Billibathynella humphreysi, Cho, 2005

Cho, Joo-Lae, 2005, A primitive representative of the Parabathynellidae (Bathynellacea, Syncarida) from the Yilgarn Craton of Western Australia, Journal of Natural History 39 (39), pp. 3423-3433 : 3425-3432

publication ID	https://doi.org/ 10.1080/00222930500345806
persistent identifier	https://treatment.plazi.org/id/2A5C9103-FF97-051C-F2E4-E2F4FDE21EB5
treatment provided by	Felipe
scientific name	Billibathynella humphreysi
status	sp. nov.

Treatment

Billibathynella humphreysi sp. nov.

( Figures 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 )

Material examined

Holotype: adult „, dissected on five slides, coll. Western Australian Museum , Perth ( WAM C 34441) . Allotype: adult ♀, dissected on five slides, coll. Western Australian Museum , Perth ( WAM C 34442) . Paratypes: two „„ ( WAM C 34443, C 34487) and three ♀♀ ( WAM C 34488–34490 View Materials ) each as whole specimen in a slide. Locus typicus. Gascoyne , Mt Padbury Station, Western Australia, Australia, irrigation well at homestead site 411 (26 ° 419440S, 118 ° 429530E), BES 9305. 6 June 2002, leg. W. F. Humphreys and L. Leijs. Other locality: Mt Padbury Station, irrigation well at homestead site 410 (26 ° 419420S, 118 ° 429510E) .

Description of male

Body length 6.30 mm (other males: 6.16, 5.45 mm), approximately 13 times as long as wide. Head as long as anterior three thoracic segments combined ( Figure 1 View Figure 1 ).

Antennule ( Figure 2A View Figure 2 ) seven-segmented. First segment with one seta on inner distal margin, with five simple dorsal setae, and with one dorsal, two lateral and one ventromedial plumose seta. Second segment with one group of four plumose setae and with six simple setae on inner distal margin. Third segment with two lateral setae and six setae on inner distal margin. Inner flagellum of third segment with three simple setae. Fourth segment with one stub seta and one plumose seta on dorsal margin, and with two stub setae and two plumose setae on outer distal apophysis. Fifth segment with four setae on inner margin, with two aesthetascs and one simple seta dorsally. Sixth segment with four setae on inner margin, with two aesthetascs and one seta dorsally, and with one aesthetasc. Seventh segment with three subterminal aesthetascs and four simple setae.

Antenna ( Figure 2B View Figure 2 ) seven-segmented, as long as the length of antennular segments 1–4. Setal formula: 0 + 0/0 + 0/1 + 1/1 + 1/0 + 0/2 + 1/4(1).

Labrum ( Figure 2C View Figure 2 ) flat with 28 teeth. Fourteen frontal teeth of more or less equal size flanked by small teeth on both sides. Inner surface with numerous combs of ctenidia and teats.

Mandible ( Figure 2D View Figure 2 ) with incisor process of six teeth. Tooth of ventral edge triangular. Spine row consisting of 14 spines. Palp of two segments, distal segment with two apical setae and one subterminal seta.

Maxillule ( Figure 2E View Figure 2 ) two-segmented. Proximal segment with four claw-like setae on inner distal margin. Distal segment with two terminal claws, eight claws on inner edge, and three simple setae on outer distal margin.

Maxilla ( Figure 2F View Figure 2 ) four-segmented, setal formula 5-10-17-11.

Thoracopods I–VII ( Figures 3A View Figure 3 , 4A, B, F View Figure 4 , 5A, B View Figure 5 ). Thoracopods I–IV increasing in size posteriorly. Thoracopods IV–VII similar in size. Thoracopods I–VII each bearing one epipod on protopod. Three setae on the basipod of thoracopod I, two setae on thoracopods II–VI, and one seta on thoracopod VII. The number of segments of the exopod of thoracopods I–VII: 7-9-10-9/10-10-9-9. The endopod of thoracopods I–VII foursegmented, setal formulae:

Thoracopod I 5 + 1/7 + 2/4 + 1/5(3)

Thoracopod II 3 + 1/8 + 2/5 + 1/6(4)

Thoracopod III 2 + 1/8 + 3/5 + 1/6(4)

Thoracopod IV 2 + 1/8 + 3, 9 + 2/5 + 1/6(4)

Thoracopod V, VI 2 + 1/7 + 2/4 + 1/5(3)

Thoracopod VII 2 + 1/6 + 2/4 + 1/5(3)

Thoracopod VIII ( Figure 2H, I View Figure 2 ) rectangular in lateral view, 1.5 times longer than wide. Protopod massive, with prominent penial region with a distal opening. Inner margin of the distal opening with tiny denticles. Epipod large, triangular, distal part covering penial region of protopod. Basipod without setae, inner margin of basipod drawn out into projection. Exopod one-third of basipod in size, triangular, bearing two subterminal setae. Endopod as large as exopod, with three distal setae.

First pleopod absent.

Uropod ( Figure 3B–D View Figure 3 ) bearing 28 spines of equal size on inner margin of sympod. Endopod 40% as long as sympod, with two distal spines and five small spines on inner margin, with two dorsal plumose setae near the base, and with two terminal plumose setae and one subterminal plumose seta on outer margin. A protrusion exists between terminal and subterminal plumose setae. Exopod slightly longer than endopod, with one basi-ventral seta, and with 22 setae on outer and terminal margin.

