Disentangling an Asian puzzle: Two new bathynellid (Crustacea, Syncarida, Parabathynellidae) genera from Vietnam

In this paper the 17 asiatic species of the family Parabathynellidae are reviewed from a taxonomic point of view using bibliographic data. Two new genera and two new species are described from caves in Vietnam. Paraeobathynella n. g. presents a unique combination of characters (antennule: seven segments; antenna: six segments; mandible: pars incisiva with five teeth and pars molaris with 10 teeth; maxillule: distal endite with seven claws; exopod of the thoracopods with three or more segments and epipods present in thoracopods 3–7; pleopods absent; endopod of the Th 8 male with two setae; seven spines on the sympod and two spines on the endopod of the uropod and the small minimum size of the adult body) and exclusive characters like the Th 8 female having two small spines and one long seta and the general size and aspect of the Th 8 male and its lobes. Sketinella n. g. presents a unique combination of characters (antennule: eight segments; antenna; six segments; mandible: pars incisiva with six teeth and pars molaris with eight teeth: maxillule: distal endite with seven claws; exopod of the thoracopods with three or more segments and epipods present in Th 3–7; endopod of the thoracopod 8 male with two setae; 12 spines on the sympod and two spines on the endopod and seven setae on the exopod of the uropod) and characters exclusive to the asiatic species are: a pair of pleopods reduced to a single seta; a very distinctive basipod and outer lobe of the Th 8 male and an exopod of the Th 8 male with a small tooth or protuberance. This is the first time that the Parabathynellidae has been found in Vietnam, this find extends the range of distribution of this family in Asia.


Introduction
The asiatic family Parabathynellidae includes 17 known species to date, belonging to six genera. Two of these genera, Allobathynella Morimoto and Miura, 1957 (eight species and subspecies) and Eobathynella Birstein and Ljovuschkin, 1964 (five species and subspecies), contain most of these species. Current taxonomy is not satisfactory as the generic diagnoses are not complete and there is great morphological variability amongst the species which have been described. When the first species were found (Parabathynella malaya Sars, 1929; 1. The genus Eobathynella is a valid taxon and distinct from the genus Parabathynella, but both the initial diagnosis and that provided by  were insufficient for the identification of the species. 2. The description of the species mesasiatica contains useful information about important morphological characters, but there is no information about the thoracopod eight (Th 8) male whose characteristics are very important in the taxonomy of Bathynellacea. Owing to this, it is impossible to make the right decision about the allocations of the species to the genus Eobathynella as did Birstein and Ljovuschkin (1968) and . 3. The diagnosis of the taxon Eobathynella cannot be made definitive until the type material can be studied in detail.
Bearing in mind these questions, Serban (1994) created a new genus, Issykkulibathynella, for the species E. tianschanica. As well as this, thanks to his study of specimens of Allobathynella japonica (provided by Mrs A. I. Jankowskaja), he was able to describe the Th 8 male very well, providing very important data for the diagnosis of this genus. Noodt (1965) mainly based the taxonomy of the genus Eobathynella on the characters of the uropod and  mainly based the taxonomy on characters of the antenna (A.II) (he includes in the genus Eobathynella all the species which lack a seta on the antepenultimate segment); neither of them was able to resolve the taxonomic problems for the group of asiatic species, although they each contributed important information. The lack of complete diagnoses of the different genera resulted in many species with very different characters being included in the same genus.
In short, species have been assigned to genera in an arbitrary way. In the Appendix we show the accepted taxonomy to date with an asterisk next to those species which, according to Serban (1994), do not belong to the genus to which they are currently assigned, but probably to a new genus.
We have reviewed all the literature on asiatic species and consider that the two new species described in this paper belong to two new genera, Paraeobathynella vietnamensis n. g. n. sp. and Sketynella trontelji n. g. n. sp. The species Allobathynella gigantea and A. gigantea pluto (Morimoto, 1963) could belong to the latter, but using only the data found in the literature it is impossible to assign them to the correct genus.

Material and methods
The specimens studied were found in four samples from caves sampled by Boris Sket and Peter Trontelj according to a contract with Fauna and Flora International-Vietnam; the subject of investigation was the cave fauna in the World Heritage area of Vinh Ha Long (5 Ha Long Bay) in northern Vietnam.
The material was collected with a 0.2 mm mesh hand net. All samples containing bathynellaceans are from small puddles of percolated water. A complete dissection of all anatomical parts of all type series was made, and kept as permanent preparations (special metal slides, glycerine gelatine-Kaiser methods-stained with methylene blue as the mounting medium). Anatomical examinations were performed using an oil immersion lens (1006; Zeiss microscope). The descriptions were based on the type series. The material is deposited in the Museo Nacional de Ciencias Naturales, Madrid (MNCN).

