Orobdella minima, Lai, 2025

Lai, Takafumi Nakano and Yi-Te, 2025, A New Small-Sized Species of Orobdella (Hirudinea: Erpobdelliformes: Orobdellidae) from Mount Hehuan, Central Taiwan, Species Diversity 30 (2), pp. 127-134 : 128-132

Orobdella minima sp. nov.

( Figs 1–4 )

Diagnosis. Body length of mature individuals reaching ~ 3 cm. Somite IV uniannulate, somites VIII–XXV quadrannulate. Male gonopore in middle of somite XI b6, female in middle of somite XIII a1, behind gastropore, gonopores separated by 1/2 + 4 + 1/2 annuli. Pharynx reaching to somite XIII/XIV. Gastropore conspicuous, in middle of somite XIII a1. Gastroporal duct bulbous. Paired sperm duct bulbs, minute, in somite XVIII b6–XIX a1. Paired epididymides in somites XVI–XVIII, occupying 10–11 annuli. Atrial cornua developed, ovate.

Material examined. Holotype: KUZ Z1471 , dissect- ed, near High Altitude Experimental Station of Endemic Species Research Institute , Hehuanshan National Forest Recreation Area , Renai Township, Nantou County, Taiwan (24.16203°N, 121.28622°E; elev. 2960 m), by Yi-Te Lai, 9 February 2013 . Paratype: ASIZW0001115 , dissected, same locality and collector as for holotype (24.16439°N, 121.28571°E; elev. 2900m), 21 March 2019 .

Description. Body firm and muscular, elongate, with constant width in caudal direction, dorsoventrally compressed, BL 31.7 mm, BW 2.8 mm ( Fig. 1A, B ). Caudal suck- er ventral, elliptic, CL 1.2 mm, CW 1.4 mm ( Figs 1B , 2D ).

Somite I completely merged with prostomium. Somite II (= peristomium), III, IV uniannulate ( Fig. 2A ). Somite V biannulate, (a1 + a2) = a3; a3 forming posterior margin of oral sucker ( Fig. 2A, B ). Somites VI and VII triannulate, a1 = a2 = a3 ( Fig. 2A, B ). Somites VIII–XXV quadrannulate, a1 = a2 = b5 = b6 ( Fig. 2A–E ). Somite XXVI triannulate, a1 = a2 <a3; a3 being ventrally last complete annulus ( Fig. 2C, D ). Somite XXVII uniannulate ( Fig. 2C ). Anus behind somite XXVII; post-anal annulus absent ( Fig. 2C ).

Male gonopore in middle of somite XI b6 ( Fig. 2E ). Female gonopore in middle of somite XIII a1, inconspicuous, located posterior to gastropore ( Fig. 2E, F ). Gonopores separated by 1/2 + 4 + 1/2 annuli ( Fig. 2E ).

Anterior ganglionic mass in somite VI a2 and a3 and somite VII a1. Ganglion VII in a2 and a3. Ganglia VIII–XV and XIX, of each somite, in a2 and b5 ( Fig. 3B ). Ganglia XVI–XVIII and XX–XXIII, of each somite, in a2 ( Fig. 3B ). Ganglion XXIV in a1 and a2. Ganglion XXV in somite XXIV b6 and somite XXV a1. Ganglion XXVI in somite XXV b5. Posterior ganglionic mass in somite XXV b6 and XXVI a1 and a2.

Eyes in 3 pairs, 1st pair dorsally, slightly anterior to middle of somite III, 2nd and 3rd pairs dorsolaterally on posterior margin of somite V (a1+ a2) ( Fig. 2A ). Papillae numerous, minute, hardly visible, 1 row on every annulus.

Nephridiopores in 17 pairs, each situated ventrally at posterior margin of a1 of each somite in somites VIII–XXIV ( Fig. 2B, D, E ).

Pharynx agnathous, euthylaematous, reaching to somite XIII/XIV ( Fig. 3A ). Crop tubular, acecate, reaching to somite XX a1/a2. Intestine tubular, reaching to XXIV a2/b5. Rectum tubular, thin-walled, straight. Gastropore conspicuous, ventral, in middle of somite XIII a2 ( Fig. 2E, F ). Gastroporal duct thick, bulbous, winding at junction with gastropore, reaching to somite XIV a1 ( Fig. 3A ).

