Isotopic variability of black corals: which scale matters? An intracolonial, intra– and interspecific comparison of two shallow-water species

This study investigates isotopic variability in two abundant black coral species from the Great Reef of Toliara (GRT), southwestern Madagascar: Cirrhipathes anguina, a tall whip coral, and Cupressopathes abies, a shorter, branched bottlebrush coral. Using stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S), we examined inter- and intraspecific dietary ariatvion and intracolonial isotopic gradients. Colonies were collected on the outer slope (20–28 m depth) of the northern pass of the GRT, an area influenced by tidal currents and terrestrial sediment discharge. Sulfur and carbon isotopes provided limited resolution for distinguishing the isotopic niches of the two species but indicated reliance on oceanic planktonic production rather than lagoonal or terrestrial sources. Significant isotopic differences occurred between species: Ci. anguina exhibited higher δ¹⁵N values, reflecting a higher trophic position and consumption of larger prey (mesozooplankton) than Cu. abies, which likely feeds on smaller plankton and particulate organic matter. Ci. anguina displayed
a broader trophic niche and greater δ¹⁵N variability, suggesting a more flexible diet. No vertical isotopic gradients were detected within colonies, implying no trophic specialization along colony height. However, δ¹⁵N variability within Ci. anguina colonies suggests opportunistic feeding behavior. Overall, this study highlights the role of morphology in shaping feeding strategies and demonstrates clear trophic niche partitioning that facilitates coexistence. Intracolonial isotopic homogeneity supports single-point sampling, although intercolonial variability warrants caution in interpreting δ¹⁵N-based trophic ecology.