Amynthas reductus, Nguyen & Lam & Nguyen, 2022

Nguyen, Tung T., Lam, Dang H. & Nguyen, Anh D., 2022, Two new earthworm species (Annelida, Oligochaeta, Megascolecidae) from the Mekong delta, Vietnam, Zootaxa 5093 (4), pp. 483-492 : 484-487

Amynthas reductus sp. nov.

( Fig. 2 )

Material examined. Holotype: 1 mature, ( CTU-EW.051.h01) Vietnam, Ben Tre Province, Binh Dai District, Thanh Tri Village , banana plantation, (10°11’18.2N - 106°38’51.8E), 2 m a.s.l., 03 October 2009, coll. Nguyen Thi Cam Ly. Paratypes: 9 matures, ( CTU-EW.051.p02), same data as for holotype . Non-types: 6 matures, ( CTU-EW.051.03), same data as for holotype .

Etymology. “ reductus ” is used to emphasize the reduction of spermathecal diverticula.

Diagnosis. Small-sized worm, length 51–71 mm, diameter 2.4–2.7 mm, segments 81–112. Prostomium epilobous. First dorsal pore in 11/12. A pair of spermathecal pores in ventral intersegment 6/7. Spermathecal diverticula strongly reduced. Male pores in xviii, without copulatory pouches; ventral distance between two male pores about 0.30X body circumference. Genital markings: two post-setal lines of 4–5 in vii and viii, but numerous in the male region including three surrounding male pore, and many arranged in two to three lines behind the setal ring of xvii, intersegment 17/18 and in front of the setal ring of xviii. Holandric. Testis sacs not connected. Intestinal caeca simple. Septa 8/9/10 absent.

Description. External characters: Body cylindrical; small size, length 51–71 mm, diameter 2.4–2.7 mm, segments 81–112. Body colouration uniformly light brown, clitellum dark brown. Prostomium 1/3 epilobous. First dorsal pore in 11/12. Setae perichaetine, pre-clitellar setae stouter and denser than post-clitellar ones; setal numbers: 63–67 in viii, 44–50 in xxx, 11–15 between male porophores in xviii; setal distance: aa=ab, zz>zy. Clitellum annular, xiv–xvi, without dorsal pores and setae. Female pore single, in mid-ventral xiv.

One pair of spermathecal pores located ventrally in intersegment 6/7; ventral distance between two spermathecal pores about 0.35-0.38X body circumference. Two to six genital markings arranged in transverse lines located medioventrally behind the setal rings of vii and viii. Male pores lying on porophores located ventrally on the setal ring of xviii; no copulatory pouches; ventral distance between two male pores about 0.30X body circumference. Genital markings numerous, including three located surrounding male pore, and many arranged in two to three lines behind the setal ring of xvii, along intersegment 17/18 and in front of the setal ring of xviii.

Internal characters: Septa 5/6/7/8 thick, 8/9/10 absent, 10/11/12/13 thick. Oesophageal gizzard between 7/8 and 10/11. Intestinal origin at xvi; caeca simple, originating at xxvii and extending anteriorly to xxv. Last hearts in xiii. Pharyngeal micronephridia developed in 5/6/7/8. Typhlosole simple, lamelliform. Lymph glands absent.

One pair of spermathecae in vii. Ampulla oval-shaped, with many sutures; ducts about 1/2 ampulla, robust. Diverticula strongly reduced to a short, round lobe. Accessory glands mushroom-shaped with long ducts, in association with genital markings.

Holandric. Testis sacs developed, separated in x–xi. Seminal vesicles well developed in xi–xii enclosing the second pair of testis sacs laterally. Ovaries well developed in xiii, on vental posterior face of septum 12/13. Ovisac invisible. Prostate glands simple, within xviii; ducts robust, enlarged basally and directly attached to body wall. Accessory glands numerous, in association with genital markings.

DNA character. DNA materials were extracted from paratypes, but the amplification of the COI fragment failed.

Habitats and ecology. The species was found at the depth of 0–10cm of sandy loam soils in banana plantations.

Distribution: Only known from Vietnam.

Remarks. The new species can be assigned to the Amynthas youngi species group characterized by having only one thecal segment in vii and holandric ( Sims & Easton 1972). To date, this group contains eight species, A. hilgendorfi ( Michaelsen, 1892) (one thecal segment form), A. glabrus ( Gates, 1932) , A. tenellulus ( Gates, 1932) , A. youngi ( Gates, 1932) , A. malacus ( Gates, 1936) , A. vietus ( Gates, 1936) , A. andersoni doettrani Blakemore, 2006 , and A. grandiglandularis ( Do, 1995) ( Michaelsen 1892; Gates 1932; Gates 1936; Do 1995; Blakemore 2006). Within the A. youngi group, A. reductus sp. nov. is clearly different from others (except A. malacus ) by having numerous, small genital markings in 17/18, strongly reduced spermathecal diverticulum, and the simple prostate gland within only xviii; while other species have large genital markings paired in the male region, well developed spermathecal diverticulum, and lobuled prostate gland in several segments.

Amynthas reductus sp. nov. is fairly similar to A. malacus by presence of genital markings in the male and spermathecal regions, and simple intestinal caeca. However, Amynthas reductus sp. nov. has strongly reduced spermathecal diverticulum, first dorsal pore in 11/12, numerous lines-arranged genital markings in vii and viii, and separated testis sacs; while A. malacus has well-developed spermathecal diverticulum, first dorsal pore in 12/13, single genital markings in the spermathecal region, and connected testis sacs ( Gates 1936).

In Vietnam, Amynthas reductus sp. nov. is somewhat similar to A. aspergillum ( Perrier, 1872) re-described by Chang et al. (2009), A. kinfumontis ( Chen, 1946) ( Chen 1946) , A. vuongmontis (Thai, 1984) , and A. tienphucanus ( Pham, 1995) ( Perrier 1872; Thai 1984a; Pham 1995) by having numerous genital markings in transverse lines in the male region. However, the new species is characterized by one pair of spermathecal pores in 6/7, genital markings in and next to 17/18, separated testis sacs, strongly reduced diverticulum, and short, simple, poorly developed prostate glands in xviii while these species have more than one pair of spermathecal pores (7/8/ 9 in A. aspergilum and A. kinfumontis , 5/6/ 7 in A. tienphucanus , and 6/7/8/ 9 in A. vuongmontis ), no genital markings in 17/18, developed diverticulum, and deeply lobed and well developed prostate glands ( Table 1 ).

vuongmontis

Figures

FIGURE 2. Amynthas reductus sp. nov. Holotype (CTU-EW.051.h01). A1 & A2. Ventral view of the male region. B1 & B2. Left prostate gland. C1 & C2. Intestinal caecum. D. Ventral view of the spermathecal region. E. Dorsal view of testis sacs and seminal vesicles. F1 & F2. Left spermatheca. G. Male pore region transverse body section. H. Genital markings transverse body section. I. Body wall transverse section (Non-type: CTU-EW.051.03). Scale bar = 1mm.