Systematic account
Paraeobathynella n. g.

Genus diagnosis
Antennule (A.I) has seven segments and subterminal aesthetascs on the last segment. Antenna (A.II) has six segments, the fourth without setae. Mandible (Md) with protuding pars molaris. Maxillule (Mx.I) with proximal endite with four teeth and distal endite with seven teeth. Exopods of Th 1-7 with more than two segments; basipod of Th 1 with two setae. Thoracopod (Th) 8 male: large with very well-developed endopod which has two terminal setae; exopod very large, longer than wide and overhanging the basipod and is on the distal face of the basipod; inner lobe completely integrated into the basal region; rounded outer lobe is not fused with basipod; basipod trapezoidal with a protuberance on the internal lateral edge. Th 8 female: large and almost square, with a long terminal seta and small teeth. No pleopods. Ventral seta of the pleotelson in the basal part of the furca. Sympod of uropod with subequal spines; endopod with two spines and two apical setae.

Description
Body. Total length of holotype male 1.56 mm (species range type locality: males n515: 0.90-1.56 mm; females n513: 0.96-1.33 mm; population 1: males 1.16-1.71 mm and females 1.28-1.55 mm; population 2: female 1.38 mm). Body elongated, segments progressively widening towards posterior end of body (Figures 1, 2-female paratype and male holotype, respectively). Length of head greater than width. All drawings are of the holotype (male) and the Th 8 female is of one paratype (female selected).
Antennule ( Figure 3A). Antennule has seven segments; no sexual dimorphism; length of first four segments greater than that of last three; setation as in Figure 3A (segment 5 with two aesthetascs; segments 6 and 7 with three aesthetascs, on the last segment aesthetascs are subterminal). Antenna ( Figure 3B). Antenna six-segmented, last four segments similar in size, longer than the two first segments; last segment with three smooth and one plumose terminal setae; segments 1 and 4 without setae; setation in the other segments as in Figure 3B.
Labrum ( Figure 3C). Labrum concave, with eight main teeth, and three lateral teeth at either end. Figure 3D). Pars incisiva with five well-developed teeth and small proximal tooth as in Figure 3D; pars molaris with 10 teeth, eight being strong and distal with small spines and two small joined proximal teeth with a large number of fine setae; mandibular palp does not exceed pars incisiva in length. Figure 3E). Proximal endite with four claws (spines of different sizes); distal endite with seven claws, two smooth and apical, the other five with strong spines and three subterminal smooth setae on outer distal margin. Figure 3F). Four-segmented, with two setae on basal segment; segment 2 with two long setae and one shorter one at distal end; segment 3 elongated with 12 setae and last segment with a strong terminal seta and two lateral setae.   Thoracopods 1-7 (Figures 4A-C, 5, 6). Well developed, length gradually increasing from 1 to 5, last three similar in size; well-developed epipodite on 3-7, measuring more than half length of basipod; basipod with one lateral, barbed terminal seta in Th 2-7; exopods of the Th 1 and 2 shorter than endopods; exopods of the Th 3-7 longer than endopods. Thoracopod 1 ( Figure 4A): has two lateral smooth terminal setae on basipod; exopod three-segmented, with two barbed setae on each segment and one group of ctenidia at the base of setae; endopod four-segmented: first segment has three barbed dorsal setae and one plumose seta, second has three barbed dorsal setae and one plumose seta, third has three barbed dorsal setae and one small, smooth, terminal seta, terminal segment has two slightly  barbed claws of similar length and one smooth seta. Thoracopod 2 ( Figure 4B): exopod four-segmented, with two barbed setae on each segment and with one group of ctenidia at the base of setae; endopod four-segmented, first segment short and with one barbed and one plumose seta; second with two groups of lateral ctenidia and three barbed dorsal setae and one plumose seta; third with two barbed dorsal setae and one tiny terminal seta; and fourth with one smooth seta and two strong slightly barbed claws of different length. Thoracopod 3 ( Figure 4C): exopod five-segmented, with two barbed setae on each segment and one group of ctenidia at the base of setae; endopod four-segmented, first segment short with one barbed and one plumose seta; second with two groups of lateral ctenidia and three barbed dorsal setae and one plumose seta; third with two barbed dorsal setae and one tiny terminal seta; and fourth with one smooth seta and two strong slightly barbed claws of different length. Thoracopods 4-7 ( Figure 5A, B, 6A, B): exopod five-segmented, with two barbed setae on each segment and with one group of ctenidia at the base of setae; endopod four-segmented, first segment short with one barbed and one plumose seta; second with four groups of lateral ctenidia and two barbed dorsal setae and one plumose seta; third with two barbed dorsal setae and one tiny terminal seta; and fourth with one smooth seta and two strong slightly barbed claws of different length. Thoracopod 8 male ( Figure 4D, E, F): large, massive, almost square; basal region massive; inner lobe completely integrated into the basal region, barely exceeds distal end of dentate lobe; basipod with one distal protuberance on the internal lateral edge which exceeds the end of internal lobe; endopod almost square, with two long setae; large exopod, longer than wide and overhanging the basipod and the outer lobe; rounded outer lobe is not fused with basipod and exceeds the end of the external side of the basipod; dentate lobe with small teeth and rounded distal end. Thoracopod 8, female paratype ( Figure 4G): large with smooth cuticle, almost square, has one long terminal seta and two small teeth. Dorsal margin of pleotelson ( Figure 7A, B): pronounced anal operculum; one barbed ventral seta.

