The “minute diving beetles” of southern Australia – taxonomic revision of Gibbidessus Watts, 1978, with description of six new species (Coleoptera, Dytiscidae, Bidessini)

Abstract Morphology and mitochondrial DNA sequence data are used to reassess the taxonomy of Australian diving beetles previously assigned to the genera Uvarus Guignot, 1939 and Gibbidessus Watts, 1978. Gibbidessus was described as a monotypic genus for Gibbidessus chipi Watts, 1978. The genus is significantly extended here. Based on molecular systematic evidence, Uvarus pictipes (Lea, 1899) is transferred to Gibbidessus. Gibbidessus chipi and Gibbidessus pictipescomb. nov. are redescribed, and six new species are described: Gibbiddessus atomussp. nov. (SW Australia, Northcliffe area) [the smallest epigean diving beetle in Australia], G. davidisp. nov. (SW Australia), G. drikdrikensissp. nov. (Victoria), G. kangarooensissp. nov. (SA Kangaroo Island), G. pederzaniisp. nov. (SW Australia, Nannup area), and G. rottnestensissp. nov. (SW Australia). Species are delineated using characters such as male genital structure and beetle size, shape and colour pattern. Mitochondrial Cox1 data for 27 individuals, representing five species, were generated, and revealed clusters congruent with the morphological evidence. Gibbidessus occur in southern Australia, with the centre of diversification in the isolated peat- and wetlands of SW Australia. All species occur in very shallow water of seasonal, exposed or half-shaded wetlands and flooded meadows.


Introduction
With 726 described species the Bidessini belong to the most diverse tribes of the Dytiscidae (Nilsson and Hájek 2020). Bidessini genera have to date been justified mainly based on a diagnostic combination of structural features (Biström 1988;Miller and Short 2015), rather than apomorphies. This had to lead to the recognition of genera that render others paraphyletic (Balke and Ribera 2004). Some of these features, such as presence / absence of an elytral plica or occipital line, have been shown to vary within clades of closely related species (Balke et al. 2015). In this context, the use of phylogenetic reconstructions based on DNA sequence data offers a source of information that helps to delineate monophyletic entities (Hendrich and Balke 2009;Balke et al. 2013). In Australia, the situation is currently rather stable. Most Australian genera have been revised or will be revised in the near future (Balke and Ribera 2004;Watts 1978;Watts and Humphreys 2001Watts and Leys 2005;Hendrich and Wang 2006;Hendrich and Balke 2009).
In this work we focus on the genus Gibbidessus Watts, 1978. These are widespread diving beetles of south-western and south-eastern Australia, but rarely collected, supposedly due to their small size. In fact, some of the species belong to the smallest epigean Australian Dytiscidae. We use molecular systematic evidence to redefine the genus and taxonomically treat all species now assigned to Gibbidessus, two known ones and six new species. We provide mitochondrial 3' cox1 sequence data for five species.

Materials and methods
Material: This study is based on the examination of 767 specimens. Types of the two previously known species were examined. Most of the specimens were collected in the past 25 years by LH and CHS Watts. Additional material was collected by Melita Pennifold of the Department of Parks and Wildlife in many parts of south-western Australia, and by Australian Water Quality in South Australia. Furthermore, the authors have studied all available specimens stored in relevant Australian museums.
Descriptions: Beetles were studied with a Leica M205C dissecting microscope at 10-100×. Male genitalia were studied and figured in dry condition. The terminology to denote the orientation of the genitalia follows Miller & Nilsson (2003). Abbreviations used in the text are: TL (total length), TL-H (total length without head), and MW (maximum width). Label data of type material are cited between quotation marks.
Photos and illustrations: Images were taken with a Canon EOS 5DS camera fitted with a Mitutoyo 10× (habitus) or 20× (genital structures) ELWD Plan Apo objective attached to a Carl Zeiss Jena Sonnar 3.5/135 MC as focus lens. Illumination was with two to three LED segments SN-1 from Stonemaster (https://www.stonemaster-onlineshop. de). Image stacks were generated using the Stackmaster macro rail (Stonemaster), and images were then assembled with the computer software Helicon Focus 4.77TM.
Coordinates are given in decimal notation unless cited verbatim from labels. Besides various Australian road maps, we also used Google Earth (http://earth.google.com) to locate several localities, and their coordinates are given in Degrees, Minutes (DDD° MM'). Our maps are based on "MICROSOFT ENCARTA World-Atlas 2000".
