Aporcelaimidae Heyns, 1965

Gusakov, Vladimir A. & Gagarin, Vladimir G., 2017, An annotated checklist of the main representatives of meiobenthos from inland water bodies of Central and Southern Vietnam. I. Roundworms (Nematoda), Zootaxa 4300 (1), pp. 1-43 : 25-26

Family Aporcelaimidae Heyns, 1965

71. Aporcelaimidae gen. spp.— {25, 55, 57, 59, 64, 65, 69} (0, 4, 23, 27; 10)

Population structure and abundance. 1–5 juvenile specimens and females in each sample (up to 2 x10 3 /m2). Remarks. The taxon unites approximately three to four representatives of the family that owing to various reasons were not identified to the rank of genus or further.

72. * Aporcelinus cf. granuliferus (Cobb, 1893) Andrássy, 2009 — {55, 63, 68, 70} (0, 0, 8, 27; 6)

Population structure and abundance. At site 55—21 juvenile specimens, nine females and one male (11 x10 3 /m2), at the other sites—1 to 4 juvenile specimens and females (up to 1 x10 3 /m2).

Ecology and distribution. Likely an amphibiont. Dwells predominantly in wet mosses and soil but also occurs in water bodies. Considered a widespread species. Reported from all continents except for Australia and Antarctica ( Andrássy 2009a). However, there are doubts concerning conspecificity of populations from different geographic regions (see Remarks). Not previously reported from Vietnam.

Remarks. In 2009, Andrássy proposed for Aporcelaimidae the new genus Aporcelinus , in which he included new species and species of other genera that had been previously attributed both to this family and to Qudsianematidae ( Andrássy 2009a) . Aporcelinus granuliferus belongs to the latter group of species and was previously assigned to the qudsianematid genus Allodorylaimus Andrássy, 1986 ( Andrássy 2009a, b). Further studies on some representatives of Aporcelinus revealed that some details of their morphology are in fact closer to the characteristics of Qudsianematidae , but molecular analysis is needed to finally solve the problem of attribution ( Vinciguerra et al. 2014; Nguyen et al. 2016a). The species Aporcelinus granuliferus itself is problematic, with a complicated taxonomic history. Some populations earlier believed to represent this species most likely belong to different species ( Andrássy 2009a; Peña-Santiago & Abolafia 2016). The Vietnamese females and male of Aporcelinus that we found are certainly close to A. granuliferus as well as to a group of other species separated from A. granuliferus by Peña-Santiago & Abolafia (2016) in their proposed new concept of the species. Considering the complete set of morphological features, our material can be definitely attributed to neither of these taxa, and an in-depth comparative analysis is beyond the scope of the present paper. Peña-Santiago & Abolafia (2016) note that many descriptions of the representatives of the " granuliferus group" are incomplete, some contain incorrect data, and the majority of them are based only on a few females. This does not allow for an assessment of the magnitude of morphological variability of populations from different geographic zones and habitats. The above reasons indicate the necessity for the analysis of additional material of this group of species both from Vietnam and from other regions. In addition, we should note that to date three species of Aporcelinus have been found in Vietnam: A. paramamillatus Nguyen, Abolafia, Bonkowski & Peña-Santiago, 2016 , A. paraseychellensis Nguyen, Abolafia, Bonkowski & Peña-Santiago, 2016 and A. falcicaudatus Nguyen, Abolafia, Bonkowski & Peña- Santiago, 2016 ( Nguyen et al. 2016a, b). All of these species were collected from forest soils in the north of the country and are clearly distinct from the specimens we found.

73. * Aporcella maitai ( Yeates, 1967) Gagarin & Gusakov, 2013 — {3, 23, 50, 69} (11, 4, 0, 9; 6)

Population structure and abundance. This species is very abundant at site 50—more than 90 specimens of both juveniles and females (69 x10 3 /m2). At the other sites—1 to 16 juveniles and females (up to 6 x10 3 /m2).

