Saturday 22 January 2022

Languidipes lithophagus: A rock-boring Mayfly from the Bago River system of Myanmar.

Rock-borings in marine environments have been extensively studied, both in modern and fossil environments, with the organisms which make these borings known to include Molluscs, Crustaceans, Polychaete and Sipunculid worms, Sea Urchins, Sponges, and Bryozoans. These organisms are considered to be major shapers of their environments, extensively modifying rocky marine substrates and playing an important role in coastal erosion and the evolution of coastal profiles. In contrast to this, freshwater rock-borers have been very little studied. Most known freshwater rock-borers are members of marine rock-boring groups which have adapted to the freshwater environment. For example, bioerosion in the shells of freshwater Molluscs in Southeast Asia, India, and Sri Lanka is known to be caused by Caobangiid Worms, which are Sabellid Polychaetes. Freshwater Mollusc shell borings are also known to be caused by endolithic Cyanobacteria in North America and Argentina, and shell borings of unknown origin have been reported in subfossil and recent freshwater Bivalve shells from North America. Macroborings in rocks from freshwater environments are well recorded in the fossil record, predominantly from calcareous hardground environments, although these are less well studied than their marine counterparts. 

The first modern Invertebrate boring into silicate rocks in a freshwater environment was not discovered until 2018, when the Pidock (rock boring Bivalve) Lignopholas fluminalis was discovered in the middle reaches of the Kaladan River in Myanmar. This species had previously been recorded from estuarine environments with variable salinities, but had not been recorded in a fully freshwater environment till this time. In 2019 a second Bivalve, the Shipworm Lithoredo abatanica, was found boring into limestone in the lower reaches of a small river on the Bohol Island, the Philippines. This species does appear to be found exclusively in freshwater environments, although its closest relatives are all marine. The larvae of a number of Insect species, including Mayflies, Ephemeroptera, Nonbiting Midges, Chironomidae, and Caddisflies, Trichoptera, have been reported burrowing into firmgrounds such as claystones, sandstones, shales, and marls.

In a paper published in the journal Materials Degradation on 20 January 2022, Ivan Bolotov and Alexander Kondakov of the Laverov Federal Center for Integrated Arctic Research oVasily Yapaskurtf the Ural Branch of the Russian Academy of Sciences a d the Northern Arctic Federal University, Grigory Potapov, also of the Laverov Federal Center for Integrated Arctic Research, Dmitry Palatov of the Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences, Nyein Chan of the Myanmar Programme of Fauna & Flora International, Zau Lunn, also of the Myanmar Programme of Fauna & Flora International, and of the Biology Department at the University of New Brunswick, Galina Bovykina, also of the Laverov Federal Center for Integrated Arctic Research and the Northern Arctic Federal University, Yulia Chapurina, Yulia Kolosova, Elizaveta Spitsyna, Vitaly Spitsyn, and Artyom Lyubas, again of the Alexander Kondakov of the Laverov Federal Center for Integrated Arctic Research, Mikhail Gofarov and Ilya Vikhrev, again of the Federal Center for Integrated Arctic Research and the Northern Arctic Federal University,  of the Faculty of Geology at the Lomonosov Moscow State University, Andrey Bychkov, also of the Faculty of Geology at the Lomonosov Moscow State University, and of Vernadsky Institute of Geochemistry and Analytical Chemistry at the Russian Academy of Sciences, and Oleg Pokrovsky of Geosciences and Environment Toulouse, and the BIO-GEO-CLIM Laboratory at Tomsk State University, describe a new species of Mayfly with rock-boring larvae from the Bago River basin of Myanmar.

Bioerosion was observed in siliceous rocks at two locations on the Bago River, one on the middle reaches and one on the upper part, and both where the water was freshwater and free from tidal influence. The site on the middle reaches of the river was about 40 m above sealevel, where the river crosses a continuous outcrop of Miocene siltstone. Borings can be seen on the right bank, and across the river bottom.

Study area and sampling localities. (a) Map of Myanmar with location of the study area. (b) Map of the Bago River basin (light red filling). The red stars indicate freshwater macrobioerosion sites. Bolotov et al. (2022).

The borings are elongate tunnel-like structures, penetrating 20-30 mm into the rock, oriented horizontally, sharing two circular end openings (apertures) at the outer surface. Many of the borings contained Mayfly larvae, others were empty, and often filled with clay. The borings were lined with a silky material. The rock surface was covered by elongate groves, apparently formed by the erosion of burrows. These groves extended onto the surface of the youngest river terrace, above the level of the present floodplain.

Examples of macrobioerosion in the Bago River, Myanmar. (a) Right shore with continuous siltstone outcrop at the middle reaches of the river. The red arrows show the location of submerged rocks with bioerosion traces. (b) Submerged part of the siltstone rock outcrop (indicated by the red arrows) at the middle reaches of the river. (c) Emerged siltstone rocks with bioerosion traces (indicated by the red arrows) at the upper reaches of the river. (d) Close-up view of siltstone outcrop with multiple bioerosion traces at the upstream section of the river. (e) Siltstone fragment densely covered by sub-recent bioerosion traces from the upper reaches of the river (plan view). The yellow arrows show crusts of a freshwater Sponge species. (f) Siltstone surface with a dense network of sub-recent bioerosion traces from the upper reaches of the river (plan view). The blue arrows show the macroborings filled with clay. The green arrow shows a pea clam in a bioerosion groove (Bivalvia: Sphaeriidae). Scale bars are 10 mm (e(, (f). Mikhail Gofarov & Ilya Vikhrev in Bolotov et al. (2022).

