Spatio-temporal variability and benthic drivers of marine sponge assemblages at Nosy Be, northwest Madagascar

Marine sponges (phylum Porifera) are key reef organisms, yet they remain understudied in the western Indian Ocean region. This study provides the first quantitative assessment of sponge assemblages in the bay between Nosy Be island and mainland Madagascar by examining their spatio-temporal diversity patterns and relationship with different substrate types. Photo-quadrat surveys to determine percentage sponge cover and substrate types were conducted annually from 2020 to 2022 at three reef stations (Gorgone, Heloise and Pirogue). The data were analysed with generalised linear models (GLMs), non-metric multidimensional scaling, PERMANOVA, SIMPER and AICc-based model averaging. We identified 26 operational taxonomic units (OTUs), representing 15 families and 10 orders. Sponge cover was low (mean 5.9 ± 0.6% SE) and showed strong spatial differences (GLM, p < 0.001) but limited interannual variation. Pirogue exhibited the highest sponge abundance (4.97 ± 0.6 ind. M2) and taxonomic richness (2.73 ± 0.24 OTU m-2), whereas Gorgone (closest to the coast of Nosy Be) had the lowest values. The sponge assemblages were dominated by a few taxa (e.g. Pseudoceratina sp., Dendroceratida sp. 1, Callyspongia spp. and Amphimedon sp.). Model averaging revealed a positive association between hard coral cover and sponge cover (p = 0.001). These results establish an ecological baseline for reefs at Nosy Be and highlight the need to integrate sponges into reef monitoring and conservation strategies for maintaining Madagascar’s coastal ecosystems.