Boring of Intertidal Sandstones by Isopod Sphaeroma triste in NW Borneo (Sarawak, Malaysia)

Abstract

© 2020 Coastal Education and Research Foundation, Inc. Sphaeromatid isopods are known for their ability to bore into wood and friable rock and to cause damage to mangrove plant roots, wooden structures, and polystyrene dock floats in the intertidal zone. The ability of isopods to bore extensively into rock and accelerate coastal erosion is less well known and has not been previously reported in Malaysia. This study investigated the presence, the identity, and the erosive effect of rock-boring isopods in sandstones of the NW Borneo coastal region (Sarawak, East Malaysia). A multidisciplinary approach was used, including field and laboratory observations (geological and biological) of rocks and wood. This study revealed that abundant cylindrical borings in soft intertidal rock are created by the boring isopod Sphaeroma triste (S. triste). Bioerosion by this species can result in the direct removal of up to 50% of the exposed surface of the rock and penetrate the rock up to a few centimeters depth. This has a significant but localised impact on coastal erosion, contributing to the development of concavities in the rock, enlargement of joints, deepening of wave cut notches, widening of rock pools, and erosion of fallen blocks and sea-cave walls. There is evidence of modification of the isopods' mandible incisor processes by abrasion during rock boring. Although several Sphaeromatid species are known to bore into soft rocks, this is the first report and comprehensive description of boring into sandstone substrates by S. triste. The S. triste borings are compared with those made by other species reported elsewhere. In terms of neoichnology, the borings belong to deep-tier Trypanites ichnofacies, and fossil equivalents may be useful in palaeogeographic reconstructions of ancient shorelines, although they may have poor preservation potential.