Ecological interactions on shells of Mactra isabelleana d'Orbigny, 1846 (Mollusca: Bivalvia) from southern Brazil: first record of a unique host–parasite interaction

Mollusk shells are common substrates for several groups of marine invertebrates, which use them as housing or refuge, often producing traces in the form of bioerosion. In dead associations (current or fossil), it is difficult to determine if bioerosion was produced postmortem or not. The objective of this study was to describe anomalous internal growths in shells of the bivalve Mactra isabelleana, relating these occurrences to bioerosion on the external face, inferring if it was produced while the mollusk was still alive or not. To do so, 552 valves of M. isabelleana from the Cassino, Hermenegildo and Concheiros do Albardão beaches, southern Brazil, were examined for external face bioerosion caused by spionid polychaetes—U-shaped horizontal perforation (Caulostrepsis isp.)—and internal shell alterations. The preferred location for bioerosion was analyzed based on a sectorization of valves into four quadrants (Q1 and Q3 posterior, and Q2 and Q4 anterior) and by counting external perforations and internal alterations in each. From the total M. isabelleana valves, 199 presented bioerosion Caulostrepsis on their external face. Inner face structures found were attached pearls, blisters and sheets. Sheets-type alterations are described herein for the first time. The most commonly affected quadrants by Caulostrepsis showed correspondence to internal alterations (Q1 and Q3) in the posterior region of the shell, indicating that the bivalve was alive when perforated. This is the first description of a parasite–host interaction in M. isabelleana shells for southern Brazil and the first time that parasitism is related to the ichnogenus Caulostrepsis.