"The hidden Filter": Quantifying the ecological role of Lithophaga lithophaga (Linnaeus, 1758) in Mediterranean...

The current impact of global warming in marine habitats is rapidly altering ecosystem functioning at different levels. Marine heat wave and massive mortality events are compromising the survivor of active and passive filter feeders. Nevertheless, the scarcity of data on the filtration rate and selectivity of ecologically crucial understudied filter feeders is compromising our understanding of the consequences. The endolithic bivalve Lithophaga lithophaga has been largely overlooked in this context. Because of this, we experimentally assessed its feeding behaviour by measuring clearance rates with two diatom species (Chaetoceros muelleri and Conticribra weissflogii) and a control microalga (Isochrysis galbana) under different concentrations (high 1.1·10⁶ cells/ml and low 2.5·10⁵ cells/ml) and across two L lithophaga size classes. On average, commonly sized individuals (∼8 g) exhibited clearance rates of 0.24 L h^-1·g^-1, suggesting that dense populations (>100 ind·m^-2) could filter up to 192 L h^-1·m^-2. Notably, we observed for the first time the production of pseudofeces by L. lithophaga when fed diatoms, especially at high concentrations, whereas I. galbana was almost entirely ingested, indicating pre-ingestive selection of particles. Moreover, pseudofeces analysis revealed enrichment in silica and carbohydrates, pointing to active rejection of frustule-bearing cells and involvement in the biogenic silica cycle in Mediterranean ecosystems. These findings confirm the ecological role of L. lithophaga as a relevant filter feeder, contributing to particulate organic matter flux and nutrient cycling. This underscores the importance to broad L. lithophaga future conservation and restoration actions in a changing ocean, since its crucial involvement in ecosystem functioning.