Acrostichus medius, Tahseen, Qudsia, Ahlawat, Shikha, Asif, Mohammad & Mustaqim, Malka, 2016

Tahseen, Qudsia, Ahlawat, Shikha, Asif, Mohammad & Mustaqim, Malka, 2016, Description of a new species of Acrostichus Rahm 1928 (Nematoda: Diplogastridae) from India with a note on its position and relationship with the congeners, Biodiversity Data Journal 4, pp. 8029-8029 : 8029

Acrostichus medius sp. n.

Materials

Type status: Holotype. Record Level: type: Acrostichusmedius; modified: NOQ; language: English; rightsHolder: Aligarh Muslim University; bibliographicCitation: Tahseen et al.; institutionID: amu.ac.in; collectionID: urn:lsid:zoobank.org:author:E2679573-3F6B-4221-8817-AFB63266528D; institutionCode: AMU; collectionCode: NOQ

Description

Hermaphrodite female (Figs 2, 3, 4, 5, 6, 7 and Table 1): Body slender, medium to large-sized; almost straight after fixation, tapering at both extremities. Cuticle with fine transverse striations; longitudinal ridges delicate, striated (Figs 3e, 4a, e, 5b). Punctations faint, dot-like, running along longitudinal ridges (Fig. 4a). Lateral fields demarcated with two conspicuous ridges about 3-5 µm apart. Lip region continuous with adjoining body. Lips amalgamated; labial sensilla slightly raised. Amphidial apertures elliptical, 2.5-3.5 µm wide and situated about 5-7 µm from anterior end of stoma (Fig. 2c). Cheilostom cuticularised, converging anteriorly with six adradial plates, gymnostom anisomorphic with dorsal wall thickened than sub-ventrals. Stegostom anisotopic, anisomorphic. Dorsal metastegostomal wall with a triangular tooth, each sub-ventral wall provided with relatively smaller tooth (Figs 2c, 3a, b). Pharyngeal corpus muscular, swollen 70-88 µm long; metacorpus, rounded to ovoid strongly developed with thickened lumen, more or less oblong in few specimens; isthmus conspicuously differentiated from metacorpus, 33-45 µm long. Basal bulb small, pyriform, glandular in nature, continuous with isthmus, 16-22 µm x 13-17 µm in dimension (Figs 2d, 3d). Nerve ring encircling isthmus at 67-74% of pharyngeal length from anterior end. Secretory-excretory pore placed posterior to nerve ring or at 81-85% of pharyngeal length from anterior end (Figs 2d, 3e). Body at pharyngeal end 3.5-4.4 times labial diam. wide. Cardia 5-7 µm long. Intestine thin-walled, often with a bacterial pouch present in anterior part. Rectum 1.2-1.5 times anal body diam. long, with three rectal glands. Reproductive system didelphic, amphidelphic; ovaries reflexed, not reaching the level of vulva (Fig. 4b, c); anterior ovary on right side and posterior on left side of intestine. Posterior genital branch usually smaller as compared to anterior branch (Fig. 6b, c, d). In several individuals, the posterior branch represented by a post-uterine sac (Fig. 6e). Oocytes arranged in double row in proliferation zone of each ovary. Oviduct a narrow tube leading to a wider ovoid spermatheca (Fig. 4b). The proximal end of spermatheca showing cluster of sperms (sh) developed within the hermaphrodite’s gonad. Uteri containing 2-4 eggs of 50-56 µm x 29-32 µm in dimension occasionally in early stage of segmentation. Each uterus with a large ovoid to kidney-shaped dorsal bi- or trilocular pouch connected to vagina mostly filled with larger sperms (sm) transferred as a result of copulation (Fig. 7). Vagina 17-21 µm, cuticularised, forming an ovijector with thick lumen occupying about 1/4th of corresponding body diam.; epiptygma present (Figs 4c, 7b). Vulval lips slightly protruding. Distance between vulva-anus 0.7-0.9 times tail length. Tail long filiform with a fine pointed terminus.

Male (Figs 2b, f, 5 and Table 1): Similar to female in general appearance but shorter in length and strongly curved in posterior region. Testis single, laterally reflexed, reflexed part 45-52 µm long. Vas deferens a long tube with a tapering ejaculatory duct joining with rectum to form cloaca. Spicules strongly built, heavily cuticularised, strongly arcuate in proximal half, 1.3-1.5 times anal body diam. long with elongated hood-like capitula, a distinct neck and distal part deeply bifurcated with fine extensions and an attenuated ventral arm separated from main body of spicule (Figs 2f, 5a, d, e). Gubernaculum stout, heavily built, 70-82% of spicule length, proximally tapering and curved and distal end with slight protuberances (Figs 2f, 5e). Tail in two parts, an anterior short, conoid part and a posterior long, filamentous part. Genital papillae ten pairs with three precloacal, one adcloacal and six postcloacal pairs. Precloacal pairs GP1 and GP2 closely placed, subventral; GP3 lateroventral. GP4 at level of cloaca. Postcloacal pairs GP5 closely posterior to cloaca; GP6 subventral, more or less one cloacal body diameter posterior to cloaca and nearly at level of phasmids. GP10 subdorsal pair placed slightly posterior to the group of subventrals GP7, GP8 and GP9 (Figs 2f, 5b, c). A membranous rudiment indicating bursa, occasionally confined to posterior most genital papillae including three subventral pairs and one dorsally directed pair (Figs 2f, 5c). Copulatory muscles representing 5-6 pairs of broad bands. Phasmids pore-like, about one anal body diam. posterior to anal opening.

