Abstract
Teredinid bivalves, commonly referred to as shipworms, are known for their propensity to inhabit, bioerode, and digest woody substrates across a range of brackish and fully marine settings. Shipworm body fossils and/or their borings, which are most allied with the ichnotaxon Teredolites longissimus, are found in wood preserved in sedimentary sequences ranging in age from Early Cretaceous to Recent and traditionally they have been regarded as evidence of marginal marine or marine depositional environments. Recent studies associated with the Philippine Mollusk Symbiont International Collaboration Biodiversity Group (PMS-ICBG) expedition on the island of Bohol, Philippines, have identified a new shipworm taxon (Lithoredo abatanica) that is responsible for macrobioerosion of a moderately indurated Neogene foraminiferal packstone cropping out along a freshwater reach of the Abatan River. In the process of drilling into and ingesting the limestone, these shipworms produce elongate borings that expand in diameter very gradually toward distal termini, exhibit sinuous or highly contorted axes and circular transverse outlines, and are lined along most of their length by a calcite tube. Given their strong resemblance to T. longissimus produced in wood but their unusual occurrence in a lithic substrate, these shipworm borings can be regarded as incipient Gastrochaenolites or, alternatively, as Apectoichnus. The alternate names reflect that the borings provide a testbed for ideas of the appropriateness of substrate as an ichnotaxobasis. The discovery of previously unrecognized shipworm borings in lithic substrates and the co-occurrence of another shipworm (Nausitora) in submerged logs in the same freshwater setting have implications for interpreting depositional conditions based on fossil teredinids or their ichnofossils. Of equal significance, the Abatan River study demonstrates that macrobioerosion in freshwater systems may be just as important as it is in marine systems with regard to habitat creation and landscape development. L. abatanica serve as ecosystems engineers in the sense that networks of their abandoned borings provide habitats for a variety of nestling invertebrates, and associated bioerosion undoubtedly enhances rates of mechanical and chemical degradation, thus influencing the Abatan River profile.
Highlights
Teredinid bivalves are colloquially referred to as shipworms and, more informally, as the “termites of the sea” [1]
New work completed as part of the Philippine Mollusk Symbiont International Collaboration Biodiversity Group (PMS-ICBG) expedition on the island of Bohol, Philippines, has led to the description and naming of a novel shipworm taxon that is responsible for extensive macrobioerosion of a lithic substrate cropping out in the bed and banks beneath freshwaters of the Abatan River
This new taxon is described in detail elsewhere [12]. The objectives of this contribution are to: (1) describe the character of the substrates and the borings therein; (2) compare these shipworm borings with previously defined ichnotaxa commonly ascribed to bivalves; (3) discuss the implications of this new finding for paleoenvironmental inferences traditionally drawn from fossil occurrences of shipworms and/or their biogenic structures; and (4) address the potential roles of shipworms as ecosystem engineers and geomorphic agents in freshwater aquatic systems
Summary
Teredinid bivalves are colloquially referred to as shipworms and, more informally, as the “termites of the sea” [1]. New work completed as part of the Philippine Mollusk Symbiont International Collaboration Biodiversity Group (PMS-ICBG) expedition on the island of Bohol, Philippines, has led to the description and naming of a novel shipworm taxon that is responsible for extensive macrobioerosion of a lithic substrate (limestone) cropping out in the bed and banks beneath freshwaters of the Abatan River This new taxon is described in detail elsewhere [12]. Limestone macroborings at the Abatan River site are all attributed to shipworms, which on the basis of hard part morphology, soft tissue anatomy, and molecular phylogenetic data represent a recently described genus and species—Lithoredo abatanica [12] Like most teredinids, this new form is characterized by a long vermiform body, small valves at the anterior end, calcite tube linings, and siphons equipped with a pair of calcite pallets that allow the tight sealing of tunnel entrances (Fig 4). Both the interior bumps and exterior dimples can be considered xenoglyphs
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