Micromicron cephalatum Cobb, 1920

Gagarin, Vladimir G. & Thanh, Nguyen Vu, 2015, Subsphaerolaimus minor sp. n. and Micromicron cephalatum Cobb, 1920 (Nematoda) from the Yen River Estuary of Vietnam, Zootaxa 3994 (3), pp. 396-410 : 403-408

Micromicron cephalatum Cobb, 1920

( Figs 4 , 5 ; Table 3 )

Material. 10 mature males and 10 mature females. Slides deposited in the nematode collection in the Institute of Ecology and Biological Resources, Vietnamese Academy of Sciences and Technology, Hanoi, Vietnam. One male, slide reference number TY 2.1.17; two females, slide reference number TY 2.1.14.

Measurements. Table 3 .

Type habitat and locality. Vietnam, Quang Ninh Province, Donghg Rhi, Yen River Estuary, mangrove forest. Latitude: 21º12.525ʹ–21º14.123ʹ N; Longitude: 107º22.269ʹ–107º23.105ʹ E, depth 1.0– 1.5 m, silted sand, with 4.2– 4.9 ‰ salinity.

Description. Male. Body short, cylindrical. Cuticle finely annulated and evenly thick (about 2 µm) over entire body. Cervical setae absent. Somatic setae present only on the posterior body end. Lateral alae 3.5–4.3 µm wide (12–14 % of comparative body width) beginning just behind the posterior edge of pharynx and extending to the level of cloaca. Anterior body end narrowed. Labial region 0.4 times as wide as body width at region of the posterior pharynx end. Labial region narrow and “sits” as a flattened head on the cephalic capsule. Six inner labial sensillae in the shape of thin setae about 1 µm long and barely visible. Six outer labial sensillae and four cephalic sensillae in the shape of thin setae 1.5–2.0 µm long. Cephalic capsule well developed, smooth, 7.5–8.0 µm high. Amphidial fovea situated at cephalic capsule, in the shape of circle 5.0–6.0 µm in diameter, in which is inserted a circle of smaller diameter. Stoma in the shape of a thin tube, with a large dorsal tooth and two smaller subventral teeth located in its anterior part. Pharynx muscular, with well-developed basal bulb measuring 22– 25 x 18–20 µm. Cardia small, narrow. Ventral gland not visible. Excretory pore located at distance 22–30 µm from anterior edge body.

Testes simple, outstretched, situated to the right side of the intestine. Spicules bent, with large capitulum and internal stripes. Spicule length at 1.5–1.6 times larger than cloacal body diameter. Gubernacula paired, “boat”- shaped. 23–29 comparatively small, cup-shaped supplements situated anterior to cloaca, ventrally. Adcloacal supplements located on cuticular hillocks. Around the cloaca is the characteristic, narrow, strongly sclerotized slender. 10–13 setae 3.0–4.0 µm long arranged before cloaca subventrally on each body side. Tail slender, gradually narrowing, ventrally curved. Three pairs of setae arranged on tail subdorsally and four pairs of thornshaped setae 3.5–4.0 µm long arranged subventrally. Caudal glands inconspicuous. Spinneret well developed, tubeshaped, 4.5–5.0 µm long.

Females. Similar to males in general characteristics. Structure of cuticle and anterior body end as in males. Cuticle finely annulated. Cervical and somatic setae absent. Lateral alae extending from posterior pharynx end to tail. Anterior body end narrowed. Labial region narrowed and “sits” as a flattened head on the cephalic capsule. Labial and cephalic sensillae in the shape of short, thin setae. Cephalic capsule well developed, smooth, 7.0–8.0 µm high. Amphidial fovea situated at cephalic capsule, circular, 4.0–5.0 µm in diameter, in which is inserted a circle of smaller diameter. Stoma in the shape of thin tube and armed with one dorsal and two subventral teeth. Pharynx muscular, with well-developed basal bulb. Rectum length is equal to or slightly less than anal body diameter.

