after Camacho, Watiroyram and Brancelj, 2011.

Antennule seven-segmented, without aesthetascs on the fifth article and with subterminal aesthetascs on the last article, sexually dimorphic, with a male antennal organ on the second article of the antennule. Antenna seven-segmented, first, second and fifth articles without setae. Mandible with expanded distal claw on pars molaris. Maxilla with four articles. Exopod of thoracopods II to VII each two-segmented, that of ThI one-segmented; basipod of thoracopods I to VII with one smooth seta; epipod of ThI absent. Male thoracopod VIII: almost square, with small endopod integrated into basipod and one or two smooth long setae; inner lobe incorporated into basal region, slightly shorter than dentate lobe; basipod very large, almost square and with a row of small denticles on the distal part, proximal side with two lobes, one of which is curved inwards and almost completely covers the exopod, without setae. Female thoracopod VIII: with two long setae. Pleopods absent. Ventro-lateral seta of pleotelson located adjacent to the insertion of the furca. Inhomonomous uropod sympod with a row of spines occupying the distal half; endopod with a spinous projection on the distal outer corner and with one thick spine covered with setules.

Siambathynella laorsriae Camacho, Watiroyram & Brancelj, 2011.

Tham Chiang Dao Cave (19°23'38.44"N, 98°55'41.00"E – Z 455 m), Thailand. Collected by Philippe Leclerc (17/07/1985).

Holotype male (MNCN 20.04/20552), allotype female (MNCN 20.04/20553) and type series comprised of seven males (MNCN 20.04/20554 to MNCN 20.04/20560) and seven females (MNCN 20.04/20561 to MNCN 20.04/20567), each on individual slides.

Body (Fig. 1). Total length of holotype 1.25 mm; allotype 1.5 mm; male length, 1.1–1.6 mm; female length, 1.0–1.8 mm. Body elongated, segments slightly widen towards posterior margin; head about as long as broad. Pleotelson with one plumose ventro-lateral seta on each side. All drawings are of the (male) holotype except for the female ThVIII and second article of the antennule, which were drawn from the female allotype.

Antennule (Fig. 2A). Seven-segmented; first three articles slightly longer than the other four; first two articles similar in length and the third is the longest; sexual dimorphism manifested by the presence of a well-developed antennal organ on the inner distal margin of the second article in males (Fig. 2A), organ represented as a protrusion with a smooth, moderately long seta in its middle lateral area. On the female antennule, only a short seta is present in the homologous position as the male antennal organ (Fig. 2C); inner flagellum almost square; fourth article slightly longer than the fifth, with two aesthetascs, which is the smallest of all the articles at half the length of the seventh, which is slightly longer than the penultimate articles; sixth and seventh articles each with three aesthetascs, which are placed sub-terminally on the last article; setation pattern as shown in Fig. 2A.

Antenna (Fig. 2B). Seven-segmented; approximately 70% the length of AI; first three articles small, all similar in size to the fifth and about 50% the length of the fourth article, which is similar in size to the last two articles; distal article with four setae, one of which is plumose. Setal formula: 0/0/1+0/1+1/0+2/4(1).

Labrum (Fig. 2D). Almost flat with eight main teeth, with the two central teeth that are slightly different from the others, more rounded, plus two lateral teeth on each side. Ventral surface ornamented with rows of fine spinules.

Mandible (Fig. 2E). Pars incisiva with four teeth and a well-developed triangular tooth on the ventral edge; pars molaris with five claws, one strongly modified distal claw that is slightly separated from the rest, with small denticles, and four claws (the two proximal ones are joint) with fine long setules; mandibular palp one-segmented, with a distal seta that does not exceed the length of the pars incisiva.

Maxillule (Fig. 2F). Proximal endite with four long serrulate claws; distal endite with seven claws, the basal one very thin, the two distal ones smooth and the other five with denticles, and with three sub-terminal smooth setae on the outer distal margin.

Maxilla (Fig. 2G). Four-segmented, first two articles with an elongated endite, with two and four setae, respectively, one setae on each endite plumose; third article rectangular with one strong claw and eight smooth setae; fourth article reduced with five setae. Setal formula: 2, 3+1, 9, 5.

