Amynthas minimus (Horst, 1893)

Shen, Huei-Ping & Yeo, Darren C. J., 2005, Terrestrial Earthworms (Oligochaeta) From Singapore, Raffles Bulletin of Zoology 53 (1), pp. 13-33 : 21-24

Amynthas minimus (Horst, 1893)

( Fig. 4 )

Perichaeta minima Horst, 1893b: 66 .

Perichaeta pusilla Ude, 1893: 63 .

Amyntas minimus – Beddard, 1900a: 649.

Amyntas pusillus – Beddard, 1900a: 649.

Pheretima minima – Michaelsen, 1900: 284; Gates, 1961: 298; 1972: 201.

Pheretima enchytraeoides Michaelsen, 1916: 33 .

Pheretima humilis Gates, 1942: 120 .

Amynthas minimus – Sims & Easton, 1972: 236; Easton, 1979: 119; 1982: 728; 1984: 118.

Material examined. – 7 mature (1 dissected) and 15 immature specimens ( ZRC), Kent Ridge Campus of National University of Singapore, 6 Mar.2003 ; 1 immature specimen ( ZRC), Sungei Buloh , 7 Mar.2003 ; 1 mature specimens ( ZRC), Rifle Range Rd. opposite to Murnane Reservoir, 25 Jun.2003 . All the above specimens were collected by K. K. P. Lim, D. C. J. Yeo & K. L. Yeo.

Description. – Small worm, length (mature) 24-32 mm. Segments numbering 81-90. Prostomium epilobous. First dorsal pore 12/13. Setal number 58 in VII, 39 in XX, 6-9 between male pores in XVIII. Clitellum XIV-XVI, 1.03 mm in length, 1.64 mm in width, dorsal pores absent, setae absent or 1-3 on ventral XVI. Spermathecal pores one pair in 5/6, distance between paired pores 0.48 body circumferences ventrally apart. Female pore single, medio-ventral in XIV. Male pores inconspicuous, paired in XVIII, about 0.26 body circumferences ventrally apart, each on a round, conical porophore about 0.3 mm in diameter ( Fig. 4A ). Genital papillae absent in both preclitellar and postclitellar regions. Preserved specimens whitish grey with yellowish tan clitellum.

Septa 5/6-7/8 and 10/11-13/14 thickened, 8/9 and 9/10 absent. Gizzard in IX-X. Intestine enlarged from XIV. Intestinal caeca paired in XXVII, simple, long, extending anteriorly to XVIII or XIX ( Fig. 4D ). Esophageal hearts X-XIII. Meronephridia bush-like mass in intersegmental spaces anterior to 7/8. Spermathecae one pair in VI, ampulla elongated oval, about 0.5 mm long, 0.27 mm wide, with a slender stalk about 0.45 mm in length. Diverticulum with a slender stalk about 0.4 mm long and an elongated oval seminal chamber about 0.28 mm in length ( Fig. 4B ). Holandry: testis sacs in X and XI. Seminal vesicles paired in XI and XII, vestigial, first pair included in testis sacs. Prostate glands paired in XVIII, smooth, extending anteriorly to XVI or XVII and posteriorly to XIX. Prostatic ducts large, U-shaped ( Fig. 4C ). Accessory glands absent.

Remarks. – Amynthas minimus is a small, holandric and bithecal earthworm belonging to the minimus species-group of the genus Amynthas (sensu Sims & Easton, 1972) . Gates (1942) described Amynthas humilis a small earthworm with one pair of spermathecae in VI. It has 6-12 setae on ventral XVI (clitellum) but no genital markings. Amynthas humilis was only found in soil of flower pots in the Judson College campus, Rangoon, in spite of extensive collecting throughout Burma. It was probably imported from somewhere else ( Gates, 1942). Gates (1961) later found that its spermathecal diverticula were similar to that of A. minimus figured by Horst (1893b). After examination of the Hawaii worms, Gates (1961: 301) considered that the indistinctness of some markings and the condition of associated tissues warrant a suspicion that glands as well as the markings may be disappearing in some lineages, and individuals without genital markings can be expected. Therefore, Gates (1961, 1972) considered A. humilis as a synonym of A. minimus .

Easton (1981: 55) placed Amynthas zoysiae ( Chen, 1933) and Amynthas ishikawai ( Ohfuchi, 1941) in the brace of species synonyms of A. minimus . Amynthas minimus is fairly similar to A. zoysiae of China in external characters, such as body size, segment number (see Table 4), and male porophore structure. However, A. minimus has higher setal number and smaller prostate glands in XVI-XIX, whereas A. zoysiae has lower setal number and larger prostate glands occupying six to seven segments in XV-XXII (see Table 4). The prostate glands of A. minimus were described as well-developed in the original description ( Horst, 1893b), but Gates (1961) stated that prostates appear to be confined within XVII-XIX after examining the type specimen. Amynthas minimus was described based on four specimens from Java ( Horst, 1893b). Gates (1961) reported that three of the type specimens have been lost.

Amynthas zoysiae occurs in uncultivated land at the foot of hills in Zhejiang (= Chekiang), China ( Chen, 1933). The presence of the A. minimus [as A. humilis ] specimens from Burma and the A. minimus specimens from Singapore with the external characters so similar to A. zoysiae suggest a close relationship between A. minimus and A. zoysiae . In fact, both species were represented by bithecal, monothecal, and athecal morphs with more or less abnormal spermathecae ( Gates, 1961).

Amynthas ishikawai was reported from a cave in Japan ( Ohfuchi, 1941). It has one pair of spermathecae in VI, and its male porophore structure is similar to A. minimus . Gates (1961, 1972) and Easton (1981) considered it synonymous to A. minimus . However, A. ishikawai is of a larger size, and has lower setal number that shows a slightly increasing trend from anterior toward posterior region (see Table 4), whereas the setal number of A. minimus is higher in the preclitellar region and lower in the postclitellar region (see Table 4). Furthermore, A. ishikawai has well-developed seminal vesicles and large prostate glands (occupying seven segments) with straight, muscular ducts, whereas A. minimus has small, vestigial seminal vesicles and normal prostate glands (occupying three or four segments) with U-shaped ducts. Therefore, A. ishikawai is considered as a valid species different from A. minimus in this study.

Amynthas minimus probably has been transported by man as evidenced by its presence in flower pots in Rangoon, Burma ( Gates, 1942) and in banana plantations in Hawaii ( Gates, 1961). The original home of this species is unknown ( Gates, 1972). This species is by far the smallest pheretimoid earthworm recorded in Singapore.

Figures

Fig. 4. Amynthas minimus (Horst, 1893). A, ventral view of male porophore (mp) region; B, dorsal view of right spermatheca (amp, ampulla; dv, diverticulum); C, dorsal view of prostate glands (pg); D, dorsal view of right intestinal caecum.