Reproductive system didelphic, amphidelphic; ovaries antidromous. Gonadal flexures comparatively short. Anterior ovary situated to left side of intestine, posterior ovary situated to right side. Vulva pre-equatorial, lips not cuticularized and not protruding beyond the body contour. Vagina comparatively short, with thick muscular walls. Both uteri spacious, filled with numerous spermatozoa and 1–2 mature eggs measuring 44– 46 x 23–27 µm. Tail slender, gradually narrowing. Caudal setae absent; caudal glands inconspicuous. Spinneret well developed, tubeshaped, 4.0–5.0 µm long.

Discussion. The genus Pseudochromadora Daday, 1899 was established for the species Pseudochromadora quadripapillata Daday, 1899 based on one female collected from saline water bodies in New Guinea ( Daday 1899). The female is briefly described morphologically and characterized by the presence of lateral cuticular alae, wide cephalic capsule in centre of which are located the amphidial fovea, and cephalic setae situated before the cephalic capsule. Micoletzky (1922) regarded this species as species inquirenda. Andrássy (1959) redescribed Pseudochromadora quadripapillata from the female holotype and reinstated the species as valid. He also synonymised the species Micromicron cephalatum Cobb, 1920 with P. quadripapillata . Gerlach described and illustrated the species Pseudochromadora cazca Gerlach, 1956 . The females of this species are similar to females of P. quadripapillata and males lack precloacal supplemental organs but have a subventral group of 10–14 copulatory thorns. Subsequently some other species of the genus Pseudochromadora with similar male structures have been described.

The genus Micromicron Cobb, 1920 was established for the species Micromicron cephalatum , found on the Pacific coast of Costa Rica (Cobb 1920). The genus is characterized by a well-developed cephalic capsule, on which are situated circular amphidial fovea; presence of lateral alae; presence of 20 precloacal supplementary organs and dorso-caudal apophysis of the gubernaculum in males (Cobb 1920). More detailed studies of the spicular apparatus of males of this species have shown that a dorso-caudal apophysis of the gubernaculum is absent, but there is a strongly sclerotized cylinder around the cloaca. This species has also been found in mangrove forests of the coast of Brazil ( Gerlach 1957). A second species of the genus Micromicron , M. luticola Timm, 1952 , was described from the Atlantic coast of the USA ( Timm 1952) and subsequently found on the coast of East Pakistan ( Timm 1967) and of Canada ( Hopper 1969). It was transferred to the genus Pseudochromadora ( Timm 1952) : M. luticola morphologically does not differ from M. cephalatum and is considered a synonym of M. cephalatum (Veschelde et al. 1998; table 4). Desmodora vietnamica Gagarin & Nguyen, 2010 has wrongly been assigned to the genus Desmodora de Man, 1889, but is actually the species M. cephalatum ( Gagarin & Nguyen, 2010) , comb. nov.

Gerlach (1963) conducted a revision of the genus Desmodora and included in this genus several subgenera, including the subgenus Pseudochromadora Daday, 1899 . Micromicron cephalatum and M. luticola he synonymised with Desmodora (Pseudochromadora) quadripapillata . Gerlach and Riemann in The Bremerhaven checklist of aquatic nematodes accepted the division of the genus Desmodora into subgenera but included the species P. quadripapillata , P. luticola and P. cephalata as valid species in the subgenus Pseudochromadora ( Gerlach & Riemann 1973) .

In 1985, Coomans et al. described males of Desmodora (Pseudochromadora) quadripapillata from a freshwater pool on a coral island in the Solomon Islands. These have 31–36 precloacal cup-shaped supplementary organs and, in consequence, evidently belong to the genus Micromicron (new comb.) Verschelde et al. (1998) revised the genus Desmodora and reinstated its subgenera as valid genera, including Pseudochromadora . They included in this genus the species P. quadripapillata with synonyms Micromicron cephalatum and M. luticola ( Verschelde et al. 1998) . Verschelde et al. (2006) presented a dichotomous key for the determination of valid species of the genus. The species P. quadripapillata in this key is presented as a separate branch on the basis of the presence in males of this species of precloacal supplementary organs. However, these supplementary organs had been previously only reported in males assigned to the genus Micromicron .

