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.
Emerging infectious diseases are a worldwide problem and are believed to play a major role in coral reef degradation. The study of coral diseases is difficult but the use of culture-independent molecular techniques has been, and will continue to be, useful in a system where a limited number of visible signs are commonly used to define a “coral disease”. We propose that coral “diseases”, with rare exception, are opportunistic infections secondary to exposure to physiological stress (e.g. elevated temperature) that result in reduced host resistance and unchecked growth of bacteria normally benign and non-pathogenic. These bacteria are from the environment, the host, or the coral mucus layer and become opportunistic pathogens. While difficult and time consuming, we do not advocate abandoning the study of disease-causing pathogens in corals. However, these studies should include comprehensive efforts to better understand the relationship between coral diseases and environmental changes, largely anthropogenic in nature, occurring on coral reefs around the world. These environmental insults are the cause of the physiological stress that subsequently leads to coral mortality and morbidity by many mechanisms including overwhelming infections by opportunistic pathogens.