Mesophotic coral reefs (MCEs) are ecologically unique components of coral reef ecosystems that occur at depths from ~30 to 150 m where they support a high number of depth-endemic species. One ecologi- cally important taxonomic group that can, especially in the Caribbean basin, dominate these habitats are sponges where they occur throughout the shallow (<30 m) to mesophotic depth range. There are an increasing number of studies on MCEs generally, and sponges have become a focal area for many of these studies as they exhibit a number of ecological and functional traits that vary with increasing depth. Here, we use an analysis of both historical and contemporary data to test the recently described “sponges increase with depth” hypoth- esis. While this hypothesis has recently been rejected without benefit of any quantitative analysis, we show that the density or percent cover of sponges increases over the shallow to mesophotic depth range for multiple reef sites in the Caribbean, and also in the Pacific at selected sites. The proximate cause for this pattern appears to be the increasing availability of trophic resources, and the ability to differentially use those resources, with increasing depth. The increase in sponge density or percent cover with depth is potentially global in nature and results in diverse, and unique, sponge-dominated communities at mesophotic depths.
The detrimental effects of invasive lion- fishes (Pterois volitans and Pterois miles) on western Atlantic shallow reefs are well documented, including declines in coral cover and native fish populations, with disproportionate predation on critically endan- gered reef fish in some locations. Yet despite individ- uals reaching depths [100 m, the role of mesophotic coral ecosystems (MCEs; reefs 30–150 m) in lionfish ecology has not been addressed. With lionfish control programs in most invaded locations limited to 30 m by diving restrictions, understanding the role of MCEs in lionfish distributions remains a critical knowledge gap potentially hindering conservation management. Here we synthesise unpublished and previously published studies of lionfish abundance and body length at paired shallow reef (0–30 m) and MCE sites in 63 locations in seven western Atlantic countries and eight sites in three Indo-Pacific countries where lionfish are native. Lionfish were found at similar abundances across the depth gradient from shallow to adjacent MCEs, with no difference between invaded and native sites. Of the five invaded countries where length data were avail- able three had larger lionfish on mesophotic than shallow reefs, one showed no significant difference while the fifth represented a recently invaded site. This suggests at least some mesophotic populations may represent extensions of natural ontogenetic migra- tions. Interestingly, despite their shallow focus, in many cases culling programs did not appear to alter abundance between depths. In general, we identify widespread invasive lionfish populations on MCE that could be responsible for maintaining high densities of lionfish recruits despite local shallow-biased control programs. This study highlights the need for manage- ment plans to incorporate lionfish populations below the depth limit of recreational diving in order to address all aspects of the local population and maximise the effectiveness of control efforts.