Gibbidessus atomus, Hendrich & Watts & Balke, 2020

Hendrich, Lars, Watts, Chris H. S. & Balke, Michael, 2020, The " minute diving beetles " of southern Australia - taxonomic revision of Gibbidessus Watts, 1978, with description of six new species (Coleoptera, Dytiscidae, Bidessini), ZooKeys 975, pp. 11-49 : 11

Gibbidessus atomus sp. nov. Figs 1 , 14 , 23 , 27

Type locality.

Western Australia, Windy Harbour Road, 11 km south of Northcliffe, small pool in sedge swamp [34°48'51S, 116°4'8E].

Type material.

Holotype, male: "Australia: SW WA, D´Entrecasteaux NP, 11 km S, Northcliffe, 77m, 4.I.2007, 34.44.048S, 116.05.354E [34°44'0S, 116°5'13E], L. & E. Hendrich (WA 162)", "Holotype Gibbidessus atomus Hendrich, Watts & Balke des. 2020" [red printed label] (WAM). Paratypes (20 exs.). 7 specimens with same data as holotype, three specimens with "DNA M. Balke 1729", "DNA M. Balke 2780", "DNA M. Balke 2781" [green printed labels]; 13 exs., "AUSTRALIA/WA: D´Entrecasteaux N.P., 11 km south of Northcliffe, Windy Harbour Road, 50 m, 3.1.2000, 34°42'S, 116°05'E [34°44'0S, 116°5'13E], Hendrich leg. (loc. WA 10c/156)" (ANIC, CLH, SAMA, ZSM). All paratypes with red printed paratype labels.

Additional material.

1 ex., "Australia, WA, RVDLE03 Riverdale Wetland [32°59'22S, 115°47'7E], 23/09/2008, South West Catchment Council Mon." (DPAW).

Diagnosis.

Very small species, externally characterised by widely rounded body, with less pronounced habitus disruption between pronotum and elytron, shiny non-microreticulate dorsal surface and vague ferruginous markings on elytra. Dorsoventrally rather domed. Cervical line present (Fig. 1 ).

Measurements.

Holotype: TL = 1.15 mm, TL-H = 1.05 mm, MW = 0.6 mm. Paratypes: TL = 1.15-1.30 mm, TL-H = 1.0-1.05 mm, MW = 0.6-0.65 mm.

Head: Dark brown, around eyes almost black. Cervical line present. Strongly and coarsely punctate, rather shiny, microsculpture almost absent. Punctures weakly distributed anteriorly, strong posteriorly between eyes. Antennae relatively short, stout. Antennomeres ferruginous, darkened anteriorly.

Pronotum: Ferruginous, anterior and posterior margins darker. Disc of pronotum somewhat darkened, broadest at posterior corners. Punctation very weakly, almost evenly distributed, shiny, microsculpture absent. Sides of pronotum margined and almost evenly rounded. Angle between pronotum and elytra less pronounced, basal pronotal plicae present. Striae moderately defined, on almost 1/2 length of pronotum, moderately incurved.

Elytra: Dark brown with vague basal area ferruginous (Fig. 1 ). Coarsely and densely punctate, shiny, microsculpture absent. Striae weakly impressed, slightly straighter but shorter than basal pronotal striae.

Ventral side: Ferruginous. Prothorax and apex of abdomen paler than other parts. Metacoxae and metaventrite covered with numerous larger punctures, surface shiny, without microreticulation. Abdominal ventrites with dense and finer punctures, shiny, microreticulation absent. Metacoxal lines almost straight, anteriorly slightly divergent. Epipleuron ferruginous, coarsely punctate, shiny, lacking microsculpture. Legs ferruginous with meta-/mesotibiae and meta-/mesotarsi set in black.

Male. Dorsal surface with coarse punctures but otherwise with shiny surface (Fig. 1 ). Median lobe of aedeagus as in Fig. 14A, B . Shape of median lobe fairly uniform, bent evenly, apex straight and pointed. Paramere, as in Limbodessus Guignot, 1939, with hook or bent finger-like apical part with tiny setae on tip (Fig. 14C ).

Affinities.

This species is similar to G. davidi sp. nov. but readily separated by its smaller size, the different colour pattern and the form of the median lobe and parameres (Figs 14 , 16 ).

Etymology.

From Latin atomus (smallest particle), as it is the smallest epigean diving beetle in Australia described so far.

Distribution.

South-western Australia. Known only from the type locality in the D´Entrecasteaux National Park, south of Northcliffe and the Riverdale Wetland [32°59'22S, 115°47'7E] (Fig. 23 ).

Habitat.

Most specimens were obtained from an exposed, shallow and small roundish puddle without any vegetation, except some algae (Fig. 27B ). The remaining specimens were collected in a half-shaded pool in a Melaleuca blackwater swamp (Fig. 27A ), with few clumps of Juncus spp. and extensive beds of macrophytes; depth up to 20 cm; bottom of sedge-filled peat (pH 5.5), twigs and rotten leaves. The heathlands south of Northcliffe are seasonally flooded, with some permanent water bodies in the summer. At Northcliffe the species is syntopic with G. davidi sp. nov. ( Hendrich 2001a), at the Riverdale Wetland it was collected with G. davidi sp. nov. and G. rottnestensis sp. nov.

Apart from the Gibbidessus , the water beetle coenosis at Northcliffe included the following species: Dytiscidae : Limbodessus inornatus (Sharp, 1882), Antiporus hollingsworthi Watts, 1997, A. mcraeae Watts & Pinder, 2000, Brancuporus gottwaldi (Hendrich, 2001), Sternopriscus minimus Lea, 1899, S. eikei Hendrich & Watts, 2007, Exocelina ater (Sharp, 1882); Hydrophilidae : Enochrus eyrensis (Blackburn, 1895), Limnoxenus zealandicus (Broun, 1880), Paracymus pygmaeus (Macleay, 1871).

Figures

Figure 27. Habitat of Gibbidessus atomus sp. nov. and Gibbidessus davidi sp. nov. A Seasonally flooded Melaleuca sedge swamp B small and shallow heathland pool along Windy Harbour Road, south of Northcliffe, south-western Australia.

Figures 1 - 4. Habitus of 1 Gibbidessus atomus sp. nov. 2 G. chipi, 3 G. davidi, male 4 G. davidi female. Scale bar: 1.0 mm.

Figures 14, 15. 14 Gibbidessus atomus sp. nov. 15 G. chipi A median lobe in ventral view B median lobe in lateral view, left side C left paramere in lateral view. Scale bar: 0.2 mm.

Figures 22, 23. Distribution of 22 Gibbidessus chipi (black dots), G. drikdrikensis sp. nov. (yellow square), G. kangarooensis sp. nov. (red square) 23 G. atomus sp. nov. (red square), G. davidi sp. nov. (yellow square).

Figures 16, 17. 16 Gibbidessus davidi sp. nov. 17 G. drikdrikensis sp. nov. A median lobe in ventral view B median lobe in lateral view, left side C left paramere in lateral view, D right paramere in lateral view. Scale bar: 0.2 mm.