Parasphaerolaimus brevisetosus Gagarin, 2014

Tchesunov, Alexei V., Eskandari, Ali, Hosseinvand, Manouchehr & Hajializadeh, Parima, 2024, Free-living marine nematodes of the family Sphaerolaimidae (Chromadoria, Monhysterida) from the mangroves of the Qeshm Island, Persian Gulf, Zootaxa 5468 (3), pp. 523-540 : 533-538

Parasphaerolaimus brevisetosus Gagarin, 2014

Figures 5–6 , 7C–E and Table 3

Material studied. Two males.

Locality and habitat. Silty sediment at distance of about 10 m from the nearest Avicennia marina tree, salinity of interstitial water: 35.2 ppt, temperature on the sediment surface: 31.4 °C north coast of Qeshm Island, Strait of Hormuz, Hormozgan Province, Iran, November 2021. (26°49’16.6”N 55°44’28.7”E) .

Descriptions.

Males. Body elongated fusiform. Cuticle with very fine but distinct cross annulations. A clear lateral ridge 6–7 µm wide extending along the body from about the level of the nerve ring to the cylindrical portion of the tail. The ridge rises up slightly over the body cuticle surface. Fine body cuticle annulations continue also onto the ridge.

Anterior end rounded conical. Mouth opening surrounded with six small lips. Six minute inner labial papillae at the bases of the lips. Circle of ten small setae (six outer labial + four cephalic setae) situated posterior to the inner labial papillae. There are two successive crowns of eight groups of short subcephalic setae. The anterior crown of subcephalic setae (I) situated at the level of the circle of light-refracted granules in the anterior cheilostoma; each group consists of three or four setae varying in length and arranged within the group at the same level horizontally. The posterior crown of subcephalic setae (II) situated at the level of anterior edge of the shagreen band; each group consists of two setae. There are also two more circles of eight singular subcephalic setae, at the level of posterior edge of the shagreen band and at the level of amphideal fovea, respectively. These last two crowns, especially the posteriormost one, are less regular than the anterior crowns I and II. Further thereafter, somatic setae arranged in eight loose longitudinal rows. The somatic setae are nearly equal to or longer than subcephalic setae (up to 8 μm); posterior to the nerve ring, the somatic setae become short and sparse. Amphideal fovea round with cuticular rim interrupted postero-dorsally; the fovea situated at the level of the anterior stegostoma.

Buccal cavity voluminous, the complex cheilostoma makes up the major part of the buccal cavity. Anteriormost portion of the cheilostoma looking soft-walled and unarmed, with only slight indistinct longitudinal striations. Middle portion of the cheilostoma shaped as truncate cone armed by a circle of light-refracted granules with fine longitudinal ribs descending and widening to the shagreen band anterior edge. The shagreen band rather thin-walled and fine-granulated; the shagreen band extends to the edge of the gymnostoma. Posterior edge of the shagreen band even, none-lobated. Stegostoma cup-shaped; internal lumen of the pharynx widened just posterior to the stegostoma. Pharynx strongly muscular along its entire length. Complex internal lumen visible as several cuticular bands.

Nerve ring at the anterior third of the pharynx. Ventral gland cell ampulla and pore located just posterior to the nerve ring. Major part of the ventral gland neck not discernible; ventral gland cell body small, situated laterally to the left of the anterior intestine.

Males reproductive system diorchic. Anterior outstretched testis located to the right of the intestine, while reflected posterior branch situated to the left of the intestine. Vas deferens consists of three sections from anterior backwards: short dark-granular striated section, short light-transparent striated section, long granular section. Vas deferens right to the intestine. Ejaculatory gland cell bodies with granulated content, three glands on both sides of the intestine and vas deferens. Spicules short arcuated, proximal (anterior) ends non-differentiated, distal (posterior) ends with small lateral hook (projection). Gubernaculum with short dorso-caudal apophysis. Three (or more?) minute precloacal midventral supplementary papillae.

Tail conico-cylindrical, terminal end slightly inflated and provided with three long terminal setae, one ventral and two latero-dorsal. A loose lateroventral row of several latero-ventral setae and a few other irregular lateral setae on both tail sides.

