(Figures 4–6)

Material examined

Type locality. Department of Environment, Climate Change and Water (DECCW) groundwater monitoring bore number 30168 (31 ◦ 10 ′ 44 ′′ S, 151 ◦ 04 ′ 03 ′′ E), 5–9 m deep, 6 December 2005, in the alluvial aquifer of the Peel River floodplain, Tamworth, NSW, Australia. Two males and two females were collected. The species was also collected from the same aquifer approximately 20 km away in DECCW monitoring bore 30150 (31 ◦ 04 ′ 56 ′′ S, 150 ◦ 54 ′ 45 ′′ E) from a depth of 6–12 m. Six females, including two juveniles.

Details of the descriptions are based on all adult specimens, two males and six females. The holotype is a male and the allotype is a female. The type series contains six additional specimens (one male and seven females, including two juveniles) (Holotype MNCN 20.04 / 8561, Allotype MNCN 20.04 / 8562 and type series MNCN 20.04 / 8563).

Description

Body. Total length of holotype (male) 1.7 mm and allotype (female) 2.3 mm. Largest male total length 2.4 mm. Longest female 2.3 mm, smallest 1.9 mm. Body elongated, segments slightly widening towards posterior end; approximately 10 times as long as wide. Head as long as wide. Pleotelson with one long barbed ventral seta on each side. All drawings are of the holotype (male) except for female Th VIII that belong to the allotype.

Antennule (Figure 4A). Seven-segmented; not sexually dimorphic, without antennal organ on segment two; combined length of first three segments similar to other four segments together; seventh segment narrower than sixth; inner flagellum almost square; setation as in Figure 4A; segment three with three setae, including one that is barbed; segments five and six with two and three similarly-sized terminal aesthetascs respectively; segment seven with three aesthetascs different in size.

Antenna (Figure 4B). Five-segmented; half the length of the A.I; first three segments are one-third the length of the last two segments, and the fifth is a little longer than the fourth; the last segment has four setae, three smooth (two apical and one subapical), and one plumose subterminal; the fourth segment has inner and outer seta; setal formula: 0 / 0 / 1 + 0 / 1 + 1 / 4(1).

Labrum (Figure 4C). Almost flat with eight main teeth, the central four slightly different from the others, and four lateral teeth at each side. Ventral surface with rows of fine spinules.

Mandible (Figure 4D,E). Pars incisiva with six teeth and triangular tooth of ventral edge; pars molaris with six claws, the two distal ones with subdistal spinules and two small joined proximal claws with a large number of fine hairs; the mandibular palp, one segmented, almost four times as long as wide, with a distal seta which does not exceed pars incisiva in length.

Maxillule (Figure 4F). Proximal endite with four long serrulate claws; distal endite has six claws with denticles and fine long, almost basal setules and three subterminal smooth setae on outer distal margin; just below the claws is a group of fine setules.

Maxilla (Figure 4G). Four-segmented, segments one and three massive, segment four very small; smooth setae on all segments; setal formula 3, 4 + 1, 9, 6.

Thoracopod I–VII (Figure 5A – 5G). Well developed, length gradually increasing from Th I to Th V, last three similar in size; epipod absent on Th I, present on Th II to Th VII, measuring almost half length of basipod; basipod with one small smooth seta at distal inner corner in Th I to VII. Exopod one-segmented, shorter than the two first segments of endopod on all thoracopods, with two unequal barbed terminal setae, one of which is small; one additional barbed seta present on dorsal side of Th II to VII. Endopod four-segmented, the first segment is almost half as long as the following two, which are similar in length, and the fourth segment is small (with two barbed, similar claws and one smooth seta); all setae on segments one to three are smooth on Th I; first and third segments without setae in Th II to VII; setae absent from all thoracopods in the distal outer corners of all segments. Thoracopod endopod setal formula: Th I, 2 + 0 / 2 + 0 / 1 + 0 / 3(1); Th II to VII, 0 + 0 / 1 + 0 / 0 + 0 / 3(1).

Thoracopod VIII male (Figure 6B – D). Massive, almost globular; basal region of the penial complex rectangular with three lobes: inner lobe (I. Lb.), outer lobe (O. Lb.) and dentate lobe (D. Lb.); rectangular inner lobe completely integrated into the basal region; outer lobe almost rectangular and similar to inner lobe and both exceed the end of the external side of the basipod (Bsp); dentate lobe slightly longer than the inner lobe and with many terminal and lateral denticles; endopod (Endp.) small, as exopod, bending backward with two unequal smooth setae and a group of terminal small denticles; exopod (Exp.) small, completely covered by basipod and with one small apical denticle; basal segment almost spherical with the basipod inserted, with a smooth seta on the internal side and with an elongated, finger-like protuberance, bending backward in frontal view.

Thoracopod VIII female (Figure 6A). Large, very elongated and with rounded tip, lacking denticles.

First pleopods (Figure 5H). One-segmented, five times as long as wide, with one long apical seta and one short subapical seta, both smooth.

Uropod (Figure 6E). Sympod almost eight times as long as wide, almost twice as long as endopod; with 12 barbed spines of similar size occupying two-thirds of length of sympod; endopod with three spines in the distal third, the distal spine stronger than the others, two long plumose setae near inner margin, one subterminal, that exceeds the distal end of endopod, and two barbed lateral setae of equal length; exopod very thin, nine times as long as wide, has four barbed setae.

Pleotelson (Figure 6F). With one plumose ventral seta on either side near the base of the furca. Anal operculum flat, not protruded.

Furca (Figure 6F). Distally pointed, elongate, twice as long as wide, with 9 or 10 barbed spines (the two terminal ones a little longer than the others); two similarsized small dorsal setae; lateral furcal organ apparent spherical, with two distal elongated projections.

Variability

The number of spines on the furca varies between 9 and 10, the sympod of the uropod has 12 or 13 spines.

Etymology

The species name “ digitus ” is a noun in apposition and refers to finger-like protuberance on the basipod of the male Th VIII.

Remarks and discussion

Chilibathynella digitus sp. nov. is the first known species of the genus with six teeth on the pars incisiva of the Md (see Table 1) and the least number of 23 setae on Mx.II. The Th VIII of the female is different from other known species (see Figure 6A), as the pair is not fused and is large. Another unique character of this new species is the elongated finger-like protuberance of the basipod of the male Th VIII. The setal formula of some thoracopods is also unique and more closely resembles that of C. kotumsarensis (see Table 1). All these characters, considered together with a unique combination of others (see Table 1) leave no doubt that the specimens studied belong to a new species.

Chilibathynella australiensis Schminke, 1973 and C. joshuai sp. nov., the other Australian species of the genus, have five segments on A.II as does the new species, and all three have a similar setal formula (see Table 2); all Australian species have more teeth on the pars incisiva of the Md (six or seven) than any other species in the genus and lack an epipod on the Th I; the setal formula of thoracopods differs among the three Australian species. The new species has a female Th VIII elongated and not fused as in C. australiensis, but larger. Despite these differences, C. digitus sp. nov. is closely related to C. australiensis (from Victoria) and C. joshuai sp. nov., as described above.