The upper section of the river is 195 m above sealevel, and cuts through Miocene clays, with outcrops of siltstone rock. These were exposed during the dry season, when the site was visited, but would have been covered in the rainy season when waters were higher. The surfaces of these rocky outcrops were completely covered by grooves and borings similar to those seen at the lower site. These burrows were mostly abandoned, although some contained dead Insect larvae, Eroded burrows had often been occupied by an unknown freshwater Sponge. Some burrow-bearing surfaces were covered by clay, suggesting the burrows on them were of some age.

Siltstone rock substrate with borings and living nymphs of the rock-boring Mayfly species Languidipes lithophagus from the middle reaches of the Bago River, Myanmar. (a) Rock fragment with borings (lateral view). (b) Rock fragment showing apertures (circular openings) and partly eroded borings (plan view). (c) Rock surface with apertures and partly eroded abandoned borings. (d) Living nymphs in their borings. (e) Living nymph in its boring. The white arrow shows a fragment of a silky substance covering the inner side of the boring. Scale bars are 10 mm (a), (c), 20 mm (b), and 5 mm (d), (e). Ilya Vikhrev in Bolotov et al. (2022).

Genetic analysis of the larvae confirmed that they belong to the genus Languidipes, but not within either of the two species currently assigned to that genus; Languidipes corporaali, which has wood boring nymphs and which has been recorded from Indonesia, Malaysia, and Thailand, and Languidipes taprobanes, which is known from India and Sri Lanka, and which also has wood-boring nymphs. A third, undescribed, species of Languidipes is also known from a single female specimen collected in Assam, India, over a century ago. The genus Languidipes was split from the African wood-boring genus Povilla in 1984, upon the basis of morphological features; Bolotov et al.'s genetic study confirmed that these genera are related, but clearly distinct.

Maximum likelihood phylogeny of boring and burrowing Mayflies (Ephemeroptera) based on the COI gene sequences. Bolotov et al. (2022).

The new species is named Languidipes lithophagus, where 'lithophagus' means 'rock-eater'. The description is based entirely upon mature nymphs, with the adults being unknown. These are 11.3–17.9 mm in length, with a head semicircular in shape when seen from above, with bristles on its labrum (upper lip) and mandibles. The abdomen is also covered in setae (bristles), and the limbs are biramous (branching), with the upper branches forming gills, as in many Crustaceans. The nymphs are pale yellow or white in colour, with two brown stripes on the upper part of the head, and black mandibles. 

Nymph of the rock-boring Mayfly species Languidipes lithophagus from Bago River, Myanmar. (a) Holotype RMBH N54_8 (dorsal view); (b) holotype (lateral view); (c) left and (d) right mandible (dorsal view). Scale bars are 5 mm (a), (b) and 1 mm (c), (d). Elizaveta Spitsyna in Bolotov et al. (2022).

The nymphs are filter feeders, obtaining food from water pumped through their burrows. The species is restricted to the Bago River Basin, which is impacted by a variety of anthropogenic loads arising from agricultural and urban land use, deforestation, sewage inputs, garbage littering, and general habitat degradation, causing Bolotov et al. to suggest that it may be Endangered under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

Morphological features and neoichnological model of the rock-boring Mayfly species Languidipes lithophagus traces from the middle section of the Bago River, Myanmar. (a) Close-up view of the apertures (plan view). The white arrows show minute fractures tracing the upper layer of the borings. The yellow arrows show larger fractures corresponding to the initial stage of rock erosion leading to the origin of grooves (i.e. borings, the upper layer of whose was lost). (b) Plan view of a groove with concave bottom originated via partial erosion of the boring. The groove bears fragments of a silky substance produced by the tracemaker (shown by white arrows). (c) Transverse cross-section of the borings showing their circular tunnel-like shape. Neoichnological model of the boring: lateral view of the longitudinal section (d) and plan view of the two apertures at the rock surface (e). Orange filling indicates the boring. Scale bars are 10 mm (a) and 5 mm (b)–(e). Artyom Lyubas and Ivan Bolotov in Bolotov et al. (2022).

As far as Bolotov et al. are aware, this is the first recorded instance of freshwater Insect larvae tunnelling into hardground substrates anywhere in the world; burrowing into firmground substrates has been recorded, but such substrates soften when wet, unlike to rocky siltstones of the Bago River Basin. Languidipes lithophagus also appears to be the first known rock-boring freshwater Animal not descended from a rock-boring marine Animal anywhere in the world, with both previously recorded species with this habit being Bivalves belonging to primarily marine groups. 

Siltstone outcrop at the midstream bioerosion site of the Bago River. The insert illustrates a rock surface with bioerosion traces of the rock-boring mayfly species Languidipes lithophagus  (indicated by the yellow arrow) at the youngest (sub-recent) river terrace. Mikhail Gofarov in Bolotov et al. (2022).

The relatives of Languidipes lithophagus do, however, do share some ecological similarities with the relatives of the two Bivalve species. The other members of the genus Languidipes bore into wood and bamboo, as do the closest relatives of Lignopholas fluminalis, which comes from a group of primarily wood-boring Piddocks (most Piddocks are rock-boring), and the Shipworm Lithoredo abatanica (almost all Shipworms are wood-boring). The shift from wood-boring, which requires considerable mechanical strength, to rock boring, which requires even more, is plausibly easier than switching directly from free-living to wood-boring.

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