Does seaweed–coral competition make seaweeds more palatable?

Seaweed–coral interactions are increasingly common on modern coral reefs, but the dynamics, processes, and mechanisms affecting these interactions are inadequately understood. We investigated the frequency and effect of seaweed–coral contacts for common seaweeds and corals in Belize. Effects on corals were evaluated by measuring the frequency and extent of bleaching when contacted by various seaweeds, and effects on a common seaweed were evaluated by assessing whether contact with coral made the seaweed more palatable to the sea urchin Diadema antillarum . Coral–seaweed contacts were particularly frequent between Agaricia corals and the seaweed Halimeda opuntia , with this interaction being associated with coral bleaching in 95 % of contacts. Pooling across all coral species, H. opuntia was the seaweed most commonly contacting corals and most frequently associated with localized bleaching at the point of contact. Articulated coralline algae, Halimeda tuna and Lobophora variegata also frequently contacted corals and were commonly associated with bleaching. The common corals Agaricia and Porites bleached with similar frequency when contacted by H. opuntia (95 and 90 %, respectively), but Agaricia experienced more damage than Porites when contacted by articulated coralline algae or H. tuna . When spatially paired individuals of H. opuntia that had been in contact with Agaricia and not in contact with any coral were collected from the reefs and offered to D. antillarum , urchins consumed about 150 % more of thalli that had been competing with Agaricia . Contact and non-contact thalli did not differ in nutritional traits (ash-free-dry-mass, C or N concentrations), suggesting that Halimeda chemical defenses may have been compromised by coral–algal contact. If competition with corals commonly enhances seaweed palatability, then the dynamics and nuances of small-scale seaweed–coral–herbivore interactions at coral edges are deserving of greater attention in that such interactions could scale-up to have important consequences for coral resilience and the persistence of reef structure and function.