DNA sequencing and data analysis: Our laboratory protocol has been explained in Hendrich et al. (2010). We used the 3' end of the cox1 gene, widely used in diving beetle research. Each of our 27 individual vouchers bears a green cardboard label that indicates the DNA extraction number of M. Balke (e.g., "DNA M. Balke 7247"). This number links the DNA sample to the dry-mounted voucher specimen, deposited in Zoologische Staatssammlung München (ZSM). We used a simple approach to calculate a neighborjoining tree (p-distances) in Geneious (11.0.4.) software (Fig. 26), and subsequent visual inspection of the tree to learn whether there was any hidden diversity or haplotype sharing.
GenBank accession numbers are provided in Table 1.

ANIC
Australian National Insect Collection, Canberra, Australia CFP Collection Fernando Pederzani, Ravenna, Italy CGC Collection Gilbert L. Challet, Florida, United States compactly built. Head with or without cervical line; frontally not bordered. Palpi rather slender, apically very finely bifid, in one species broad. Pronotum with a pair of basal striae. Elytron with a basal stria but without sutural striae. Punctation of elytra does not form rows. Epipleura lack a basal cavity posteriorly limited by a transverse carina. Prosternal process rather elongate, narrow, laterally distinctly marginated and with ventral surface not medially excavated. Prosternal process reaches metaventrite, which is not distinctly depressed posterior to mesocoxae. Metacoxal lines comparatively short, only slightly longer than distance between them posteriorly. Very fine punctures on either side of midline of metaventrite, not forming distinct rows. Metatrochanters and metafemora not distinctly modified (Biström 1988;Hendrich et al. 2019). Parameres symmetric. Larvae unknown.
Gibbidessus was described as a monotypic genus to accommodate G. chipi Watts, 1978. A molecular phylogenetic investigation covering all Australian Bidessini genera (Hendrich et al. 2010)  Diagnosis. Very small species, externally characterised by widely rounded body, with less pronounced habitus disruption between pronotum and elytron, shiny nonmicroreticulate dorsal surface and vague ferruginous markings on elytra. Dorsoventrally rather domed. Cervical line present (Fig. 1) 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.
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.
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.
Pronotum: Ferruginous, anterior and posterior margins darker. Disc of pronotum somewhat darkened, broadest at posterior corners. Punctation very weakly punctate almost evenly distributed, shiny and microsculpture absent. Sides of pronotum margined and almost evenly rounded. Angle between pronotum and elytra not pronounced, basal pronotal plicae present. Striae well defined, almost 1/2 length of pronotum, strongly incurved.
Male. Dorsal surface with coarse punctures but otherwise with shiny surface Fig. 2. Median lobe of aedeagus as in Fig. 15A, B. Shape of median lobe fairly uniform, apex in lateral view straight and pointed, in ventral view very broad and rounded at apex. Parameres bi-segmented and elongated with few setae at apex (Fig. 15C).
Affinities. This species is similar to G. drikdrikensis sp. nov. but readily separated by its smaller size, and the form of the median lobe (Figs 15,17).
Habitat. The type specimens were collected in an old farm dam and its flood zone, overgrown by rich vegetation. In Victoria and South Australia most of the specimens were collected in small shallow pools and seasonal wetlands. A single specimen from South Australia (Mount Crawford State Forest, Watts Gully) has been found in a shallow, slow flowing temporary forest creek. At Dri Drik, in Victoria, the species is syntopic with G. drikdrikensis sp. nov. Diagnosis. Small species which externally is characterised by a wide rounded body, shiny non-microreticulate dorsal surface, vague testaceous markings on elytra, and without habitus disruption between pronotum and elytron. Dorsoventrally rather domed. Cervical line present (Fig. 3).
Pronotum: Ferruginous, anterior and posterior margins darker. Disc of pronotum somewhat darkened, broadest at posterior corners. Punctation of pronotum very weak, almost evenly distributed, shiny and microsculpture absent. Sides of pronotum margined and almost evenly rounded. Angle between pronotum and elytra less pronounced, basal pronotal plicae present. Striae moderately defined, almost 1/2 length of pronotum, strongly incurved.
Male. Smaller and more elongate than female (Fig. 4). Median lobe of aedeagus as in Fig. 16A, B. Shape of median lobe in lateral view, straight and fairly uniform, in ventral view broad, with a thorn on each side, and rounded at apex. Parameres bisegmented and elongated with few setae at apex (Fig. 16C, D).
