Research Article |
Corresponding author: Michael Balke ( balke.m@snsb.de ) Academic editor: Oana Teodora Moldovan
© 2020 Michael Balke, Ignacio Ribera.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Balke M, Ribera I (2020) A subterranean species of Exocelina diving beetle from the Malay Peninsula filling a 4,000 km distribution gap between Melanesia and southern China. Subterranean Biology 34: 25-37. https://doi.org/10.3897/subtbiol.34.50148
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We describe a new subterranean species of the genus Exocelina Broun, 1886 (Coleoptera: Dytiscidae) from the Malay Peninsula. Almost all of the 196 species of that genus are epigean and distributed mainly in New Guinea, Australia, Oceania and New Caledonia. One epigean species is, however, known from China. The discovery of a species on the Malay Peninsula fills that distribution gap to some degree.
Beetles, blind subterranean species, disjunct distribution, new species
Here we report the discovery of a new subterranean diving beetle from the Malay Peninsula. This species was placed in the Dytiscidae, subfamily Copelatinae based on morphological characters using the key of Miller and Bergsten (
Specimens were studied with a Leica M205C stereo microscope at 10–160x. Images were taken with a Canon EOS 5DS camera fitted with a Mitutoyo 10x ELWD Plan Apo objective attached to a Carl Zeiss Jena Sonnar 3.5 / 135 MC as focus lens. Illumination was with two to four 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.
Drawings were produced with a camera lucida, first sketched with pencil on paper, then photographed and digitally inked using an iPad Pro and the Concepts as well as MediBang Paint APPs.
One paratype male of the new species (voucher number IBE-AN1160) was used for a non-destructive DNA extraction using a commercial kit (Qiagen DNeasy Tissue Kit). We successfully amplified six mitochondrial and nuclear genes in five sequencing reactions, two cytochrome c oxidase subunit I fragments (COI-5’ -the "barcode"- and COI-3’), 5’ end of rrnL RNA plus leucine tRNA transfer (tRNA-L1) plus 5’ end of NADH dehydrogenase subunit I (NAD1), and one internal fragment of both small ribosomal unit (18S RNA) and Histone 3 (H3) (see
The combined dataset was analysed with a fast maximum likelihood search as implemented in IQ-TREE v1.6 (
IBE Institute of Evolutionary Biology, Barcelona, Spain
KSc Kazuki Sugaya collection, Zama, Japan
NMW Naturhistorisches Museum Wien, Austria
ZSM Zoologische Staatssammlung München, München, Germany
Malaysia, Pahang, Cameron Highlands, Tanah Rata, 4.474705, 101.384043.
Holotype male (ZSM): Malaysia, Pahang, Cameron Highlands, Tanah Rata, Mount Berembun, 4.474705, 101.384043, 1,500m, 27.–29.ii.2012, K. Sugaya leg.
Paratypes : 4 males (1 used for DNA extraction and sequencing, voucher No. IBE-AN1160) and 2 females, same label data as holotype (IBE, KSc, NMW, ZSM).
Size and shape
: Smallest Exocelina known (length of holotype including head 2.7 mm, length without head 2.4 mm, greatest width 1.0 mm). Abdomen comparably parallel sided; pronotum also comparably parallel sided, slightly constricted before base, hind angles produced backwards (Fig.
Testaceous and slightly translucent (Figs
Head and pronotum with distinct microreticulation formed by small regular cells and fine moderately dense punctation. Elytra with distinct microreticulation formed by small regular cells and dense, coarse, setiferous punctation (Fig.
Eyes fully reduced, with only small black scars remaining on surface of head (Figs
Antennomeres filiform to slightly moniliform (Fig.
Variation. Length of beetle including head 2.4–2.8 mm. Two paratypes are darker orange (see Fig.
Named after Kazuki Sugaya, the discoverer of this species.
This species differs from all other Dytiscidae by: Copelatinae with reduced eyes; beetle length < 3 mm; body with well visible microreticulation; prosternal process short and deflexed; metacoxal processes small, more elongate oval (in other Copelatinae, including the groundwater species Exocelina abdita
Collected from two helocrenes on a slope in forested area. The beetles were observed creeping around and were not swimming when observed (K. Sugaya personal communication 2019) (Fig.
The best evolutionary model fitting the data according to Modelfinder was a GTR+F for all partitions. Exocelina sugayai sp. nov. was recovered deeply subordinated within Exocelina, as the sister of the Chinese E. shizong Balke & Bergsten, 2003 and the New Caledonian E. nehoue Balke et al., 2014. These three species are part of a clade (“C4” in
Exocelina sugayai sp. nov. male A eye in lateral view B detail of head and pronotum C surface sculpture on base of head and anterior margin of pronotum D detail of posterior angle of pronotum E detail of surface sculpture on base of elytron F detail of lateral view of elytral and pronotal base and head.
Most species of Exocelina inhabit stream associated (lotic) habitats, specifically areas of stagnant water at the edge of streams and creeks, the interstitial and tiniest of water holes on riverbanks, as well as small puddles in intermittent creeks including the source area that might only have occasional water flow after rainfalls (see habitat photos in
The lotic beetles often hide in the gravel when disturbed, and observations of M. Balke in New Guinea suggest that the interstitial of riverbanks is often utilized by these beetles, possibly to avoid downstream drift. The beetles seem to avoid habitat with fine, dense substrates, which we suggest make it hard to hide as such substrate clogs the space between stones and pebbles (see also
This lifestyle could be interpreted as a preadaptation for interstitial or stygobitic life. In fact, some Australian species seem to mainly inhabit the interstitial, and have been suggested to provide a scenario for the transition from epigean to stygobitic life (
Biogeographically, the occurrence of Southeast Asian and a Chinese species of Exocelina remains enigmatic. The origin of the clade containing these species was estimated as at least 10 million years ago (“C4”
Simplified phylogenetic tree obtained with IQ-TREE using the DNA sequence dataset of
We express our sincere thanks to Kazuki Sugaya for sending the specimens studied here to the senior author, and Anabela Cardoso for laboratory work. Helena Shaverdo and Günther Wewalka (Vienna) provided very valuable reviews of the submitted manuscript. This research was supported by DFG Ba2152/4-1, 7-1, 11-1, 11-2 and 24-1. Michael Balke acknowledges support from the EU SYNTHESYS program projects FR-TAF-6972 and GB-TAF-6776.