A checklist of earthworms ( Annelida : Oligochaeta ) in southeastern Vietnam

The earthworms of southeastern Vietnam are reported based on literature and samples collected during the period of 2012–2019. A total of 41 earthworm species of 12 genera in six families are recorded in the southeastern part of Vietnam. Of these, Polypheretima elongata and Dichogaster affinis are recorded for the first time in the region. The dominant family is Megascolecidae with 35 species of seven genera while each of the other families is represented by only one species, except for Octochaetidae which has one genus and two species. The comprehensive checklist also provides data of each species including examined specimens, distributions and remarks. An identification key to species is compiled for southeastern Vietnam.


INTRODUCTION
Southeastern Vietnam has an area of 23,607.8km 2 , located between 10.316-12.283 0 N & 105.800-107.583 0 E. This region is the territory of six provinces and city: Ho Chi Minh, Ba Ria -Vung Tau, Dong Nai, Binh Duong, Binh Phuoc, and Tay Ninh. The terrain changes from mountainous areas and midlands to coastal deltas. The region is located in a humid subtropical climate zone featured by the rainy season, which starts in May and lasts till October (average rainfall counted for 90% of the whole year), and the dry season from November to April. The region has different soil types, but mainly dominated by ferralsols and acrisols (Sterling et al. 2008).
The earthworms of Vietnam in general and of the southeastern part in particular was first reported by Perrier (1872Perrier ( , 1875 with descriptions of four new species. There were no reports for the southeastern part of Vietnam until 1956 when Omodeo (1956) described six new species. After that, there were no reports on earthworms in the region for approximate 60 years. Recently,  and Nguyen et al. (2015) reported list of earthworms recorded in small parts of Binh Duong Province. Nguyen et al. (2015a,b) also described four new species from Dong Nai Province. All data were synthesized into a comprehensive checklist of earthworms in Vietnam by Nguyen et al. (2016). After 2016, the earthworms of southeastern Vietnam have been intensively studied, and 14 new species were described from this region (Nguyen & Lam 2017;Nguyen et al. 2018Nguyen et al. , 2019Nguyen et al. , 2020a. Together with discoveries of new species, taxonomic acts have also been undertaken.  and Nguyen (2020) corrected Metaphire magophila (Nguyen, 2011) as a senior synonym of M. easupana (Thai & Huynh, 1993). Samples of M. neoexilis (Thai & Samphon, 1988) found in Binh Duong province were misidentified as Amynthas modigliani (Rosa, 1896).
This work aims to provide comprehensive information on the earthworms of southeastern Vietnam. An identification key is also provided to facilitate further studies on earthworms in this region.
Earthworms were collected by digging and handsorting method following Górny & Grum (1993). After collecting, specimens were cleaned by tap water, killed in 2% formalin, temporally fixed in formalin 4% for 12 hours, then transferred to new formalin 4% for longterm preservation. All specimens were deposited in the Laboratory of Zoology, Department of Biology, Can Tho University.
The specimens were examined under a motic digital microscope (Model: DM143-FBGG-C) and dissected from the dorsal side for internal observation. Colour images were taken using a camera attached directly to the microscope, then improved and grouped into plates using Photoshop CS6.

RESULTS
Until date, a total of 41 earthworm species of 12 genera in six families (Almidae, Megascolecidae, Moniligastridae, Octochaetidae, Ocnerodrilidae, and Rhinodrilidae) have been recorded in southeastern Vietnam. All information of each species is presented in the checklist. Polypheretima elongata and Dichogaster affinis are reported for the first time in the region. Megascolecidae was the dominant family in terms of the number of species and genera (35 species of seven genera). It also corresponded to the earthworm distribution in the Oriental region (Hendrix & Bohlen 2002). Other families had only one species each except Octochaetidae which had two species in one genus. Particularly, the genus Metaphire had 20 species while Amynthas was poorly known with only seven species. Thai (2000) also indicated that Metaphire was the most diverse genus in the south of Vietnam.

