LAUSR

Sponges are a crucial component of Caribbean coral reef ecosystem structure and function. In the Caribbean, many sponges show a predictable increase in percent cover or abundance as depth increases from shallow (< 30 m) to mesophotic (30–150 m) depths. Given that sponge abundances are predicted to increase in the Caribbean as coral cover declines, understanding ecological factors that control their distribution is critical. Here we assess if sponge cover increases as depth increases into the mesophotic zone for three common Caribbean reef sponges, Xestospongia muta, Agelas tubulata, and Plakortis angulospiculatus, and use stable isotope analyses to determine whether shifts in trophic resource utilization along a shallow to mesophotic gradient occurred. Ecological surveys show that all target sponges significantly increase in percent cover as depth increases. Using bulk stable isotope analysis, we show that as depth increases there are increases in the δC and δN values, reflecting that all sponges consumed more heterotrophic picoplankton, with low C:N ratios in the mesophotic zone. However, compound-specific isotope analysis of amino acids (CSIA-AA) shows that there are speciesspecific increases in δCAA and δNAA values. Xestospongia muta and P. angulospiculatus showed a reduced reliance on photoautotrophic resources as depth increased, while A. tubulata appears to rely on heterotrophy at all depths. The δCAA and δNAA values of these sponges also reflect species-specific patterns of host utilization of both POM and dissolved organic matter (DOM), its subsequent re-synthesis, and translocation, by their microbiomes. As coral reefs around the world continue to show declines in biodiversity and health (Hoegh-Guldberg et al. 2007; Hughes et al. 2017, 2018) there has been increased interest in understanding processes affecting the structure and function of coral reef communities in the Anthropocene (Waters et al. 2016). The decline in coral cover due to anthropogenic stressors such as pollution, ocean acidification and increases in sea surface temperature (e.g., Hoegh-Guldberg et al. 2007) has resulted in ecological phase shifts in the benthic communities of many coral reefs (McManus 2000). These community shifts are predicted to cause an increase in the abundance of sponges, and changes in the functional attributes of coral reefs as sponge biomass exceeds coral biomass Caribbean-wide (McMurray et al. 2010; Bell et al. 2013, 2018). In the Caribbean, sponges provide an important source of food and habitat for a variety of coral reef species (Diaz and Rützler 2001; Bell 2008). Sponges also play an important role in benthic food webs through benthic-pelagic coupling due to their consumption of live particulate organic material (POM) (Pile et al. 1997; Lesser 2006; Lesser and Slattery 2013) and dissolved organic material (DOM) (de Goeij et al. 2013, 2017; Mueller et al. 2014), thus coupling water column productivity to benthic secondary productivity (Gili and Coma 1998; Lesser 2006). While many ecological studies on the trophic ecology of sponges have been conducted on shallow coral reefs (de Goeij et al. 2017), studies in the mesophotic zone, while increasing (Loya et al. 2016), are not as common (Lesser et al. 2018). The mesophotic zone is found between 30 and 150 m and is defined primarily by gradients of abiotic factors, particularly irradiance (Lesser et al. 2018). Along the shallow to mesophotic depth gradient, sponges exhibit increased growth rates, abundances, and diversity (Lesser 2006; Trussell et al. 2006; Lesser and Slattery 2013; Slattery and Lesser 2015). Additionally, their primary particulate food sources, autoand heterotrophic picoplankton, increase with depth (Lesser 2006; Lesser and Slattery 2013). As sponges consume particulate and dissolved food in proportion to its availability (Lesser 2006; de Goeij et al. 2008; Slattery and Lesser 2015), mesophotic sponges should therefore consume more POM with increasing depth (e.g., Slattery and Lesser 2015). Despite the fact that dissolved organic carbon (DOC) declines with increasing depth *Correspondence: [email protected] Additional Supporting Information may be found in the online version of this article.