Thoracopods I–VII (Fig. 3A–G). Well developed, gradually increasing in length from ThI to ThV, last two thoracopods similar in length; ThI without epipod, ThII–VII with large epipod, each about the same length as the corresponding basipod; basipod of ThI–VII with one smooth seta on inner distal corner. Exopod of ThI one-segmented, two-segmented in ThII–VII; exopod longer than first two endopod articles in ThII–VI and equal in length in ThVII; two barbed setae on each exopodal article and with one group of strong ctenidia at the base of the inner setae. Endopod four-segmented; first article short, second and third long and similar in length, both with a cluster of strong spinules along the outer margin; fourth article reduced, with two smooth, strong claws of similar length and one seta; pair of smooth inner setae on first article present only on ThI; inner setae on article two always barbed; outer small seta on third article of ThI–VII smooth; outer distal setae on second article of all Ths plumose. Setal formula of endopod as follows:

Thoracopod I 2+0/2+1/1+1/3(1)

Thoracopod II 1+0/2+1/1+1/3(1)

Thoracopod III and IV 0+0/2+1/1+1/3(1)

Thoracopod V to VII 0+0/1+1/1+1/3(1)

Thoracopod VIII (Fig. 4A, B). Almost square; basal region of the penial complex with three lobes: inner lobe (I.Lb.), outer lobe (O.Lb.) and dentate lobe (D.Lb.); rectangular inner lobe completely integrated into basal region, a little shorter than dentate lobe; trapezoidal outer lobe as dentate lobe; endopod (Endp.) integrated on basipod, with one long smooth seta; reduced exopod, with two spinules; basipod very large, square, slightly recurved caudally, with a distal row of small denticles, with two lobes on the frontal side, one of which is recurved inwards (crest-like protuberance), without setae. The ThVIII of the type species, S. laorsriae, is shown for comparison (Fig. 4C).

Thoracopod VIII female (Fig. 4D). One-segmented with two long smooth setae and two small denticles at the base.

First pleopods . Absent.

Uropod (Fig. 4E). Sympod five times as long as wide and twice as long as endopod, with nine barbed spines about equal in length except the distal-most, which is twice as long as the others, occupying the distal half; exopod 30% longer than endopod, with four barbed setae, two terminally and unequal in length; distal outer corner of endopod forming a spinose process, with two row of setules and one strong spine that is 20% longer than the endopod, with setules, one plumose seta near the base and two small barbed terminal setae of different lengths.

Pleotelson (Fig. 4F). With one small, plumose ventro-lateral seta on each side close to the insertion of the furca. Anal operculum slightly concave.

Furca (Fig. 4F). Rectangular, with three barbed spines, outer two spines twice as long as the innermost one; two equally long dorsal plumose setae; lateral furcal organ “cork”-like.

The number of spines on the uropod sympod varies between nine and 12, with the larger specimens showing a higher number of spines. Variation is also observed in the setal formula of the endopod of the thoracopods: ThI, 2+0/2–3+1/1–2+1/3(1); ThII, 1+0/2–3+1/1–2+1/3(1); ThIII, 0–1+0/2–3+1/1–2+1/3(1); ThIV-V, 0–1+0/2+1/1+1/3(1); ThVI, 0–1+0/1–2+1/1+1/3(1) and ThVII, 0+0/1–2+1/1+1/3(1).

Siambathynella janineana sp. nov. is morphologically similar to the type species S. laorsriae in generic features (Camacho et al. 2011) such as the number of articles on AI and AII, presence of a male antennal organ on the second article of AI, number of exopod articles on the thoracopod, basipod of male ThVIII with teeth and without setae, the presence of a large endopodal spine on the uropod sympod. The two species are also similar in size, ranging between 1 and 1.8 mm, with females being larger than males in both species. However, both species can be easily distinguished by the antennal organ and the morphology of the lobes on ThVIII in both males and females. The other differences are numerous but more subtle, making them potentially more difficult to discern; however, considered altogether, they reveal two distinct species. In general, these differences relate to relative proportions among articles, specific patterns of setation and relative sizes of setae and spines on AI and AII, mouthparts and other appendages such as the thoracopods, uropod and furca.

