Morphologically similar, coexisting hard corals (Porites lobata and P. solida) display similar trophic isotopic ratios across reefs and depths

Recent studies using stable isotope analysis in scleractinian corals have highlighted strong inter-and intra-specific variability in isotopic ratios, but few have excluded the effects of morphology, which affects resource acquisition, potentially confounding this with metabolic differences among species. Differences in the stable isotopic (delta C-13 and delta N-15) ratios of the coral host tissue and photosymbionts of two co-existing, morphologically similar Porites corals (P. lobata and P. solida) were examined across nested spatial scales (inter-reefs and intra-reef) and across depths in Zanzibar, Tanzania. There were few differences between species in either coral host or photosymbiont isotopic ratios, but the two tissues showed different spatial patterns. Photosymbionts showed variation only in their delta C-13 ratios, which differed among reefs, but not by depth. In contrast, the coral hosts differed in delta C-13 and delta N-15 values among reefs and also by depth. Within-reef differences among sites occurred only for photosymbionts at one reef. The absence of differences in isotopic ratios between the two Porites species across reefs and depths, confirms that highly related and morphologically similar scleractinian corals may occupy similar ecosystem niches, metabolising resources in a similar fashion. This suggests that resource partitioning among corals, and subsequent isotopic variability, is most likely driven by resource acquisition, rather than being inherently species-specific.