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Sponges are a major component of benthic ecosystems across the world and fulfil a number of important functional roles. However, despite their importance, there have been few attempts to compare sponge assemblage structure and ecological functions across large spatial scales. In this review, we examine commonalities and differences between shallow water (<100 m) sponges at bioregional (15 bioregions) and macroregional (tropical, Mediterranean, temperate, and polar) scales, to provide a more comprehensive understanding of sponge ecology. Patterns of sponge abundance (based on density and area occupied) were highly variable, with an average benthic cover between ~1 and 30%. Sponges were generally found to occupy more space (percentage cover) in the Mediterranean and polar macroregions, compared to temperate and tropical macroregions, although sponge densities (sponges m–2) were highest in temperate bioregions. Mean species richness standardised by sampling area was similar across all bioregions, except for a few locations that supported very high small‐scale biodiversity concentrations. Encrusting growth forms were generally the dominant sponge morphology, with the exception of the Tropical West Atlantic, where upright forms dominated. Annelids and Arthropods were the most commonly reported macrofauna associated with sponges across bioregions. With respect to reproduction, there were no patterns in gametic development (hermaphroditism versus gonochorism), although temperate, tropical, and polar macroregions had an increasingly higher percentage of viviparous species, respectively, with viviparity being the sole gamete development mechanism reported for polar sponges to date. Seasonal reproductive timing was the most common in all bioregions, but continuous timing was more common in the Mediterranean and tropical bioregions compared to polar and temperate bioregions. We found little variation across bioregions in larval size, and the dominant larval type across the glo

Coral reef ecosystems have been degrading globally for decades due to global climate change and anthropogenic pressure, and corals are expected to continue declining in the future. Some coral reef sponges, although also vulnerable to warmer seawater, are more tolerant than corals to the combined effects of projected ocean warming and acidification. As a result, sponges have been proposed as potential winners on future coral reefs. Shifts towards sponge dominance on coral reefs may result into altered reef trophodynamics. However, there is limited information on how ecological processes on coral reefs will be affected by an increase in sponge dominance. Therefore, the potential ecological functioning of sponge-dominated reefs is unclear. To understand how reef ecosystems could function if sponges will become dominant, it is important to clarify: how perturbations that have a negative effect on corals propagate through the reef ecosystem; how sponge assemblages can develop over time; and how increased sponge dominance will affect other coral reef organisms. Focusing on Indo-Pacific coral reefs, this thesis investigates the temporal dynamics and the ecological processes that characterise reefs where sponges form abundant and highly diverse assemblages. Aims of this thesis were: 1) to explore the ecosystem-level effects of climate change-induced shifts towards sponge dominance on coral reef ecosystems; 2) to quantify the temporal variability of sponge abundance and biodiversity on coral reefs; 3) to understand how benthic sessile organisms benefit from free substrate that becomes available when corals die; 4) and to determine whether sponges can provide structural complexity similar to that of corals. In my first data chapter, I use qualitative mathematical modelling to identify the key ecological processes that determine the functioning of a sponge-dominated reef. With a set of qualitative models, I obtain qualitative predictions of the ecosystem-level effects of incre