Rhithrodytes pantaleonii, Toledo & Bilton & Balke & Schizzerotto & Villastrigo, 2023

Toledo, Mario, Bilton, David T., Balke, Michael, Schizzerotto, Antonio & Villastrigo, Adrián, 2023, A new, putatively semisubterranean, Rhithrodytes diving beetle from southwestern Sardinia (Coleoptera: Dytiscidae), Acta Entomologica Musei Nationalis Pragae (Acta. Ent. Mus. Natl. Pragae) 63 (1), pp. 125-133 : 127-130

Rhithrodytes pantaleonii sp. nov.

( Figs 1–5 )

Type locality. Italy, Sardinia, Carbonia-Iglesias province, Riu Siuru 4 km NW Domusnovas, 39°21.55′N 08°37.14′E, 260 m a.s.l.

Type material. HOLOTYPE: J, labelled: “ 16.VI.2022 Italy, Sardinia, CI / Riu Siuru , 4 km NW Domusnovas / 39°21.55’ N 08°37.14’ E, 260 m / Balke, Bilton,Schizzerotto & Toledo leg.” ( ZSM), with red holotype label and DNA voucher label: “DNA voucher MB8956 ” ( ZSM) . PARATYPE: J same data as holotype and with red paratype label ( CTB).

Description. Habitus. Body shape ( Fig. 2 ) elongate- -oblong, no discontinuity between posterior angles of pronotum and base of elytra, lateral sides of elytra almost parallel, maximum width just before middle of elytral length; dorsoventrally flat ( Fig. 3A ); colouration pale on both dorsal and ventral sides, ventral segments and parts of elytra almost transparent. Appendages uniformly reddish-yellow; eyes small ( Fig. 4A ).

Head relatively large in proportion, anterior margin forming a rather prominent, regular curve. Eyes apparently functional, but small in proportion to head ( Fig. 4A ); ommatidia irregular in size, maximum 9 in longest series. Surface weakly shining, with visibly impressed isodiametric microreticulation and sparse fine punctures ( Fig. 4A ). Colouration uniformly reddish-yellow.Antennomeres 3–10 as long as or longer than wide; terminal antennomere slightly darkened.

Pronotum subrectangular; lateral sides almost subparallel posteriorly, rounded anteriorly; weakly but visibly sinuate in lateral view. Anterior angles rather prominent; lateral rim well developed along each side.Anterior margin with impressed, transverse irregular puncture row; with sparse and fine punctures between lateral rims and disc; disc and scutellar process without punctures; surface with well impressed isodiametric microreticulation. Colouration reddish-yellow, with indistinct infuscations along anterior and posterior margins.

Elytra elongated, almost parallel-sided in anterior half, apical third regularly acuminate; lateral margins straight in lateral view, gently curving anteriorly over approximately basal sixth of length ( Fig. 3A ). Microreticulation isodiametric, more impressed than on head and pronotum, with fine and irregular discal punctures arranged longitudinally, most of them bearing yellow decumbent setae. Colouration yellow with wide, poorly defined, weakly darker area posteriorly along suture and slightly expanded apically. Venation of metathoracic wings visible through elytra.

Legs. Metatibiae and metatarsi slender and quite long, with very sparse natatorial setae on inner face. Pro- and mesotarsi not strongly expanded; protarsal claws unmodified.

Venter ( Fig. 2A ) uniformly reddish-yellow; entirely isodiametrically microreticulated, with additional impressed pits. Prosternal process elongate, rounded apically, barely touching anteromedial metaventral process. Metacoxal lines clearly visible, reaching metaventrite. Epipleuron quite abruptly narrowed before mid-length; oblique carina visible near shoulder. Abdominal ventrites II and III fused with suture barely visible.

Male genitalia. Median lobe ( Fig. 2C ), straight in lateral view, with apical hook strongly developed; parallel-sided in ventral view, with acuminate tip visible at apex; acuminate tip strongly developed and clearly visible in ventro-apical view. Parameres ( Fig. 2C ) elongate, with narrow, rounded, setose apex.

Measurements. Holotype: BL = 2.7 mm; BL-H = 2.35; EW = 1.2 mm. Paratype: BL = 2.7 mm; BL-H = 2.35; EW = 1.2 mm.

