Reservoir, sink, or reactor? Repositioning marine sponges as candidate indicators for coastal antimicrobial resistance monitoring

Antimicrobial resistance (AMR) spreads through the environment, and in coastal areas it is investigated mainly in water and sediment. These matrices capture antimicrobial resistance genes (ARGs) passing through them but may fail to retain that information over time because of the dynamics inherent in these ecosystems. Filter-feeding invertebrates are obvious candidates for the role, and bivalves have already been used for this purpose. Marine sponges (phylum Porifera) surpass bivalves on multiple fronts: they retain submicrometric particles that bivalves let pass, harbor microbial communities orders of magnitude denser than the surrounding water, and are sessile and long-lived in coastal areas receiving anthropogenic waste. Even so, researchers have never evaluated them as AMR indicators because sponge microbiology has focused on bioprospecting. Resistance in sponges almost always appears as incidental antibiogram data, or as annotation in metagenomes generated for other purposes. Here, we argue that the problem is not in collecting samples but in interpreting them: a sponge signal can only be read once an ecological question is resolved. As a reservoir, the holobiont accumulates ARGs and reflects environmental abundance, making it a valid monitoring target. As a sink, it removes ARG-carrying cells without passing them on, decoupling the signal from environmental load. As a reactor, it mobilizes ARGs into the resident symbiont community, so the signal indicates a risk the organism itself generates. This perspective distinguishes each role, identifies factors that confound these observations, and defines the validation strategy required for sponges to serve as indicators in AMR monitoring.