Sphaerolaimus pumilus, Tchesunov & Eskandari & Hosseinvand & Hajializadeh, 2024

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 : 529-533

Sphaerolaimus pumilus sp. n.

( Figures 3–4 and Table 2 )

Type material. Holotype male (slide 50/18), four paratype males (slide 50/19) and three paratype female specimens (slides 50/20 and 50/21) are deposited in the nematode collection of the Center of Parasitology , A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences , Moscow, Russia. Other type specimens are kept in the nematode collection of Department of Invertebrate Zoology , Faculty of Biology , M. V. Lomonosov Moscow State University, Moscow, Russia. All the type specimens are mounted in permanent glycerin slides.

Type locality and habitat. Clay sediment under the crowns of Avicennia marina tree, salinity of interstitial water: 35.5 ppt, temperature on the sediment surface: 27.0 °C, north coast of Qeshm Island, Strait of Hormuz, Hormozgan Province, Iran, November 2021 (26°47’47.5”N, 55°43’18.1”E) .

Etymology. The species epithet means “pigmy” and reflects the minute body size of the species.

Descriptions.

Males. Very minute nematodes. Body cuticle thin and smooth, transversal striations not discernible. No lateral alae along the body. Anterior end rounded. Six small lips closed around the mouth opening. Inner labial papillae scarcely discernible. In most specimens, six setae (two lateral, two subdorsal and two subventral) visible at the level of fine-striated portion of the cheilostoma. Six groups of a little longer subcephalic setae at the level of shagreen band. They are arranged in two subdorsal and two subventral pairs, and two lateral tetrads. All setae disposed at the same level horizontally in all subcephalic groups. In the latero-median pairs, some longer setae located in a more lateral position. Cervical setae up to 5 μm long arranged in eight sparse irregular longitudinal rows which become inconspicuous shortly posterior to the amphideal fovea. Somatic setae short and sparse along the body. Amphideal fovea circular, with distinct cuticular rim, situated well posterior to the stegostoma level. Buccal cavity conspicuous and relatively voluminous. Cheilostoma, major portion of the buccal cavity, consists of three parts. Anterior cheilostoma part truncate cone-shaped and smooth-shaped. Middle cheilostoma part also truncated cone-shaped, its walls complex with fine longitudinal striations (ribbing). Posterior cheilostoma part is spacy barrel-shaped with walls forming a sclerotized shagreen band. Anterior and posterior edges of the shagreen band are even, without lobes and inlets. Gymnostoma narrow. Stegostoma thick-walled, asymmetrical. Pharynx almost cylindrical, gradually widening to the cardia, strongly muscular along its entire length. Cardia low, enveloped by intestinal tissue. Nerve ring approximately at the middle of the pharynx. Pore and ampulla of the ventral secretory-excretory gland located at a distance posterior to the nerve ring. Ventral gland cell body situated posterior to the cardia, lateral to the intestine.

Males reproductive system is diorchic, anterior testis outstretched, situated to the left of the intestine. Posterior testis is reflected, situated to the right of the intestine. In two paratypes, on the contrary, anterior testis located to the right and posterior to the left of the intestine. The testes look different. Anterior outstretched testis is filled with small spheric to ovoid cells, evidently at stage of spermatids or spermatozoa. Posterior reflected testis contains a row of large cells increasing gradually in size from the tip to the junction with the vas deferens (spermaduct); no small spermatid or spermatozoa were observed there. Spicules very short, slightly curved, distally rounded, proximally non-knobbed. Gubernaculum absent. No supplementary organs, except a tiny precloacal pore with a canal. Tail conico-cylindrical, terminally slightly clavated, with three long terminal setae and a few latero-dorsal and latero-ventral setae.

Females. In a sole specimen (female), the latero-median setae on the head are distinctly paired, therefore, the actual number of setae in this circle may be ten (6 ols+4 cs). Amphideal fovea of females are smaller than that of males. Female reproductive system monodelphic, prodelphic, an outstretched ovary situated to the left of the intestine. Uterus filled with small spherical spermatozoa. Vagina oblique.

Diagnosis. Body length 564–634 μm; a = 20–30, b = 4.1–4.5, c = 6.5–7.5, c’ = 3.4–5.3, V% = 62–65%. Outer labial and cephalic setae of the anterior crown 1–1.5 μm long, located at the level of the striated part of the cheilostoma. Subcephalic setae 2–6 μm long, arranged in eight pairs (four sublateral and four submedian) or in two lateral tetrads and four submedian pairs, situated at the level of shagreen band. Amphideal fovea located well posterior to the stegostoma. Ventral excretory/secretory pore located at a distance of 27–37 μm posterior to the nerve ring. Spicules short and almost straight, 14–15 μm long. Gubernaculum absent. Tail conico-cylindrical, cylindrical portion 35–44% of the entire tail.

Differential diagnosis. A notable feature of S. pumilus is the close distance of sublateral pairs of the subcephalic setae that forms an unusual pattern of two lateral tetrads and four submedian pairs. In some individuals, the tetrads look as having a small space separating latero-dorsal and latero-ventral pairs of setae.

S pumilus belongs to a group of species with amphideal fovea situated at a distance posterior to the stegostoma. This is a juvenile feature (see e.g. Lorenzen, 1978, 1981, 1994 and other sources cited therein) retained at adult stage in several species. Among these species, S. pumilus shares absence of a gubernaculum with S. abeskunus Tchesunov, 1980 and S. paragracilis Vitiello, 1971 . S. pumilus differs from S. abeskunus (described from the Caspian Sea, 180–185 m deep) by shorter body length (564–634 μm versus 846–1190 μm), shorter anterior setae (1–1.5 versus 5.6–5.7 μm), width of the amphideal fovea (4–7 versus 8.7–8.8 μm), and spicules length (14–15 versus 25–27 μm). S. pumilus differs from S. paragracilis Vitiello, 1971 (Mediterranean, 310–650 m deep) by slightly shorter subcephalic setae (2–6 versus 6.5–8.5 μm) arranged in eight groups and absence of even a vestigial gubernaculum.

It is worth noting that the two related species, Sphaerolaimus pumilus sp. n. and S. paragracilis dwell in remote quite different habitats, the first species in tropical intertidal mangrove sediments, while the second one in permanently cool or cold aphotic deep-sea silt in the Mediterranean Sea ( Vitiello, 1971). Such a striking difference between two habitats can strongly corroborate separateness of these two closely related species.

Figures

FIGURE 3. Sphaerolaimus pumilus sp. n., entire and anterior body. A: holotype male, entire; B: paratype female n3, entire; C: paratype female n2, anterior body. Scale bars: 50 μm.

FIGURE 4. Sphaerolaimus pumilus sp. n., anterior ends and male posterior body. A: anterior end of the holotype male; B‒D: anterior ends of paratype females; E: posterior body of the holotype male. Scale bars: 10 μm.