Abstract
Macrobioerosion of mineral substrates in fresh water is a little-known geological process. Two examples of rock-boring bivalve molluscs were recently described from freshwater environments. To the best of our knowledge, rock-boring freshwater insects were previously unknown. Here, we report on the discovery of insect larvae boring into submerged siltstone (aleurolite) rocks in tropical Asia. These larvae belong to a new mayfly species and perform their borings using enlarged mandibles. Their traces represent a horizontally oriented, tunnel-like macroboring with two apertures. To date, only three rock-boring animals are known to occur in fresh water globally: a mayfly, a piddock, and a shipworm. 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).
Highlights
A wide array of rock-boring animals and their bioerosion traces was described from marine environments[1,2,3]
Rock borers are extremely rare in freshwater habitats and the relevant organisms are usually derived from marine bioeroders[23]
Siltstone rocks with subrecent traces of the same morphology were recorded at the surface of the youngest river terrace, above the level of the present floodplain (Supplementary Fig. 1)
Summary
A wide array of rock-boring animals and their bioerosion traces was described from marine environments[1,2,3]. There were a few reports describing bioerosion structures in freshwater mollusc shells that are largely associated with caobangiid polychaetes (Annelida: Sabellidae)[25,26]. 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]. Other works describe microborings in freshwater mollusc shells produced by endolithic cyanobacteria in North America[30] and Argentina[31]
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