Affinities. This species is similar to G. atomus sp. nov. but readily separated by its larger size, the different colour pattern and the form of the median lobe and parameres (Figs 14, 16). From G. pederzanii sp. nov. it can be distinguished by the less roundish body and the form of the median lobe and parameres (Figs 16, 19).
Etymology. The beetle is named after the son of the first author, David Hendrich. The specific epithet is a substantive in the genitive case.
Distribution. South-western Australia. From Perth in the north to D´Entrecasteaux National Park in the south (Fig. 23).
Habitat. In the Northcliffe area most specimens were obtained from an exposed, shallow and small roundish puddle, without any vegetation, except some algae. The other specimens were collected in a half-shaded pool in a Melaleuca blackwater swamp, with few clumps of Juncus spp. and extensive beds of macrophytes; depth up to 20 cm; bottom consisted of sedge-filled peat (pH 5.5), twigs and rotten leaves (Figs 27,28). The whole area south of Northcliffe is seasonally flooded with some permanent central water bodies in summer. In the D´Entrecasteaux NP the species is syntopic with G. atomus sp. nov., and around Perth in the Beeliar Regional Park with G. rottnestensis sp. nov. At the Riverdale Wetland Reserve G. davidi sp. nov. was syntopic with G. atomus sp. nov. and G. rottnestensis sp. nov.
Diagnosis. Medium-sized species which externally is char acterised by a wide rounded body, shiny non-microreticulate dorsal surface, vague testaceous markings on elytra, and without slight habitus disruption between pronotum and elytron. Dorsoventrally rather domed. Cervical line present (Fig. 5).
Pronotum: Ferruginous, anterior and posterior margins darker. Disc of pronotum somewhat darkened, broadest at posterior corners. Punctation very weak, almost evenly distributed, shiny and microsculpture absent. Sides of pronotum margined and almost evenly rounded. Angle between pronotum and elytra not pronounced, basal pronotal plicae present. Striae well defined, almost 1/2 length of pronotum, strongly incurved.
Male. Dorsal surface with coarse punctures but otherwise with shiny surface (Fig. 5). Median lobe of aedeagus as in Fig. 17A, B. Shape of median lobe fairly uniform, evenly bent in lateral view, apex straight and pointed at tip in ventral view. Parameres bi-segmented, broad and with few setae at apex (Fig. 17C, D).
Affinities. This species is very similar to G. chipi but readily separated by its larger size and the form of the median lobe and parameres (Figs 15, 17).
Etymology. The species is named after the type locality. The specific epithet is a substantive in the genitive case.
Distribution. Only known from the type locality Drik Drik in south-western Victoria (Fig. 22).
Habitat. The few specimens were collected in shallow water at the edge of a large, exposed but shallow farm dam, overgrown with grasses and sedges. The species is syntopic with G. chipi. Diagnosis. Small species which externally is characterised by a more elongate body, shiny non-microreticulate dorsal surface, and with well pronounced habitus disruption between pronotum and elytron. Dorsoventrally rather flattened. Without cervical line but rather a few punctures instead (Fig. 6).
Pronotum: Ferruginous, anterior and posterior margins darker, broadest at middle. Punctation weak anteriorly but quite strong on posterior half and on lateral sides, almost evenly distributed, shiny and microsculpture absent. Sides of pronotum broadly margined and almost evenly rounded. Angle between pronotum and elytra well pronounced, basal pronotal and elytral plicae present. Striae moderately defined, almost 1/2 length of pronotum, strongly incurved.
Male. Dorsal surface with coarse punctures but otherwise with shiny surface (Fig. 6). Median lobe of aedeagus as in Fig. 18A, B. Shape of median lobe, bent evenly and fairly uniform in lateral view, in ventral view pointed at apex. Parameres bi-segmented, elongated, and with few setae at apex (Fig. 18C, D).
Female. Unknown. Affinities. This species is similar to G. pictipes but readily separated by the different colour pattern and the more flattened body. Furthermore, both species can be separated by the form of the median lobe and parameres (Figs 18, 20).
Etymology. The species is named after the type locality. The specific epithet is a substantive in the genitive case.
Distribution. A rare species, only known from the type locality on Kangaroo Island, South Australia (Fig. 22).