Maxilla (
Uropod ( Figure 7C). Sympod slightly longer than the endopod and four times longer than wide; with seven barbed spines of different size; endopod slightly shorter than exopod with two strong, barbed spines and one claw ( Figure 7D) on the distal end and two plumose setae on the external part which exceed the distal end of the endopod, and two barbed terminal setae, four groups of ctenidia on dorsal face; exopod has two barbed terminal setae, the outer longer than the inner, and four barbed setae on external face. Figure 7A, B). Furca has five barbed spines (the two terminal ones being a little longer and thicker); two dorsal plumose setae similar in size.

Etymology
The generic name is to be nearest to Eobathynella genus. The species name is dedicated to Vietnam.

Remarks
The combination of characters of Paraeobathynella n. g. (number 7 in Table I) is similar to some of the possible combinations present in the genus Allobathynella (number 1 in Table I) due to its great variability. According to  and Serban (1994) it is possible that this last genus includes species that belong to two or three different genera. The different combinations of the characters of the species of the possible different genera (numbers 6-8 in Table II) do not coincide with Paraeobathynella n. g. (number 19) (see Table II). Paraeobathynella vietnamensis n. g. n. sp. is different to the five first species in Table II which are possibly true ''allobathynels''. The most similar species is A. coreana (number 5) but this species has one pair of pleopods with one segment and two setae, whereas the new species do not have pleopods. In both species the Th 8 female and the uropod are very different and there are also other differences in almost all structures of the body. In Allobathynella the exopod of the Th 8 male is on the external face of the basipod, whereas in Paraeobathynella n. g. this structure is on the distal face of the basipod, as in the genus Issykkulibathynella (number 5 in Table I) and the other new genus, Sketinella n. g. (number 8 in Table I).
Only E. mesasiatica (number 9 in Table II) is definitely a species of the genus Eobathynella and the new species (number 19 in Table II) is very different from E. mesasiatica to belong to this genus. In E. mesasiatica the exopods of Th 1-7 have more than two segments and the A.I has seven segments, like Paraeobathynella n. g. (number 18 in Table II), whereas in the other four species assigned to Eobathynella (numbers 10 to 13 in Table II) the A.I has six segments and the exopods of all the thoracopods are always bisegmented. Paraeobathynella n. g. (number 19 in Table II) is a medium-sized species, but E. mesasiatica is a large species and both species have a lot of different characters (see Table II): number of teeth on pars molaris; number of teeth on distal endite of the Mx.I; epipodite of Th 3 present in Paraeobathynella n. g., absent in Eobathynella; number of setae in Th 1-7; ratio of exopod to endopod of the thoracopods; number of setae on the endopod of Th 8 male and size and shape of all the lobes; size and number of spines on the sympod and endopod of the uropod and number of setae on the exopod of the uropod and presence or absence of anal operculum.
In the new species the pars molaris of the Md is very well developed (with 11 teeth) as in Allobathynella gigantea pluto (number 7 in Table II), which is a very large species and very different from P. vietnamensis n. g. n. sp. in the rest of its characters.
Paraeobathynella vietnamensis n. g. n.sp has a unique combination of characters and the Th 8 female is completly different from the other asiatic species.

