Exocelina saltusholmesensis sp. n. is described from a single female collected in Holmes Jungle Reserve near Darwin in the Northern Territory, Australia. Phylogenetically, the new species is sister to a clade containing the epigean E. ferruginea (Sharp, 1882) and E. punctipennis (Lea, 1899) but well characterized by its smaller size, the much smaller eyes, vestigial wings and paler surface. Exocelina saltusholmesensis sp. n. was collected from a small pool in an intermittent and temporary small creek. The collecting circumstances suggest that this is an interstitial species, with morphological characters interpreted as adaptations to a strongly hidden if not mostly subterranean lifestyle.

Beetles, interstitial aquatic fauna, adaptation to subterranean life

The diving beetle genus Exocelina contains more than 140 described species, most of them in New Guinea and New Caledonia (Toussaint et al. 2014, 2015; Nilsson 2015). The majority of species occur in habitats associated with streams, but there were also four transitions to stagnant water habitats (Toussaint et al. 2015). Two highly derived stygobitic Australian species have been described to date, Exocelina abdita Balke et al. 2004 from Newhaven Station, Camel Bore, Northern Territory and E. rasjadi Watts & Humphreys, 2009 from Tjukurla calcrete, Lake Hopkins palaeovalley, Western Australia. Both species are blind (Fig. 2B, 3C), have vestigial wings, are depigmented and show a more or less pronounced constriction between base of pronotum and elytra (Fig. 2B) which is linked to reduction of flight muscles (Balke et al. 2004, Watts and Humphreys 2009). Another rarely collected Australian species, Exocelina australis (Clark, 1863), has smaller eyes than typical epigean species and is only lightly pigmented (Fig. 2A). This might indicate a hidden, interstitial lifestyle. The collection of the species from hyporheic environments of a number of localities in the Flinders Range also hints in this direction (Leys et al. 2010).

Here, we describe a new species collected from a small pool in the bed of a slowly flowing wet-season creek near Darwin. Morphological features such as reduced eyes and vestigal wings, light pigmentation and absence of the species from other nearby water bodies suggest this is an interstitial species providing a scenario for the transition from epigean to stygobitic life. This discovery highlights the hidden diversity of an underexplored habitat.

Morphological observations were made and measurements were taken with a Leica M205 C stereomicroscope. Specimens were illuminated by a diffuse 13W fluorescent double light tube.

A DNA sequence of the 3’ fragment of the mitochondrial cytochrome oxidase 1 gene was generated by Remko Leys at the South Australian Museum in Adelaide, using the oligonucleotide primers Jerry and Pat (TCC AAT GCA CTA ATC TGC CAT ATT A / CAA CAT TTA TTT TGA TTT TTT GG).

Digital images were taken with a Nikon D3X equipped with a bellow or expansion rings and lenses: Mitutoyo 10x ELWD Planapo or Leitz Photar 25/2.8. Illumination came from three compact Nikon flashes, and the instrument was moved on an Isel linear drive (https://www.isel.com/at/lineareinheiten-les6.html) or Cognisys Stackshot (for very small steps 1–7 µm) (www.cognisys-inc.com). Image stacks were combined using the method A in Helicon focus software (www.heliconsoft.com).

A number of West Palearctic diving beetle species are known from wells or from the interstitial of gravel banks along rivers. These were classified as semi-subterranean or interstitial species. Morphological characters hinting at this include the reduction of eyes, depigmentation, presence of long sensory setae and the reduction of wings (e.g. Fery et al. 2012; Hernando et al. 2012; Manuel 2013).

While the Australian ground water fauna is well studied and is now known to harbour a considerable diversity of diving beetle species (Watts and Humphreys 2009 and previous; Leys et al. 2003; Balke et al. 2004; Watts and Leijs 2005). Interstitial habitats remain much less explored although already known to contain specialised species (e.g. Limbodessus rivulus (Larson, 1994), apterous species of Carabhydrus Watts, 1978 (Hendrich and Watts 2009), Limbodessus occidentalis (Watts & Humphreys, 2004)). Here we add another candidate from interstitial fauna that might document the first steps into fully underground habitats and highlights the need for focused investigations of such habitats in Australia and beyond.

We thank Eduardo (“Edu”) Orduña (El Pont de Suert, Spain) and Hans Fery (Berlin) for his help composing the name properly and Remko Leys (Adelaide) for sharing DNA sequence data.