Habitat. The single specimen was collected at the edge of the Eleaner River in the southern part of Kangaroo Island. Most probably this is not the original habitat of the species. Almost all Gibbidessus inhabit more seasonal, open wetlands, overgrown with sedges and rushes.  Paratypes (13 exs.): All specimens with same data as holotype. Two specimens with "SAMA Database No 25-001593" and one with a yellow printed label "photographed" (CFP, CLH, SAMA, ZSM). All paratypes are provided with printed red paratype labels.
Diagnosis. Medium-sized species which externally is char acterised by a rounded habitus, without disruption between pronotum and elytron, and shiny, non-microreticulate dorsal surface with testaceous markings on elytra. Dorsoventrally rather arched. Without cervical line but rather a few punctures instead (Fig. 7).
Male. Dorsal surface with coarse punctures but otherwise with shiny surface (Fig. 7). Median lobe of aedeagus as in Fig. 19A, B. Shape of median lobe, almost straight and fairly uniform in lateral view, in ventral view rounded at apex. Parameres bi-segmented, elongated, and without setae at apex (Fig. 19C, D).
Affinities. This species is similar to G. davidi sp. nov. but readily separated by the different colour pattern, the more roundish body (Figs 4, 7), the larger punctation on elytra, and the form of the median lobe and parameres (Figs 16, 19).
Etymology. The species is named after our colleague, the dytiscid specialist Fernando Pederzani (Ravenna, Italy), who collected the type material. The specific epithet is a substantive in the genitive case.
Distribution. South-western Australia. A rare species, which is only known from the type locality somewhere around Nannup in south-western Australia. Most probably a more inland species and restricted to forested areas and not in heathland or coastal sedge swamps (Fig. 25).
Habitat. All specimens were collected in shallow water at the edge of a small slow flowing forest creek (F. Pederzani in litt.). (Lea, 1899) Additional material studied (320 exs.). 51 exs., "WA Lake Nalyerin 33 08S, 116 22E CHS, Watts 6/10/03", "SAMA Database 25-009282" (SAMA); 4 exs., "Nalyeen Lake  Note. Watts (1978), who moved Bidessus pictipes to Uvarus, already noticed that this species might not belong to Uvarus: "It is with considerable hesitation that I place the following species in this genus. I suspect that it will eventually prove to belong to a genus of its own".

Gibbidessus pictipes
Diagnosis. Small and dark brown species, with vague testaceous markings on elytra, and with habitus disruption between pronotum and elytron. Dorsoventrally rather flattened. Head without cervical line but rather a few punctures instead (punctures not obvious in females) (Fig. 8).
Elytra: Dark brown with vague basal area ferruginous (Fig. 8). Coarsely and densely punctate, shiny, microsculpture absent. Striae weakly impressed, slightly straighter than in female specimens and of same length as basal pronotal striae.
Male. Dorsal surface with coarse punctures but otherwise with shiny surface (Fig. 8). Median lobe of aedeagus as in Fig. 20A, B. Shape of median lobe, bent evenly and fairly uniform in lateral view, in ventral view tapering and pointed at apex. Parameres bi-segmented, elongated, and with setae inside apical hook (Fig. 20C, D).
Affinities. This species is similar to G. kangarooensis sp. nov. and the larger G. rottnestensis sp. nov. (TL = 1.5-1.7 mm) but readily separated by the different colour pattern. Furthermore, all three species can be separated by the form of their median lobes and parameres (Figs 18,20,21).  Distribution. South-western Australia. The most common and widespread species in south-western Australia and a more inland species. South of a line from 230 km north of Perth (Leeman) to Albany (Fig. 24).
Habitat. Permanent and seasonal, very shallow, sun exposed or half-shaded sedge swamps, lakeshores, larger ponds and flooded meadows on sandy bottom, with a thin layer of peat or rotten debris of sedges (Figs 30, 31). A winter and early spring breeder. Most specimens were collected in September and October, with the next generation in December and January. Apart from G. pictipes, the water beetle coenosis at Nalyerin Lake (Fig. 31) Hendrich & Watts, 2004, S. wattsi Hendrich & Watts, 2004, Necterosoma darwinii (Babington, 1841, Rhantus suturalis and Lancetes lanceolatus (Clark, 1863).