Testisacs multiple ( Fig. 3B ); on right side, in somite XIX a2 to somite XXV a1, in total ~22 testisacs, 3 in XIX, 2 in XX, 4 in each somite of XXI–XXIV, 1 in XXV; on left side, in somite XIX b6 to somite XXIV b6, in total ~20 testisacs, 1 in XIX, 4 in each somite of XX–XXIII, 3 in XXIV. Paired sperm duct bulbs minute; on right side, in somite XVIII b6; on left side, in somite XIX a1 ( Fig. 3B ). Paired epididymides; right epididymis in somite XVI a1 to somite XVIII b5/b6,

Partition Model

18S and H3 1st position K80 +I

28S and H3 2nd position GTR +I + G

H3 3rd position HKY + G

COI and ND1 1st positions GTR +I + G

COI and ND1 2nd positions GTR +I + G

COI and ND1 3rd positions and 16S GTR +I + G tRNACys – tRNAVal * and tRNALeu GTR +I + G

occupying 11 annuli; left epididymis in somite XVI a1/a2 to somite XVIII/XIX, occupying 11 annuli ( Fig. 3B ). Paired ejaculatory ducts in somite XI b5 to somite XVI a1; coiled in position posterior to ovisacs; each duct crossing ventrally beneath each ovisac, then coiled in position anterior to ovisacs; each widening from respective junction with epididymis, narrowing at junction with atrial cornua, then sharply turning proximally toward atrial cornua without pre-atrial loop ( Fig. 3B ). Pair of muscular atrial cornua developed, ovate, in somite XI b5 and b6 ( Fig. 3B–E ). Atrium short, muscular, globular in somite XI b5 and b6 ( Fig. 3C–E ).

Paired ovisacs globular, in somite XIII a2 and b5 ( Fig. 3B, F ). Oviducts thin-walled, left oviduct crossing ventrally beneath nerve cord ( Fig. 3B, F ); both oviducts converging into common oviduct in somite XIII a2. Common oviduct thin-walled, short, directly descending to female gonopore ( Fig. 3F ).

Variation. Measurements (n = 1, paratype only): BL 26.8 mm, BW 2.5 mm, CL 1.0 mm, CW 1.2 mm. Crop reaching to somite XIX/XX. Intestine reaching to somite XXIII a2/b5. Testisacs; on right side, ~14 sacs in somite XIX a2 to somite XXIV b5; on left side, ~12 sacs in somite XIX a2 to somite XXIV b6. Paired sperm duct bulbs ( Fig. 4 ); right bulb in somite XIX a1; left bulb in somite XVIII b6. Paired epididymides; right epididymis in somite XVI b5 to somite XVIII/XIX, occupying 10 annuli; left epididymis in somite XVI a1/a2 to somite XVIII b6, occupying 11 annuli. Paired ejaculatory ducts; right duct in somite XI b5 to somite XVI b5; left duct in somite XI b5 to somite XVI a1. Paired ovisacs in somite XIII a1–b6. Right oviduct crossing ventrally beneath nerve cord.

Coloration. In life, dorsal surface bluish gray ( Fig. 1C ); ventral surface yellowish or grayish white. Color faded in preservative.

Distribution and natural history. This species was collected only from the high-elevation area of Mt. Hehuan in the Central Mountain Range of Taiwan. Because both specimens possessing fully developed genital organs were collected in February and March, we estimate that the reproductive season of this species begins after March at the type locality.

Molecular phylogenetic position. The ML (not shown) and BI ( Fig. 5 ) trees had almost identical topologies, and were concordant with those of previous analyses (e.g., Nakano 2025). Orobdella minima formed a well-supported clade with the Taiwanese O. ketagalan Nakano and Lai, 2012 and O. meisai Nakano and Lai, 2017 (BS = 99%, PP = 1.0). The monophyly of O. minima and O. ketagalan was fully supported (BS = 100%, PP = 1.0).

Etymology. The specific name is given based on its small body size, which is likely to be the smallest Orobdella species in Taiwan.

Remarks. Orobdella minima clearly belongs to the genus Orobdella because this new species possesses the generic diagnostic characteristics defined by Nakano (2016). The obtained trees also corroborate that this new species is explicitly classified as a member of this genus. The clitellum was not observed in any of the present specimens of O. minima , but nonetheless, they possessed fully developed male and female genital organs. Additionally, the body length of both individuals was up to ~ 3 cm. Therefore, we concluded that O. minima can be determined as a small-type species within Orobdella .