The new species (S. janineana) differs from the type species (S. laorsriae) in the following characteristics: 1) the antennal organ of S. janineana is simple, consisting of an expanded area, or protrusion, with one smooth seta; by contrast, S. laorsriae has two “curious structures”, one resembling a miniature elephant trunk and the other small, half-ring shaped (see Fig. 2H); 2) articles three and seven of the AI of the new species are larger than the rest of the articles, and the fifth is the smallest, whereas in the type species, the first three and the last two AI articles are similarly sized, as are the fourth and fifth, which are the smallest; 3) differences in setation pattern in terms of setal number and size of some AI articles; also, the new species has two aesthetes on the fifth article, which are absent in the type species; 4) AII is 30% shorter than AI in the new species and 40% shorter in the type species; 5) the new species has fewer teeth on the labrum (12 vs 14 in the type species); 6) the distal claw of the pars molaris of the Md is slightly modified and separated from the rest of claws and expanded in the new species; the type species has the typical parabathynellida morphology usual morphology; 7) the most basal claw of the distal endite of MxI is very thin in the new species; 8) the exopod is longer than the endopod in ThII to ThV, but of equal length in ThVI and ThVII in the new species; in the type species, the exopod and endopod are similar in length in ThII, ThVI and ThVII, but in ThIII to ThV, the exopod is longer than the endopod, and in ThI of both species, the exopod is shorter than the endopod; 9) the fourth article of the endopod in ThII to ThVII is rectangular; it is only square in ThI in the new species and in all Ths of the type species; 10) two external setae are present on the second article of the endopod of ThI to ThV, and one seta on the rest of Ths, in the new species; in the type species, only ThI and II present two setae, while the rest only have one seta; 11) the presence of an external seta on the third article of the endopod of all Ths in the new species, which is only present on ThI of the type species; 12) the outer seta of the first article of the exopod is as long as the second article in all Ths of the new species, while it is always shorter than the second article in the type species; 13) the outer seta of the second article of the exopod is long, about half the length of the internal seta in ThII to VII in the new species; it is short, about one-third the length of the internal seta, in the type species; 14) the epipod is smaller in the new species; 15) female ThVIII has two denticles in the new species that are not present in the type species; 16) male ThVIII has a small exopod in the new species, and a large recurved one in the type species; 17) the outer lobe of the male ThVII is large, trapezoidal and well individualized in the new species, while it is reduced and fused to the basipod in the type species; 18) the uropod sympod is five times as long as wide; in S. laorsriae, it is only three times as long as wide; 19) the uropod exopod is 30% longer than the endopod in the new species; the two structures are similar in size in the type species; 20) the strong distal spine of the uropod endopod is 20% longer than the endopod and has with two longitudinal rows of setules in the new species, while in the type species, the spine and endopod are of equal length and the spine has an oblique row of setules; 21) the inner distal setae of the uropod exopod and endopod are half as long as the outer setae in new species; in the type species, they are short, one-third or less the length of the outer setae and 22) the two dorsal plumose setae of the furcal rami differ in length in the new species but are of equal length in the type species.

As previously mentioned, the antennal organ is only present in some Asian species of the genera Atopobathynella and Chilibathynella, in addition to Siambathynella. In Atopobathynella, the antennal organ of A. operculata and A. paraoperculata from India consists of two massive lobes without setae or other structures (Ranga Reddy and Totakura 2015), whereas in A. nelloreensis, it is digitiform (Bandari, Shaik and Ranga Reddy 2017). In Chilibathynella kotumsarensis, the antennal organ is reduced and represented by two conical, dentate and nearly contiguous hyaline structures (Ranga Reddy 2006).

The two genera of Parabathynellidae known from caves in Thailand are easily distinguishable: species of Siambathynella present fewer articles, teeth, claws, setae and spines than those of Paraeobathynella. The three species of Paraeobathynella have between seven and nine AI articles; six to seven AII articles and a between three and nine exopod articles in all the thoracopods (Camacho et al. 2018). Also, a higher number of teeth, claws and spines on the different appendages is observed in Paraeobathynella than in Siambathynella.

The discovery of Siambathynella janineana sp. nov. adds one more piece to the puzzle of the diversification of genera and species, providing relevant information on the diversity of this group of crustaceans and the biodiversity of groundwater environments.

The material of Siambathynella janineana sp. nov. studied was collected from a pool located in the upper gallery of Tham Chiang Dao Cave. In addition to these specimens, numerous others were sampled by Janine Gibert either by filtering another pool in the upper gallery using a plankton net with an 0.150 mm mesh [about 305 specimens (sp.) were collected with this method] or by using artificial substrates in the same pool (215 sp.) (Gibert 1987). A few specimens were also found in the terminal sump of the tourist gallery (1 sp.), at the outside emergence (1 sp.) and in a Bou-Rouch pump that was positioned about 25 m downstream of the emergence (40 cm depth and 20 l filtered) (1 sp.). The most abundant populations found in the cave were Bathynellacea, with a total of 523 specimens (Gibert 1987). Other taxa collected included mainly ostracods, a few cyclopoid copepods and rarely gastropods and harpacticoid copepods (Gibert 1987). The temperature of the sampled waters varied between 21 °C and 23 °C during July 1985.

The species name “janineana” (adjective) is dedicated to Prof. Janine Gibert, who passed away in 2009. She was head of the Groundwater Hydrobiology and Ecology Laboratory (UMR CNRS 5023, Laboratoire d’Ecologie des Hydrosystèmes Fluviaux, Hydrobiologie et Ecologie Souterraines) at Université Claude Bernard Lyon 1 in France. She worked in various international programs on subterranean biodiversity and groundwater ecology from the perspective of environmental sustainability, particularly in the project “Protocols for the ASsessment and Conservation of Aquatic Life In the Subsurface (PASCALIS)”. She was also a member of the biospeleological team that led Expedition Thai-Maros in 1985 and was highly active in the sampling of the caves, especially Tham Chiang Dao.

The life science identifier (LSID) number obtained for the new species is https://zoobank.org/F4366538-281F-40A4-92E5-6BAEE6D3E3E2