Differential diagnosis. An oblong, flat, almost parallel sided Rhithrodytes ( Fig. 2 ), with small eyes in proportion to the head. Colouration uniformly pale yellowish-reddish (note that the holotype in Figs 3 & 4 has darkened somewhat following DNA extraction; for more accurate representation of colour, see Fig. 2A ). Natatory setae reduced. On the basis of these characters, Rhithrodytes pantaleonii sp. nov. has a very distinctive external morphology and cannot be confused with any other species of the genus. The male genitalia ( Fig. 2C ) are rather similar to those of R. sexguttatus ( Fig. 2D ), differing by the straighter profile of the median lobe in lateral view, with a narrower apical hook, by the narrower, more strongly acuminate apex of the median lobe in ventral view, and by the narrower apex of the parameres.

Etymology. The new species is dedicated to Prof. Roberto Pantaleoni (University of Sassari), who kindly supported our trip in Sardinia. It is a noun in the genitive case, standing in apposition.

Ecology. The Riu Siuru is a small, rocky lowland stream, flowing in a North-South direction over a calcareous substrate. Due the relative drought conditions in Spring- Summer 2022, we found the stream partially dried out and therefore focused on a ca. 15 m reach which had numerous residual pools and some flow ( Fig. 5 ). Areas immediately above and below this stretch were found to be entirely dry. The new species was collected by digging between gravel and sand, very close to the water’s edge and was very rare or at least difficult to find with hand nets. Other associated aquatic Coleoptera were Gyrinidae : Gyrinus urinator Illiger, 1808 , Gyrinus caspius Ménétriés, 1832 ; Dytiscidae : Agabus binotatus Aubé, 1837 , A. rufulus Fairmaire, 1859 , Colymbetes schildknechti Dettner, 1983 , Meladema lepidoptera Bilton & Ribera, 2017 , Bidessus minutissimus (Germar, 1824) , B. saucius (Desbrochers, 1871) , Hydroglyphus geminus (Fabricius, 1792) , Hydroporus analis Aubé, 1838 , Graptodytes exsanguis (Bedel, 1925) , G. flavipes (Olivier, 1795) , Stictonectes optatus (Seidlitz, 1887) , Deronectes moestus moestus (Fairmaire, 1858) , Scarodytes fuscitarsis (Aubé, 1838) ; Hydrophilidae : Laccobius neapolitanus Rottenberg, 1874 , Anacaena globulus (Paykull, 1798) , Limnohydrobius convexus (Brullé, 1835) ; Hydraenidae : Hydraena sardoa Binaghi, 1961 , H. subsequens Rey, 1886 , Limnebius perparvulus Rey, 1884 , L. mucronatus Baudi, 1872 , Ochthebius dilatatus Stephens, 1829 ; Limnichidae : Pelochares versicolor (Waltl, 1838) . Of these, Graptodytes exsanguis and Hydraena sardoa were the most abundant.

Figures

Fig. 1. Molecular phylogeny of Rhithrodytes and related genera of Hydroporini obtained with BEAST.Values beside nodes represent posterior probabilities, and 95% credibility intervals for estimated ages are shown with grey bars. Inset at top left shows the approximate distribution of land in the Mediterranean Basin in the Oligocene/Early Miocene (modified after ANDREUCCI et al. 2017); black arrow – Corsico-Sardinia, red arrow – Sardinian Seaway. Beetles at right hand side as follows, top to bottom: Stictonectes rebeccae Bilton, 2013, Rhithrodytes pantaleonii sp. nov., Rhithrodytes sexguttatus (Aubé, 1838).

Fig. 2. Sardinian Rhithrodytes species. A – R. pantaleonii sp. nov., paratype, dorsal habitus; B – R. sexguttatus (Aubé, 1838), dorsal habitus; C – R. pantaleonii sp. nov., holotype, male genitalia; D – R. sexguttatus (Aubé, 1838), male genitalia. In C & D, images represent, from left to right: median lobe, lateral view; median lobe, ventral view; median lobe apex; paramere. Scale bars A & B = 1 mm; C & D = 100 μm.

Fig. 3. Sardinian Rhithrodytes species, lateral view.A – R. pantaleonii sp. nov., holotype; B – R. sexguttatus (Aubé, 1838). Scale bar = 1 mm.

Fig. 4. Sardinian Rhithrodytes species, details of head and compound eyes.A – R. pantaleonii sp. nov., holotype; B – R. sexguttatus (Aubé, 1838). Scale bar = 100 μm.

Fig. 5. Type locality of R. pantaleonii sp. nov., Italy, Sardinia, Carbonia-Iglesias province, Riu Siuru 4 km NW Domusnovas, 39°21.55′N 08°37.14′E, 260 m a.s.l. A – Overview of stream with (left to right) Mario Toledo, Antonio Schizzerotto & David Bilton; B – Detail of microhabitat, Rhithrodytes collected from edges of right-hand bank. Photos Michael Balke.