Description
Body. Total length of holotype male 1.70 mm (species range: n52: 1.80-1.94 mm). Body elongated, segments progressively widening towards posterior end of body. Length of head greater than width (Figure 8). All drawings are of the holotype (male).
Antennule ( Figure 9A). Antennule has eight segments; no sexual dimorphism; length of first four segments slightly longer than that of last four; setation as in Figure 1A (segments 6-8 have three aesthetascs, on the last segment aesthetascs are subterminal).   Antenna ( Figure 9B). Six-segmented, last three segments are longer than the three first segments; last segment has three smooth and one plumose terminal setae; segments 1 and 4 have no setae; setation in other segments as in Figure 6B.
Labrum ( Figure 9C). Concave, with eight main teeth, and two lateral teeth at either end.
Mandible ( Figure 9D). Pars incisiva has six well-developed teeth; pars molaris has eight teeth, five of which are strong and distal with small spines, the remaining three being small joined and proximal with a large number of fine setae; mandibular palp does not exceed pars incisiva in length. Maxillule ( Figure 9E). Proximal endite has four claws (with spines of different sizes); distal endite has seven claws, two smooth and apical, the other five with strong spines; and with three subterminal smooth setae on outer distal margin.
Maxilla ( Figure 9F). Four-segmented, with three setae on basal segment, two of which are distal; segment 2 with two long setae and one shorter one at distal end; segment 3 elongated with 11 setae, the last segment having a strong terminal seta and four lateral setae.
Thoracopods 1-7 ( Figures 10A-C, 11, 12). Well developed, length gradually increasing from 1 to 5, last three similar in size; well-developed epipodite on 3-7, measuring half length of basipod; basipod has one barbed lateral terminal seta on Th 1-7; exopods of Th 1 and 2 shorter than the endopods; exopod of Th 3 and 4 longer than the endopod, and exopods of  Th 5-7 similar in length to the endopods. Thoracopod 1 ( Figure 10A): two lateral smooth terminal setae on basipod; exopod three-segmented, with three barbed setae on the first segment and two barbed setae on the other two segments and one group of ctenidia at the base of the setae; endopod four-segmented: first segment with three barbed dorsal setae and one plumose seta, second with four barbed dorsal setae and one plumose seta, third with three barbed dorsal setae and one small, smooth, terminal seta, terminal segment with two claws of similar length and one smooth seta. Thoracopod 2 ( Figure 10B): exopod foursegmented, with three barbed setae on first segment and two barbed setae on the rest of the segments, and with one group of ctenidia at the base of the setae; endopod four-segmented, first segment short and with two barbed setae and one plumose seta; second with four groups of lateral ctenidia and three barbed dorsal setae and one plumose seta; third with three barbed dorsal setae and one tiny terminal seta; and fourth with one smooth seta and two strong claws of similar length. Thoracopod 3 ( Figure 10C): exopod five-segmented, with three barbed setae on the first segment and two barbed setae on each one of the other segments and with one group of ctenidia at the base of the setae; endopod four-segmented, first segment short with one barbed and one plumose seta; second with three groups of lateral ctenidia and three barbed dorsal setae and one plumose seta; third with two barbed dorsal setae and one tiny terminal seta; and fourth with one smooth seta and two strong claws of different length. Thoracopods 4-7 ( Figures 11A, B, 12A, B): exopod fivesegmented, with two barbed setae on each segment and with one group of ctenidia at the base of setae; endopod four-segmented, first segment short with one barbed and one plumose seta; second with groups of lateral ctenidia and three (four in Th 5) barbed dorsal setae and one plumose seta; third with two (three in Th 6) barbed dorsal setae and one tiny terminal seta; and fourth with one smooth seta and two strong claws of different length. Thoracopod 8 male ( Figure 10D, E): large, massive and square; basal region massive; inner lobe almost trapezoidal; basipod with protuded distal end on the internal lateral edge; endopod, well developed with two long plumose setae; very large exopod, terminal end sharply pointed and with one small ''tooth''; outer lobe is not fused with basipod and is tongue-like in shape and does not exceed the distal end of basipod; dentate lobe with strong teeth.
Pleopod ( Figure 13A). Pleopod present, as a single long seta, on the first segment of the pleon.
Uropod ( Figure 13C). Sympod longer than endopod and four times longer than wide; has 12 barbed spines, the three terminal ones longer than the rest; endopod shorter than exopod with two strong, barbed spines and one claw ( Figure 13C) on the distal end and two plumose setae on the external face, of these the most distal exceeds the distal end of the endopod, the basal one being small, and two barbed terminal setae have five groups of ctenidia on dorsal face; exopod has two terminal barbed setae, the outer longer than the inner, and five barbed setae on external face.
Furca ( Figure 13B). Furca has seven barbed spines (the two terminal ones being longer and thicker); and two dorsal barbed setae which are similar in size.

