Abstract
Abstract Marine sponges (Phylum Porifera) are ecologically key filter feeders playing central roles in water purification and habitat structuring across all latitudes. Their filter-feeding behaviour and established use as bioindicators of chemical pollution make them strong candidates for microplastic (MP) monitoring. However, despite a decade of research, the field lacks a protocol unification. This systematic review integrates all 39 available works (31 peer-reviewed articles, theses, datasets, and conference proceedings) on the occurrence and effects of MPs in sponges published up to December 2025. MPs have been detected across 15 countries and in over 51 taxa, predominantly within the class Demospongiae, with concentrations spanning several orders of magnitude (0.21±0.22 to 1.05×10⁶ MPs/g DW). Fibres are the most frequently reported MP type, with blue and transparent colours dominating, and smaller size fractions tending to be most abundant. At a physiological level, MP exposure compromise filtration and respiration, induces oxidative stress, alters contraction cycles and disrupts the sponge microbiome. Evidence also confirms that MPs have been accumulating in sponge tissues for at least four decades, with concentrations increasing over time. Nonetheless, extreme methodological heterogeneity severely limits cross-study comparability. This review highlights the lack of standardised protocols and a model organism as the primary barrier to advancing the field, and calls for coordinated efforts to develop harmonised methods enabling sponges to fulfil their potential as reliable bioindicators of MP pollution.