The present study extends our knowledge on bioerosion in freshwater settings via studying a previously unknown case of a siltstone (aleurolite) boring produced by mayfly nymphs from tropical Asia (Myanmar). These burrows were produced by larval stages of mayflies (Ephemeroptera), nonbiting midges (Diptera: Chironomidae), and caddisflies (Trichoptera). This unusual bivalve species, Lithoredo abatanica Shipway, Distel & Rosenberg, 2019, a member of the Teredinidae (commonly known as shipworms), drills into and ingests the limestone substrates in the lower reaches of a small river on the Bohol Island 41.Ī small body of available literature describes burrows made by recent freshwater insects in various firmgrounds such as claystones, sandstones, shales, and marls 42, 43, 44. A year later, the second freshwater rock-boring animal was described from the Philippines 40. The clavate borings of this species in siltstone rocks are covered by a microbial biofilm, the members of which could promote bivalve bioerosion through rock weathering by dissolving Mn-rich chlorites 39. This species was known to occur exclusively in estuaries 38 but the Kaladan’s population is adapted to live in fresh water 24. It is a close relative of marine piddocks and belongs to the bivalve species Lignopholas fluminalis (Blanford, 1867) (Pholadidae). The first modern silicate rock-boring freshwater organism was discovered in the middle reaches of the Kaladan River in Myanmar in 2018 24. Most of these ichnotaxa were described in calcareous hardgrounds 3. A variety of trace fossils is known to occur in freshwater deposits, including those associated with putative invertebrate macroborings 3, 36, 37. Other works describe microborings in freshwater mollusc shells produced by endolithic cyanobacteria in North America 30 and Argentina 31.Ĭonversely, available paleoichnological data reveal that fossil macrobioerosion structures are widespread in freshwater settings globally 32, 33, 34, although these settings have received little attention compared with their marine counterparts 35. Microborings in subfossil and recent freshwater bivalve shells were also discovered in North America but the producers of these boreholes remain unknown 28, 29. These minute worms are an exclusively freshwater group that contains seven species boring into shells of gastropods and bivalves in Southeast Asia, India, and Sri Lanka 26, 27. There were a few reports describing bioerosion structures in freshwater mollusc shells that are largely associated with caobangiid polychaetes (Annelida: Sabellidae) 25, 26. Until recently, macrobioerosion of rock substrates in modern freshwater environments was unknown 23, 24. Furthermore, a number of marine geological processes such as sedimentation, erosion of rock surfaces, and evolution of coastal profiles and coral reefs are greatly influenced by rock-boring animals globally 17, 18, 19, 20, 21, 22.Ĭonversely, rock borers are extremely rare in freshwater habitats and the relevant organisms are usually derived from marine bioeroders 23. Seawater rock borers play a significant role as ecosystem engineers that may greatly increase the complexity of hardground substrates and biodiversity of associated communities 16. These animals are common in recent and fossil sea beds 4 and belong to multiple invertebrate taxa such as molluscs 5, 6, crustaceans 7, polychaetes 8, sipunculids 9, sea urchins 10, 11, 12, sponges 13, 14, and bryozoans 2, 15. All the three species originated within primarily wood-boring clades, indicating a simplified evolutionary shift from wood to hardground substrate based on a set of morphological and anatomical preadaptations evolved in wood borers (e.g., massive larval mandibular tusks in mayflies and specific body, shell, and muscle structure in bivalves).Ī wide array of rock-boring animals and their bioerosion traces was described from marine environments 1, 2, 3. To date, only three rock-boring animals are known to occur in fresh water globally: a mayfly, a piddock, and a shipworm. Their traces represent a horizontally oriented, tunnel-like macroboring with two apertures. These larvae belong to a new mayfly species and perform their borings using enlarged mandibles. Here, we report on the discovery of insect larvae boring into submerged siltstone (aleurolite) rocks in tropical Asia. To the best of our knowledge, rock-boring freshwater insects were previously unknown. Two examples of rock-boring bivalve molluscs were recently described from freshwater environments. Macrobioerosion of mineral substrates in fresh water is a little-known geological process.
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