Etymology
The new genus is named after Prof. Boris Sket and consists of the surname Sket, plus ''i'', followed by the last five letters of the name of the first genus described, Bathynella, and the species is named after Peter Trontelj, with thanks for their providing me with the material.

Remarks
The combination of characters in Sketinella n. g. is unique (see Table I). Its exclusive characters are: the pleopods (it only has the first pair) are reduced to a single seta (in the asiatic species which have pleopods these consist of a segment and two long setae); the basipod, the outer lobe and the exopod of the Th 8 male are different to that found in the other known asiatic species. Sketinella n. g. presents some characters which are similar to Allobathynella (see Table I). According to  and Serban (1994), the generic diagnosis of Allobathynella is not sufficiently detailed to assign any species to it and it is possible that the genus may include species that belong to two or three different genera. The only five species (1-5 in Table II) that belong to Allobathynella (A.I with seven segments; four or five of teeth on pars incisiva, and four or nine teeth on pars molaris of the Md; exopod of the Th 8 male on latero-external face of the basipod; pleopods one-segmented and with two long setae; etc.) are very different from Sketinella trontelji n. sp. (number 20 in Table II).
Sketinella trontelji n. sp. has a unique combination of characters. This species is nearest to Allobathynella gigantea pluto in the number of segments that make up the A.I and A.II, the number of teeth on the Md and the type of uropod. But the new species (medium-sized and cave dwelling) is different from Allobathynella gigantea pluto (large and an inhabitant of wells) in that it has: seven teeth on the distal endite of the Mx.I; protruded anal operculum; epipod present on the Th 3; a simplified first pair of pleopods; different number of segments in the exopod of Th 1-3 and the exopod and endopod of different proportions; more spines on the furca in spite of it being smaller in size, and a male Th 8 which is square and in which the basipod, the outer lobe and the exopod (with a lateral ''tooth'') are exclusive in shape. The internal side of the basipod of the Th 8 male (which has protruded distal end) is similar in the two new genera, but the external side is different in both genera (bigger in Sketinella); the exopod is similar in both new genera in shape, but is smaller and less curved in Sketinella than in Paraeobathynella, and the lateral tooth does not exist in Paraeobathynella. Table I shows the variability that exists in different characters in the diagnoses of the six asiatic genera accepted to date and the two new genera proposed in this paper. It is obvious that Allobathynella and Eobathynella ought to be studied and diagnosed again.

Discussion and conclusion
According to Serban (1994), the generic diagnosis of Allobathynella is insufficient: the Th 8 male is not described and some characters show great variability (e.g. A.I with six to eight segments; A.II with five or six segments; pleopod present or absent; exopod of the Th 1-7 with two to seven segments; etc.). In all probability only five species (1-5 in Table II) belong to Allobathynella, species 6-8 in Table II (with asterisk) almost certainly belong to one or two different genera.
Within Eobathynella only the type species, E. mesasiatica (number 9 in Table II), may in reality belong to this genus, the other four species (10-13 in Table II, with asterisk) surely belong to one or two different genera because they are very different from E. mesasiatica (see Table II).
Nipponbathynella, Sabahbathynela, and Issykkulibathynella are well described, and new species can easily be assigned to these genera.
However, the diagnosis of Batubathynella is not complete and the Th 8 male is not described in enough detail.
In conclusion, the creation of the two new genera, Paraeobathynella and Sketinella, are justified by the absence of: N usable diagnoses of Eobathynella and Allobathynella; N a single set of taxonomic criteria for the Asian parabathynelids on which every specialist agrees (the taxonomic status of most of the known taxa is uncertain); N a thorough study of the Th 8 male of almost all the asiatic species in the Parabathynellidae family.
The distribution to date of the 20 asiatic species in the family Parabathynellidae now covers Japan, South Korea, Central Asia, Malaysia, Borneo and, including the two new species described here, Vietnam, where Bathynellacea has been found for the first time.