INVASIVE SPECIES III Parasitism and fitness of invaders: oligochaete Chaetogaster limnaei produces gill damage and increased respiration rates in freshwater Asian clams Florencia Liquin . Leandro A. Hu ¨ nicken . Florencia Arrighetti . Dora Davies . Esteban M. Paolucci . Francisco Sylvester Received: 8 April 2020 / Revised: 3 September 2020 / Accepted: 24 September 2020 Ó Springer Nature Switzerland AG 2020 Abstract Biological interactions between invasive and resident species are perceived as a key factor mediating impacts in invaded ecosystems, although symbiotic interactions have received relatively little attention. To evaluate the association between the invasive clam Corbicula fluminea and its oligochaete symbiont Chaetogaster limnaei, we conducted labo- ratory assessments of prevalence and intensity of infection, external and internal macroscopic alter- ations, and histological analyses of the gills of uninfected and infected clams collected from the shores of the Rı ´o de la Plata estuary in Argentina. To evaluate potential ramifications of physical alterations on physiological traits and host fitness, we conducted respiration experiments on the same clams. Worm prevalence was higher in smaller than large clams. While no macroscopical alterations were detected, several histopathological changes were observed in the gills of infected clams. Infected clams had significantly higher metabolic rates than uninfected conspecifics and smaller clams had higher metabolic rates than larger conspecifics. Increased host meta- bolic rates, even at very low infections intensities, were likely a compensatory response to the stress caused by gill damaged inflicted by the oligochaetes. The relationship between C. limnaei and invasive C. fluminea in Rı ´o de la Plata best fits as parasitism. Guest editors: Katya E. Kovalenko, Fernando M. Pelicice, Lee B. Kats, Jonne Kotta & Sidinei M. Thomaz / Aquatic Invasive Species III Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10750-020-04424-6) con- tains supplementary material, which is available to authorized users. F. Liquin (&) Á D. Davies Á F. Sylvester Instituto para el Estudio de la Biodiversidad de Invertebrados, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Av. Bolivia 5150, A4408FVY Salta, Argentina e-mail: florenciafernand09@gmail.com F. Liquin Á F. Sylvester Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas (CONICET), Salta, Argentina L. A. Hu ¨nicken Á F. Arrighetti Á E. M. Paolucci Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas (CONICET), Buenos Aires, Argentina L. A. Hu ¨nicken Á F. Arrighetti Á E. M. Paolucci Museo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia’’, Av. A ´ ngel Gallardo 470, C1405DJR Buenos Aires, Argentina Present Address: L. A. Hu ¨nicken Centro de Investigacio ´n Aplicada y Transferencia Tecnolo ´gica en Recursos Marinos ‘‘Almirante Storni’’, Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas (CONICET), Gu ¨emes 1030, San Antonio Oeste, R8520CXV Rı ´o Negro, Argentina 123 Hydrobiologia https://doi.org/10.1007/s10750-020-04424-6