Ecology and distribution. Most likely an amphibiont. The species united under the synonym A. maitai (see Remarks and Gagarin & Gusakov 2013d), have been reported previously only from soil in New Zealand and India, plus in one river in Republic of South Africa ( Yeates 1967; Baqri & Jairajpuri 1968; Botha & Heyns 1991). In Vietnam, this nematode was found in water bodies of various types, but exclusively in shallow water biotopes.

Remarks. The genus Aporcella (Aporcelaimidae) was erected by Andrássy (2002b). The cuticular layers in representatives of this genus are not visually discernible, and the vagina lacks refractive oval formations (pars refringens vaginae). These features separate the representatives of Aporcella from those of the similar genus Aporcelaimellus Heyns, 1965 in which the layers of cuticle are differentially refractive and the inner vulval lips are, as a rule, sclerotized ( Andrássy 2002b, 2009b). When we examined nematodes identified as Aporcelaimellus maitai Yeates, 1967 from our Vietnamese samples, we found that they possess the characteristic features of Aporcella noted above. In addition, analysis of species of Aporcelaimellus revealed that two additional known species, A. heynsi Baqri & Jairajpuri, 1968 and A. glandus Botha & Heyns1990 , are very similar morphologically to A. maitai ( Yeates 1967; Baqri & Jairajpuri 1968; Botha & Heyns 1991) and also possess the features of the genus Aporcella . This is why these three species were united as Aporcella maitai ( Gagarin & Gusakov 2013d) . It is worth noting that other researchers also emphasized the similarities among Aporcelaimellus maitai , A. heynsi and A. glandus and indicated their possible conspecificity ( Andrássy 2002а; Álvarez-Ortega & Peña-Santiago 2013). In fact, the authors of the latter paper, published simultaneously with our article and containing the most recent taxonomic overview to date of the genus Aporcelaimellus , attributed these three species to the genus Discolaimium Thorne, 1939 in Qudsianematidae (Álvarez-Ortega & Peña-Santiago 2013) . In an earlier paper, Álvarez-Ortega et al. (2013a) presented a new concept and corrected diagnosis of the genus Aporcella and demonstrated (using the results of a molecular analysis along with morphological characters) that this genus is close to discolaimids; they further suggested that this evidence needs to be considered in future revisions of taxonomy of dorylaimids. Thus, the taxonomic status of the nematodes from Vietnam and of species-synonyms that we consider in the present paper as Aporcella maitai , cannot be accepted as finally clear in view of the recent studies referred to above. However, until the development of new criteria for classification of dorylaimid nematodes (and assessment of the importance of certain diagnostic features) we place them in the family Aporcelaimidae (and consequently in the species list for Aporcella ) based upon the morphology of the odontostyle. According to the original descriptions of A. maitai , A. heynsi and A. glandus and our data from Vietnam ( Yeates 1967; Baqri & Jairajpuri 1968; Botha & Heyns 1991; Gagarin & Gusakov 2013d), the odontostyle apertureis clearly longer than half of the total odontostyle length, which is still considered a diagnostic feature of the family Aporcelaimidae , and was given in the revised diagnosis of Aporcella ( Álvarez-Ortega et al. 2013a) as well. In the species of Qudsianematidae (including Discolaimium ) the aperture is less than half the odontostyle length ( Vinciguerra 2006; Andrássy 2009b).

74. * Aporcelaimellus sp.— {22} (5, 0, 0, 0; 1)

Population structure and abundance. Five juvenile specimens and one female (2 x10 3 /m2).

Ecology and distribution. The majority of species of Aporcelaimellus dwell in soil but some of them also occur in water ( Vinciguerra 2006; Andrássy 2009b). The studied specimens from Vietnam were collected in the central part of a small reservoir.

Remarks. To date, three representatives of Aporcelaimellus have been found in Vietnamese water bodies: A. krygeri (Ditlevsen, 1928) , A. obtusicaudatus (Bastian, 1865) and A. obscurus (Thorne & Swanger, 1936) ( Nguyen 2007) . According to modern concepts of the taxonomy of this genus, the latter species is considered a synonym of A. obtusicaudatus (Álvarez-Ortega & Peña-Santiago 2013) . The single adult female found in our studies belongs to another, unidentified species.