Corals and macroalgae can sometimes coexist

Abstract

Reef- building corals and canopy- forming macroalgae are the main bioengineers in the shallow environments of the world’s oceans. While corals dominate in tropical waters, erect macroalgae prevail in temperate and cold seas. However, where they overlap, corals and algae have an uneasy coexistence, with algae rapidly taking over coral- dominated tropical reefs when herbivory is reduced or nutrient inputs increase. Yet we observed Cladocora caespitosa and the fucoid perennial macroalga Treptacantha ballesterosii coexisting in relatively high densities near Formentera, one of Spain’s Balearic Islands.Both species show a very low growth rate (2.5 mm year−1 for the coral and 10 mm year−1 for the alga) and are long- lived (up to 300 years for the coral and 20 years for the alga). A large percentage of the algal specimens measured more than 10 cm long and some reached up to 26 cm, which means that the corals and algae in this seabed have been closely interacting for a long time. More interestingly, the algae here did not seem to be negatively affected by the coral since most of their holdfasts were in the middle of coral colonies, and the coral colo-nies were not showing any signs of stress. This mixed seabed raises the question of whether corals and macroalgae necessarily outcom-pete each other or whether they can sometimes coexist without show-ing clear negative interactions, as they do in this Mediterranean location.