Dataset: Isotopic data from sponges collected in 2013 and 2014 from reefs in Honduras, Belize, Panama and the Florida Keys.

Sponge collection:  Sponge species were collected from at least one site within four geographic regions spanning more than 15° of latitude (~1700 km) within the Caribbean Sea. Individual regions included the Bocas del Toro archipelago of Panama, the Miskito Cays of Honduras, the Mesoamerican barrier reef of Belize, and the Florida Keys. At each site, replicate small (3–5 ml) samples of dominant and conspicuous sponge species were collected by SCUBA using a dive knife and placed into individual bags containing seawater for transport back to the laboratory. Sponge samples always included a cross section with both inner and outer tissue regions to standardize collections and sample across the entire body of the sponge. Collections frequently included eight of the ten most dominant Caribbean species and species previously designated as both HMA and LMA sponges. Samples were preserved, processed, and prepared for δ13C and δ15N analysis. Sponges were identified to species and, if necessary, identities were verified via tissue histology and spicule preparations. Replicate subsamples of each sponge species were also preserved in 95% EtOH in 5 ml cryovials and frozen at −20 °C for analyses of microbial community structure.

Stable isotope and chlorophyll a analyses: Stable isotope values (δ13C and δ15N) of bulk sponge tissue serve as a time-integrated record of the sources of carbon and nitrogen assimilated by a holobiont (including activities of both sponge and microbial cells) and any fractionation associated with symbiont or host metabolism or nutrient recycling. Within an individual reef, δ13C and δ15N values of sponge tissue therefore act as a metabolic “fingerprint” that integrates the physiological, metabolic, and ecological differences present across individual sponges. Bulk sponge tissue samples were analyzed in the Stable Isotope Ratio Mass Spectrometry Laboratory at the University of Hong Kong as in. Mean (±SE) precision during analysis was 0.1 (0.001) ‰ and 0.2 (0.03) ‰ for δ13C and δ15N, respectively. Isotope values are expressed in delta (δ) notation in units per mille (‰). Values of the elemental composition (%C, %N, and C:N) of each sample of sponge tissue were also provided. Elemental values provide important information about how biomass-associated pools of carbon and nitrogen vary across sponge species and allowed us to test whether our trends in δ13C and δ15N values were strongly influenced by structural differences in sponge tissue. Photosymbiont abundance (as determined by chlorophyll a [chl a] concentration) was quantified in sponges from sites in Honduras, Panama, and the Florida Keys as in and expressed as μg chl a [g dry sponge tissue]−1. Scopalina ruetzleri samples were not analyzed for chl a because they were too small to provide tissue for both isotope and chl a analyses.