Diagnosis amended. Parasphaerolaimus with distinct lateral alae; body length 1317–1737 μm, a = 13.0–21.7, b = 3.2–3.9, c = 7.2–8.8, V% = 71.6–72.2%; anterior subcephalic setae I 2.0–3.0 μm long and posterior subcephalic setae II 1.0‒3.0 µm; amphideal fovea 6–10 μm wide, situated at level of the anterior stegostoma; entire stoma length 28–53, midstoma width 28–30 μm, shagreen band narrow and not sectioned into discernible elements, width of the shagreen band 12–15 μm; ventral pore situated just posterior to the nerve ring, at a distance of 167–194 posterior to the anterior end; spicules arcuated, 70–84 μm long, gubernaculum with dorso-caudal apophyses 15–19 μm long.

Remarks. Our finding of Parasphaerolaimus brevisetosus is a second record of the species since the first discovery of the species was in the North Vietnam by Gagarin (2014). Our males do not differ from the type males by dimensions. An important structural difference is a lateral cuticular ridge clearly visible in Qeshm specimens but not mentioned in the original diagnosis. Our reexamination of the type specimens reveals that the ridge is present also in the types.

Originally, Parasphaerolaimus brevisetosus has been found in silty sediments of a mangrove littoral zone at water salinity 6.1‰ in the Red River mouth, North Vietnam ( Gagarin 2014). Qeshm specimens have been collected also in silty sediments in the vicinity of mangrove trees but at a higher salinity, about 38‰. Evidently, P. brevisetosus is a widely distributed Indo-West-Pacific intertidal salinity-tolerant species with preference to silty sediments of mangroves.

Further remarks. Besides P. brevisetosus , we found two other species of Parasphaerolaimus in samples from Qeshm, Parasphaerolaimus 1 ( aff. paradoxus Ditlevsen, 1918 ) ( Figure 7F ) and Parasphaerolaimus 2 ( Figure 7G ). Both additional species were recorded with only few females in our collection, and hence their identification is delayed for future until males are found. However, microscopic observation of these species contributed to reinterpretation of the Parasphaerolaimus genus diagnosis.

Ecological notes.

Species of Sphaerolaimidae are common but not numerous in nematode communities of the Qeshm intertidal mangrove sediments. Total number of sphaerolaimid individuals in our Qeshm nematode collection constitutes 117 individuals or 3.14% of 3723 nematode specimens identified up to genus level. The sphaerolaimid species were found in eleven of 19 samples processed. Percentage of sphaerolaimid specimens varied from 0 to 12.18% per sample. All the sphaerolaimid specimens were detected in silty samples of the north coast of Qeshm and neither of them in sandy samples of the south coast.

Species of Sphaerolaimidae , and particularly Sphaerolaimus , are known long time as predators preying on other nematodes (e.g. Wieser 1959, Heip et al. 1985, Moens et al. 2014). Here, we also tried to evaluate feeding habits and diet of Sphaerolaimidae observing their gut content ( Table 4 ). The most reliable data were taken for Sphaerolaimus qeshmensis sp. n. and Parasphaerolaimus brevisetosus while Sphaerolaimus pumilus sp. n. was found with too low number of specimens to make conclusion. Guts of S. qeshmensis sp. n. and P. brevisetosus were empty in more than half of the individuals, 57 and 82%, respectively. From individuals with full intestines, 63% of guts of S. qeshmensis sp. n. contained swallowed nematode prey, and only 12.5% full intestines contained swallowed nematodes in P. brevisetosus . Thus, S. qeshmensis sp. n. and P. brevisetosus evidently differ from each other in diet and ratio of food items, S. qeshmensis sp. n. is a more active predator. As for S. pumilus sp. n., all the individuals but one had no content in the alimentary tract; an only individual had a lump but in the buccal cavity, not in the gut.

Figures

FIGURE 5. Parasphaerolaimus brevisetosus, entire and anterior end of a male. A: entire; B: head. Scale bars: A—200 μm; B—10 μm.

FIGURE 6. Parasphaerolaimus brevisetosus, male details. A: anterior body; B: posterior body. Scale bars: 100 μm.

FIGURE 7. Queshm Island species of Sphaerolaimidae, photographs. A: Sphaerolaimus qeshmensis, anterior end, holotype male; B: Sphaerolaimus qeshmensis, copulatory apparatus, holotype male. C: Parasphaerolaimus brevisetosus, anterior end, male; D: Parasphaerolaimus brevisetosus, male copulatory apparatus; E: Parasphaerolaimus brevisetosus, lateral alae, male; F: Parasphaerolaimus sp. 1, anterior end, female; G—Parasphaerolaimus 2, anterior end, female. Scale bars: 20 μm.