Cobb, 1920 Micromicron cephalatum 2.25 –?? 20 Timm, 1952 Micromicron luticola 3.0 – 34 18 29–33

3.0 40–42 – – – Gerlach, 1957 Micromicron cephalatum 2.0 – 33 10 28–29

5.0 48 – – – Timm, 1967 Pseudochromadora luticola 2.0–2.5 – 26–34 19–21 24–26

3.5 46.5,50.3 – – – Hopper, 1969 Pseudochromadora luticola 2.6 – 40 17 30 2.7–3.6 40.6–48.3 – – – We consider the genera Micromicron and Pseudochromadora to be valid genera in the family Desmodoridae Filipjev, 1922 . Species of Micromicron differ from the species of Pseudochromadora in four morphological characters: 1. In Pseudochromadora , the body annuli are split up and interdigitate at the level of the lateral alae, but in Micromicron , the body annuli are not split up and not interdigitate. 2. Somatic setae in males and females Pseudochromadora are numerous and arranged in six longitudinal rows. Somatic setae are absent in females Micromicron and in males situated subventrally on the posterior body end. 3. Males of Pseudochromadora have copulatory thorns ( Fig.6 ), but males of Micromicron have cup-shaped precloacal supplementary organs. 4. Males of Micromicron have a strongly sclerotized cylinder around the cloaca, which is absent in Pseudochromadora . Therefore, the genera Pseudochromadora and Micromicron are independent and valid genera in the family Desmodoridae . Pseudochromadora quadripapillata was described based only on one female and consequently cannot be assigned at present to any genus of Desmodoridae ( Daday 1899; Andrássy 1959). In this connection, P. quadripapillata is considered as species inquirenda. Pseudochromadora cazca Gerlach, 1956 , described on females and males from mangrove forests of Brazil ( Gerlach, 1956) is established as the type species of the genus Pseudochromadora Daday, 1899 .

Figures

FIGURE 4. Micromicron cephalatum Cobb, 1920. A: male, entire body; B: female, entire body; C: male, head; D: spicule and gubernaculum; E: male, posterior body end. Scale bars: A, B— 50 µm; E— 20 µm; C— 15 µm; D— 10 µm.

FIGURE 5. Micromicron cephalatum Cobb, 1920. A: male, entire body; B, C: male, head; D: male, anterior body end; E: male, posterior body end, F, G: male, tail, H: male, precloacal body region; I: female tail. Scale bars: A— 50 µm; D, E— 20 µm; B, C, F – I— 10 µm.

FIGURE 6. Pseudochromadora galeata Verschelde et al., 2006. Male. A: entire body; B, C: head; D: anterior body end; E: posterior body end; F: spicular apparatus (from Verschelde et al., 2006, Fig. 3).

Tables

TABLE 3. Morphometrics of Micromocron cephalatum Cobb, 1920. All measurements are in µm and the form: mean ± standard deviation (range).

Character	10 males	10 females
L	645±58(544–722)	648±59(543–716)
a	21±2(18–25)	19±2(16–22)
b	7.1±0.4(6.6–7.7)	6.8±0.6(5.7–7.7)
c	14.7±1.5(11.9–17.0)	12.7±1.4(10.4–15.3)
cʹ	2.2±0.3(1.8–2.7)	3.2±0.4(2.7–3.6)
V, %	–	44.4±21.2(40.6–48.3)
diam.l.r.	13±1(11–14)	13±1(12–14)
diam.midb.	31±2(27–34)	34±2(30–37)
a.d.	20±2(17–22)	16±2(14–17)
ph.l.	91±5(77–96)	96±4(90–101)
dis.ph.cl.	510±49(432–579)	–
dis.ph.v.	–	192±22(156–221)
dis.v.a.	–	309±39(240–360)
t.l.	44±7(35–56)	51±4(45–55)
spic.	30±2(27–32)	–
gub.l.	16±2(14–19)	–
n.sup.	26±2(23–28)	–