Diagnosis. Larger species which externally is characterised by a more elongate body, shiny non-microreticulate dorsal surface, testaceous markings on elytra, and with distinct habitus disruption between pronotum and elytron. Dorsoventrally slightly flattened. Without cervical line but rather a few punctures instead (Fig. 10).
Male. Dorsal surface with coarse punctures but otherwise with shiny surface (Fig. 10). Median lobe of aedeagus as in Fig. 21A, B. Shape of median lobe, bent evenly and fairly uniform in lateral view, in ventral view strongly tapering and rounded at apex. Parameres bi-segmented, elongated, without setae inside apical hook (Fig. 21C, D).
Affinities. This species is similar to the smaller G. pictipes (TL = 1.45-1.6 mm) but readily separated by the different colour pattern on elytra. Furthermore, both species can be separated by the form of their median lobes and parameres (Figs 20, 21).
Etymology. The species is named after the type locality. The specific epithet is a substantive in the genitive case.
Distribution. South-western Australia. Widespread but always rare and in low population densities. A more coastal species, from around 100 km north of Perth to south of Augusta and eastwards to the Muir Lakes (Fig. 25).
Habitat. Seasonal, very shallow and exposed sedge swamps, pool and puddles on sandy bottom, with a thin layer of peat and rotten debris of sedges (Figs 28, 29). Gibbidessus rottnestensis sp. nov. tolerates slightly saline water as it was found at Preston Beach in a shallow lagoon near the coast. According to the data it is an early spring breeder. Most specimens were collected in September and October. In the Riverdale Wetland the species was syntopic with Gibbidessus atomus sp. nov. and G. davidi sp. nov. For the rich water beetle coenosis in Beeliar Regional Park near Perth see under Gibbidessus davidi sp. nov. In the seasonal swamps at Wongonderrah Road, near Nambung River Crossing, the species was collected with several hundred specimens of an undescribed Exocelina species and Hyderodes crassus Sharp, 1882; at Preston Beach north of Bunbury it was collected with Hyphydrus elegans (Montrouzier, 1860), Necterosoma darwinii, and Platynectes aenescens Sharp, 1882.

Discussion
South-western Australia has long been recognised as a hotspot of aquatic macroinvertebrate and microfaunal diversity (Horwitz 1997;Segers and Shiel 2003). Four of the six described species of Gibbidessus are elements of this endemic freshwater fauna, and constitute a significant qualitative contribution to the biodiversity of the region. Three species are distributed in the southeast, including one endemic species from Kangaroo Island. Most probably more intensive studies will reveal further species occurring along the lowland coastal areas of south-western and south-eastern Australia. Seven species of the genus are strictly lentic, appearing to be restricted to shallow and temporary pools, puddles, flooded meadows and seasonal sedge swamps in peatland areas or to very shallow waters at the edges of peaty lakes. One species, G. pederzanii sp. nov. was collected only at the edge of a shallow and slow-flowing forest creek. Occasionally, single specimens of G. pictipes and G. chipi have been found in slowly flowing or intermittent creeks. All species can be found in spring and early summer, and the majority of specimens have been collected between September and October. In the southern and more humid parts of south-western Australia, specimens of the new generation can be collected from November until January. Within any of their habitats, up to three species of the genus can be found (e.g., Riverdale Wetland); aggregations of several hundred specimens of at least one species are possible (e.g., Beeliar Regional Park in Perth and Lake Nalyerin). According to our experience, the occurrence of any Gibbidessus species indicates a high conservation value of the sampled water body or wetland.
The larvae of all species are still undescribed. The adults of all species seem to be capable of flight, but no specimens of any species have been obtained by operating light traps.
script. The Department of Conservation and Land Management is acknowledged for giving permission to conduct scientific research in Nature Reserves [Permit numbers: SF 003017 and NE 002348], and the Department of Sustainability and Environment in Victoria [Research Permit No. 10003840] is acknowledged for giving permission to conduct scientific research in National and State Parks. This work was supported by grants from Deutsche Forschungsgemeinschaft DFG (BA 2152/4-1, 6-1 and 7-1; HE 5729/1-1), as well as from the CLIMAQUA project funded by the Bundesministerium für Bildung und Forschung (BMBF) (grant # 01DR14001 to M. Balke). Lars Hendrich warmly thanks Emma Hendrich (Munich, Germany), Stephan Gottwald and Ingo Weckwerth (both Berlin, Germany) for their patience, assistance, and enthusiastic encouragement during several field trips.