Amynthas polychaetiferus
Distribution: Widely distributed in the study area, but more gathered in Dong Nai and Ba Ria-Vung Tau, little known in Binh Duong and Binh Phuoc, and rarely found in Ho Chi Minh City and Tay Ninh.
Remarks: The species have setae crowded ventrally in xix, varied in numbers or sometimes in usual position. The species was reported from the study area with the highest frequency and species abundance compared to other places.
Remarks: The species is similar to M. neoexilis (Thai & Samphon, 1988), but it is characterized by having large and stout spermathecal diverticula and ventrally connected testes sacs (Nguyen & Lam 2017 Remarks: The species is fairly similar to M. houlleti (Perrier, 1872), however, it is characterized by having two pairs of spermathecal pores in 7/8/9 and smaller size (Nguyen et al. 2020b).
Remarks: The present population has slight difference from the original description of Thai & Samphon (1988) in genital markings in xviii and male pores in xix.
Remarks: The populations collected in Ba Den Mountain (Tay Ninh province) and Phu Giao (Binh Duong province) lack genital markings while others have two pairs in 17/18 and 18/19 as in the original description.
Remarks: The species is somewhat similar to M. bahli (Gates, 1945), but is characterized by having large discshaped genital markings and unconcave male region.
Distribution: Widely distributed in southern Vietnam, but more commonly found in grey soils of deltas.
Remarks: The species was erroneously identified as M. californica (Kinberg, 1867), but corrected by Nguyen et al. (2020). M. planata differs from M. californica in having spermathecal pores in 5/6/7, genital marking present in the spermathecal region associated with saccular accessory glands internally, simple intestinal caeca, separated testes sacs, and smaller size. It is noted that the preservation code CTU-EW.005 (for M. californica) would be changed to CTU-EW.016 (for M. planata).
Distribution: Ba Ria-Vung Tau (Minh Dam Mts). Remarks: The species is closely similar to M. planata (Gates, 1926), but it is distinguished by lacking of genital markings and accessory glands, having waved diverticula, connecting testes sacs, and smaller size (Nguyen et al. 2020b).

J TT
Remarks: The species has very unique morphology characters among known Metaphire species, with regard to its saddle-shaped clitellum and setal arrangement in two rings (Nguyen et al. 2020b Remarks: The species is fairly similar to M. posthuma (Vaillant, 1868), but it is recognized by having spermathecal pores in the dorsum, first dorsal pore in 9/10, and ventrally connected testes sacs. There are two morphological forms. The first form found in Binh Duong Province has spermathecal pores located closely to the mid-dorsal line and four pairs of genital markings in the male region. The other form has spermathecal pores located laterodorsally and more than four pairs of genital markings (Nguyen et al. 2020b).
Remarks: The species is fairly similar to M. phaluongana (Do & Huynh, 1992), but it is distinguished by having a pair of genital markings in xviii, separated testes sacs, and presence of penial seta (Nguyen & Lam 2017).  (Perrier, 1872), it is but specialized by the presence of micronephridia attached onto the spermathecal ducts (Nguyen et al. 2018 (Beddard, 1900), but it is distinguished by lacking of genital markings, presence of seminal chambers, and the strongly coelomic copulatory pouches (Nguyen et al. 2015b).
Remarks: The species is closely similar to Po. grandisetosa (Thai, 1996), but it is specialized by number of spermathecal pores (one pair in 5/6, 2 pairs in 6/7 or a pair per segment (found in Binh Phuoc Province)), holandry, presence of copulatory pouches, and absence of stout setae in the dorsum (Nguyen et al. 2015b
Remarks: The species was recorded for the first time in southeastern Vietnam. It was usually found along with Dichogaster bolaui in soils of roadside or fallow lands.
Remarks: It was very common in southeastern Vietnam, especially in moist soils.  Table 1.

Family
Distribution: Very commonly distributed in Vietnam. Remarks: The species was known to be native to the South America region, but wide spread over the world (Brown et al. 2006). In Vietnam, this species has been known widely in all habitats except natural forests in high mountains.