Diagnosis

Acrostichus medius n. sp. is characterised by female hermaphrodites having a medium-sized body with cuticle bearing faintly striated longitudinal ridges; lateral fields demarcated with two conspicuous ridges; lips amalgamated, labial sensilla small, papilliform; cheilostom with six adradial plates; dorsal tooth moderately-built slightly larger than subventrals; posterior genital branch relatively smaller; dorsal uterine pouch large, multilocular, oval to rounded occasionally filled with sperms and males with robust, heavily cuticularised spicules with hood-like capitula, appearing deeply forked distally with fine extensions and a ventral attenuated arm; gubernaculum 2/3 of spicule length with curved and tapering proximal end and distal end with slight protuberances; bursa almost absent with rudiments confined to posterior four genital pairs.

Etymology

The species name ‘medius’ is a latin word that indicates the intermediate status of the species showing a blend of characters of Acrostichus and Diplogastrellus .

Distribution

Samples containing Acrostichus medius n. sp. were collected from soil rich in organic matter near State Bank of India at Aligarh, Uttar Pradesh, India at geographical coordinates 27°53'35"N, 78°4'27"E.

Ecology

Acrostichus is a genus with species reported from all the continents except Australia although biogeography of the genus indicates larger distribution in subtropical to temperate regions (Fig. 8). The individuals have been found to inhabit soil, fresh and polluted water and more specifically reported from aquatic mulm or slime flux or sewage; from soil to rotten decaying matter or from moist husk to rotten plants and from dung to frass of beetles. Most species are bacteriophagous but like other diplogastrids, may also feed on protozoa, fungi and nematodes ( Bento et al. 2010).

Conservation

Due to inhabiting diverse environment types, the species show extensive diversity reflecting phenotypic plasticity. The variations in the shape of buccal cavity which can be shallow and broad, or narrower and deeper, and the variations in size and shape of dorsal tooth are few such examples.

Biology

A. medius n. sp. shows hermaphrodite females and males in the population. It demonstrates a gradual reduction in posterior genital branch in several specimens up to the extent of a reminiscent post-uterine sac. Such unique feature indicates towards its transitional status in the evolutionary process showing affinities to both the related yet distinct genera Acrostichus and Diplogastrellus .

Taxon discussion

Relationship with closely related species

Acrostichus medius n. sp. most closely resembles A. consobrinus ( De Man 1920) Massey 1962 in most morphometric characteristics but differs in having smaller ‘a’ (13.0-19.6 vs 20.5-24.3) and greater 'c' (9.2-15.8 vs 7-8) values in females and smaller ‘a’ value (15.2-19.5 vs 25.7-29.5) in males; labial sensilla papilliform (vs setose); faint (vs conspicuous) adradial plates; stoma narrow tubular (vs wide tubular); dorsal tooth small (vs robust), slightly larger (vs markedly larger) than subventrals; spicules with prominently demarcated dorsal and ventral arms having fine distal extensions (vs arms not demarcated into arms and genital papillae ten pairs (vs nine pairs) in A. consobrinus .

Acrostichus medius n. sp. also resembles A. superbus ( Paesler 1946) Massey 1966 in most morphometric characteristics but differs in having smaller ‘a’ value in females (13.0-19.6 vs 20.0-24.4) and males (15.2-19.5 vs 25.5-30.2); labial sensilla papilliform (vs setose); faint (vs conspicuous) adradial plates; stoma narrow tubular (vs wide tubular); gymnostom (strongly vs weakly) cuticularised; subventral metastegostomal teeth relatively larger (vs smaller); basal bulb continuous with isthmus (vs distinctly demarcated); males with broad, massive (vs slender, arcuate) spicules having prominent dorsal and ventral arms (vs arms not demarcated) and genital papillae ten pairs (vs nine pairs) in A. superbus apud Weingärtner 1950 and Gagarin 2002).

The new species comes close to A. dendrophilus ( Weingärtner 1950) Massey 1966 in most morphometric characteristics but differs in having smaller ‘a’ value (13.0-19.6 vs 23.3-30.7) in males; smaller ‘b’ value (5.7-6.3 vs 6.8-8.6) in females; greater ‘c’ (3.1-3.6 vs 2.0-3.0) and ‘V’ (43.0-46.9 vs 33.7-36.7) values; labial sensilla papilliform (vs setose); males with broad, massive (vs slender, arcuate) spicules; hood-shaped (vs rounded) capitula; spicules having prominent dorsal and ventral arms (vs arms not demarcated) and genital papillae ten pairs (vs eight pairs) in A. dendrophilus apud Gagarin 2002.

The new species also resembles A. lazarevskajae ( Lazarevskaja 1964) Sudhaus and Fürst von Lieven 2003 in most allometric ratios but differs from it in having larger body size (L= 764-867 µm vs 365-430 µm in females and 611-715 µm vs 290-441 µm in males); greater ' ć' value (9.2-15.8 vs 6.6) and large-sized spicules (36-44 µm vs 23-25 µm) in males; spicules massive (vs slender) with elongated hood-like (vs rounded capitula) and distal part deeply bifurcated (vs bearing very fine spines at tip) and presence [vs absence of an attenuated ventral arm separated from main body of spicules in A. lazarevskajae apud Lazarevskaja 1964].

Notes

Most of the species of the genus have been described on very flimsy characteristics hence many of them are likely to be synonymous. We tried to have a comparative assessment of the morphological characteristics of most species. Of the total nominal species, few could not be included largely due to insufficient descriptions available.