Pleotelson ( Figure 3B, C View Figure 3 ) with one seta near the base of furcal rami on both sides. Anal operculum flat.

Furcal rami four times as long as wide, with two large distal spines, and 21 spines on inner margin, and with two dorsal setae.

Description of female

The female differs from the male in thoracopod VIII. Body length 6.02 mm (other females: 5.94, 5.82, 5.80 mm).

The right and left thoracopods VIII ( Figure 2G View Figure 2 ) not fused, cone-shaped, as large as the endopod of the male thoracopod VIII, with two distal denticles.

Remarks

The new species is probably the most primitive member of the Parabathynellidae . The richness of the extremities in teeth, claws and setae, the high number of segments of thoracopodal exopod and finally the large body size are the indices (Table I). From the Australian continent two genera are known as being primitive: Chilibathynella and Notobathynella . According to Schminke (1973), Chilibathynella has a balloon-shaped male thoracopod VIII, five-segmented antenna, one-segmented exopod of thoracopods I–VII and thus differs from the new species significantly. Notobathynella , in contrast, is similar to the new species in many aspects. For example, the male thoracopod VIII of Notobathynella is very similar to that of the new genus in its rectangular form, in having the massive protopod with prominent penial region, in the basipod drawn out into projection, and in the triangular exopod. Although fewer in number, the exopods of thoracopods I–VII of Notobathynella display a relatively large number of segments (2-3-4-4-4-3-3). However, Notobathynella has a five- to six-segmented antenna and lacks the basi-ventral seta on the uropodal exopod. In addition, the epipod of the male thoracopod VIII never covers the penial region of the protopod and the basipod bears a seta. Hence, we propose for the new species a new genus Billibathynella gen. nov.

In terms of the primitiveness of the extremities, Allobathynella Morimoto and Miura, 1957 , occurring in East Asia, is also comparable with the new species, B. humphreysi . According to Schminke (1973), this genus includes the most primitive members of the Parabathynellidae . However, even the basal representative of Allobathynella , A. gigantea pluto Morimoto, 1963 , is at most only 3.30 mm long and has a six-segmented antenna, a labrum with 14 teeth, a mandible with seven teeth on the incisor process and 12 spines in the spine row, an exopod formula of thoracopods I–VII of 4-5-6-6-6-6-6, 18 spines on the uropodal sympod, and six spines on the furcal rami. Except for the eight-segmented antennule (cf. Morimoto 1963), the primitiveness of A. gigantea thus does not go beyond that of B. humphreysi . The new species rather seems to resemble the hypothetical stem species of the Parabathynellidae , which Schminke (1973) has reconstructed based on the summation of primitive characters of the family known at that time. The differences of the stem species from B. humphreysi may exist only in the three-segmented mandibular palp, in the male and female thoracopods VIII with the form slightly different from the other thoracopods, the possession of two epipods on each of thoracopods I–VII, the presence of two-segmented pleopods on the first and second pleomeres, and the number and the position of the setae on the pleotelson.

Billibathynella humphreysi has been collected from irrigation wells located in the calcrete aquifer in the arid zone of Australia, where typical karst features have been developed, with sinkholes with a large water body ( Sanders 1974; Barnett and Commander 1985; Humphreys 2001). Since the sinkholes usually provide unconfined spaces, one could argue that the unusual body size of B. humphreysi has been developed secondarily in adaptation to the free water, as supposed by Noodt (1964) for Bathynella baicalensis Bazikalova, 1954 and Baicalobathynella magna Bazikalova, 1954 , the largest and the most primitive representatives of the family Bathynellidae View in CoL from the bottom of Lake Baikal. Accordingly, the marine interstitial ancestor of the Parabathynellidae View in CoL could have invaded the continental groundwater via the coastal interstitial (thalassoid origin: Boutin and Coineau 1987; Coineau 1996; Camacho 2003). As in both species from Lake Baikal, however, B. humphreysi does not differ from other species of the Parabathynellidae View in CoL in basic body plan, even lacking pleopods, and thus shows no sign of adaptation to the free water habitat. The habitat of the nominative form of Allobathynella gigantea Morimoto, 1959 also supports the primitiveness of B. humphreysi . A. gigantea is much smaller (2.35 mm) and equipped with fewer teeth, setae, or spines on the proper extremities and thus seems to be more suitably adapted to interstitial spaces than B. humphreysi . However, the former species is likely to favour free water, since it was found abundantly under stones on the bottom of a shallow dug well exposed to the faint sunlight ( Morimoto 1959). In terms of the origin of the Parabathynellidae View in CoL , on the other hand, it is worthy to mention that both B. humphreysi and A. gigantea occur in regions which have mostly not been submerged by the sea since the Palaeozoic ( Morimoto 1959; Watts and Humphreys 2000). As Schminke (1972) remarked for the two bathynellids from Lake Baikal, it does not seem to be a simple coincidence that B. humphreysi , the most primitive form within the Parabathynellidae View in CoL , occurs just in one of the oldest far inland areas of the world. This fact strongly suggests that the extant parabathynellids have been derived from a surface water limnic ancestor (limnocoid origin: Schminke 1973). It is also probable that the transition to groundwater happened in Notogaea. A way to test this hypothesis on the centres of origin of the family, however, would be an extensive investigation of the groundwater fauna of old cratons in far inland areas of the gondwanan fragments.