.6) of body length anterior to cloaca; both distal portions indistinct in ZIHU 3135, 3136. Spicules ( Fig. 2D, E , 3F ) paired, curved and almost equal in length, 1.3;1.5 (1.3–1.6, 1.4 ± 0.1) abd long on arc and 1.0;1.1 (0.9–1.3, 1.1 ± 0.1) abd on chord. Gubernaculum consisting of a well cuticularized dorsal piece, 0.2;0.2 (0.2–0.3, 0.2 ± 0.03) spic arc, and a short lateral piece 4.1;4.1(3.3–4.2, 3.9 ± 0.3) m long; weakly cuticularized apophysis­like structure extending dorso­caudal. Small postcloacal supplements arranged subventrally in tail region, 6;7 (5–8) in number.

Tail ( Fig. 2D ) conical, gradually tapering to terminal spinneret, 4.6 (3.2–5.4, 4.4 ± 0.9) abd long; some short setae or papillae present, 2 of them just anterior to spinneret; many micro­organisms adhering to ventral surface of tail in paratype ZIHU 3135, their shape and size similar to those within body. Two caudal glands distinct; micro­organisms present in dorsal side of glands.

FEMALE (Paratype ZIHU 3142; range and/or avg ± sd in parentheses). Similar to males in most features. Body longer than in males ( Fig. 2F ), cuticular annules indistinct at mid body. Head with no additional papillae posterior to cephalic setae ( Fig. 2G ). Amphids located at a similar position in males but somewhat smaller in diameter, 40;36% (36–45, 40.5 ± 2.7) cbd; the third amphid located abnormally 1.7 hd from anterior body end on right side in the paratype ZIHU 3139 ( Fig. 3D ). Rudimentary gut starting 77 (62–77, 69.5 ± 6.1) m or 7.5 (6.0–7.5, 6.8 ± 0.6) hd from anterior body end. Ovaries ( Fig. 2F, H ) paired, opposed and outstretched, both right to the intestine (both left in ZIHU 3138–3141); anterior and posterior ovaries obscure distally but traced up to 446 (151–566) m and 529 (54–529) m from vulva, respectively (posterior ovary incompletely developed in ZIHU 3138, 3139). Vagina ( Fig. 2I,J ) well cuticularized, 9 (7–9, 7.9 ± 0.8) m long.

Posterior body end rounded and lacking anus due to breakage and wound healing at level anterior to anus ( Fig. 2K , 3G ): in the paratype ZIHU 3141 ( Fig. 2L ), conical tail present but rectum, anus and caudal glands degenerated.

JUVENILE (Paratype ZIHU 3143). Similar to females in most features. A short cervical seta located in ventral and dorsal position at level of amphids and in lateral position posterior to amphids, but no other setae nor papillae present just posterior to amphid. Amphid diameter 33% cbd. Nerve ring about 4.6 hd from anterior body end. Rudimentary gut probably starting posterior to nerve ring. Posterior body end rounded and lacking anus due to breakage and wound healing at level anterior to anus

Etymology The specific name papillosum refers to the minute papilliform sensilla on the head.

Discussion

The genera Astomonema and Parastomonema are distinguishable by the arrangement of head sensilla and the number of male gonads ( Kito, 1989; Austen et al., 1993); i.e., 0+6+4 head sensilla and a single testis in Astomonema while 0+0+4 head sensilla and paired testes in Parastomonema. Regarding these features, the present species differs from members of both genera because it has the 0+6+4 head sensilla as in Astomonema and the paired testes as in Parastomonema. It may be possible that this species belongs to a third mouthless genus which differs from the two known genera. The anterior sensilla of the present species are, however, quite minute papillae and hardly visible unlike those in Astomonema. Considering this degenerate feature of the head sensilla appearing like the 0+0+4 arrangement, it is more reasonable, at present, to place this species into Parastomonema characterized by the presence of paired testes in male.

Parastomonema papillosum sp. nov. is the second member of the genus. The type species P. fijiense is known from a coral reef of Viti Levu, Fiji ( Kito, 1989). P. papillosum clearly differs from P. fijiense in the following features besides the arrangement of head sensilla; short body length 1.2–3.0 mm (vs. 6.1–13.0 mm in P. fijiense), short cephalic setae 1.5–2.6 m (vs. 7–9 m), absence of subcephalic setae, and gubernaculum with weakly cuticularized apophysis in male (vs. gubernaculum with well cuticularized one). Although a vestigial buccal cavity was recognized in the early stage juveniles in P. fijiense, its presence is unknown in P. papillosum because no early stage juveniles were examined in this study.

Besides P. fijiense and P. papillosum , undescribed species of Parastomonema are so far known from mangrove forests of Merbok in Malaysia ( Somerfield et al., 1998), Hong Kong in China ( Zhou, 2001) and Santa Catarina Island in Brazil ( Netto & Gallucci, 2003). The Chinese species is probably different from the two known species in having no gubernaculum in males (Dr. Zhou, personal communication). There is no taxonomic information about the species reported from the other two localities.

Figures

FIGURE 2. Parastomonema papillosum sp. nov. Male (A–E, holotype ZIHU 3137): A) Entire body, posterior testis reflexed at arrowed point. B) Anterior region. C) Head. D) Posterior region. E) Spicule and gubernaculum. Female (F–I, K, ZIHU 3142; J, ZIHU 3140; L, ZIHU 3141): F) Entire body, vulva (arrowed). G) Head. H–J) Vulval region. K, L) Posterior region. A–E, L, left side view; F–I, K, right side view; J, ventral view.

FIGURE 3. Parastomonema papillosum sp. nov. Male (A, E, F, holotype ZIHU 3137; H, additional specimen) and female (B–C, ZIHU 3138; D, ZIHU 3139; G, ZIHU 3142): A) Anterior region. B) Transverse section at level anterior to cephalic setae, cephalic papillary nerves (arrowed). C) Transverse section at level of amphids. D) Two amphids located in tandem. E) Anterior and posterior testes near the arrowed point in Fig. 2A. F) Cloacal region. G) Posterior region. H) Head sensilla, sequentially arranged from the surface (a) to the deeper focal plane (d). A, E, F, H, left side view; D, G, right side view; B, C, en face view, top is dorsal side. Bar scales: A, E–H = 10 µm; B–D = 5 µm. Abbreviations: amp = amphid, cgl = caudal gland, cp = cephalic papilla/papillary nerve, cs = cephalic seta, gub = gubernaculum, gut = rudimentary gut, m = micro­organisms, nr = nerve ring, spic = spicule, T1 and T2 = anterior and posterior testis, vs = vacuolated structure.