Orobdella minima differs from the nine sexannulate and two octannulate species by its mid-body somites, which are quadrannulate. Except for O. ketagalan , O. minima is clearly distinguishable from the 14 quadrannulate congeners by the possession of paired sperm duct bulbs (vs. without sperm duct bulbs in 14 congeners; see Nakano and Prozorova 2024). Orobdella minima is readily distinguished from O. ketagalan by the presence of epididymides, which are absent in the latter. It also differs in having a developed bulbous gastroporal duct and ovate, well-developed atrial cornua (vs. a simple tubular duct and undeveloped coniform cornua in O. ketagalan ) (cf. Nakano and Lai 2012).

The present phylogenies recovered the monophyly of three Taiwanese quadrannulate species, O. minima , O. ketagalan , and O. meisai . Additionally, O. minima formed a clade with the middle-type O. ketagalan , and this clade was sister to the middle-type O. meisai . A previous study stated that the small-sized body length has evolved in parallel within the Japanese Orobdella (Nakano 2016, 2021). The obtained trees confirmed that the small body length has also evolved independently in the Taiwanese Orobdella . The present results also suggested that O. minima would retain the epididymides as a plesiomorphic feature among the Taiwanese Orobdella , because both O. ketagalan and O. meisai do not possess epididymides in their male genital organs ( Nakano and Lai 2012, 2017).

The absence of epididymides is not limited to O. ketagalan and O. meisai , but also observed in O. shimadae Nakano, 2011 and O. dolichopharynx Nakano, 2011 from the central Ryukyu Islands, which formed a monophyletic group with the Taiwanese species ( Fig. 5 ). These two Ryukyu species also possess rudimentary gastroporal ducts ( Nakano 2011). Given that the gastroporal duct serves as the spermatophore-receiving organ ( Nakano 2017b), this combination may suggest a functional relationship between a reduced or undeveloped gastroporal duct and the absence of epididymides. Supporting this idea, O. kawakatsuorum Richardson, 1975 and O. brachyepididymis Nakano, 2016 , from Hokkaido and Shikoku, Japan, respectively, possess simple, tubular gastroporal ducts and also retain only short epididymides (Nakano 2012, 2016). Thus, although exceptions may exist, the presence and degree of development of epididymides may be functionally linked to the morphology of the gastroporal duct, potentially reflecting variations in spermatophore formation or copulatory behavior across species. Furthermore, the phylogenetic position of the new species suggested that the possession of sperm duct bulbs may be a synapomorphic character between O. minima and O. ketagalan that formed a fully supported clade within the genus.

Figures

Fig. 1. Orobdella minima sp. nov., holotype, KUZ Z1471 (A, B), and paratype, ASIZW0001115 (C). A, Dorsal view of the preserved specimen; B, ventral view of the preserved specimen; C, dorsal view of live animal.

Fig. 2. Orobdella minima sp. nov., holotype, KUZ Z1471. A, Dorsal view of somites I–VIII; B, ventral view of somites I–VIII; C, dorsal view of somites XXIV–XXVII and caudal sucker; D, ventral view of somites XXIV–XXVI and caudal sucker; E, ventral view of somites XI–XIII; F, ventral view of gastropore. Abbreviations: an, anus; fg, female gonopore; gp, gastropore; mg, male gonopore; np, nephridiopore.

Fig. 3. Orobdella minima sp. nov., holotype, KUZ Z1471. A, Ventral view of gastroporal duct; B, dorsal view of reproductive system, including ventral nervous system; C, dorsal view of male atrium including positions of ganglia XI and XII; D, left lateral view of male atrium; E, ventral view of male atrium; F, dorsal view of female reproductive system, including position of ganglion XIII. Abbreviations: ac, atrial cornu; at, atrium; cod, common oviduct; cp, crop; ed, ejaculatory duct; ep, epididymis; gd, gastroporal duct; gp, gastropore; od, oviduct; ov, ovisac; ph, pharynx; sdb, sperm duct bulb; ts, testisacs.

Fig. 4. Orobdella minima sp. nov., paratype, ASIZW0001115, dorsal view of male sperm ducts in somites XVIII and XIX including ventral nervous system. Abbreviations: ep, epididymis; sdb, sperm duct bulb; ts, testisac.

Fig. 5. Bayesian inference tree for 8100 bp of nuclear 18S, 28S, and H3, and mitochondrial COI, tRNACys, tRNAMet, 12S, tRNAVal, 16S, tRNALeu and ND1 markers. Numbers on nodes indicate bootstrap (BS) values for maximum likelihood≥60% and Bayesian posterior probabilities (PP)≥0.90. Asterisks denote nodes with BS=100%, PP =1.0. Numbers and abbreviations in parentheses indicate, respectively, the mid-body somite annulation and the body length of mature individuals for each species.