Synergistic effects of algal overgrowth and corallivory on Caribbean reef‐building corals

Abstract

Abstract. Indirect biotic interactions play a crucial role in structuring ecological communities, but many of these interactions have not been explored. Algal competition and corallivory are two major stressors contributing to the decline of coral reefs. Here, we provide the first evidence of algal-induced corallivory and synergistic effects between the two stressors on corals. When corals (Montastraea faveolata) were placed in contact with algae (Halimeda opuntia) together with corallivorous fireworms (Hermodice carunculata) in aquaria, corals suffered high tissue mortality. This mortality was reduced in the presence of algae only, and no mortality occurred in the presence of fireworms only or when excluding both algae and fireworms. These findings were supported by field observations showing a predominance of fireworms inside algae contacting live corals, and by an in situ experiment demonstrating higher coral mortality in contact with algae left undisturbed than with algae from which all mobile epifauna were periodically removed. Among the main contributing mechanisms, we suggest that algal contact produces decaying coral tissue that attracts the corallivore and enhances its aggregation behavior. Our study demonstrates an indirect effect pathway by which algae can impact corals, which shares similarities with the classic models of apparent competition and habitat facilitation.

Spatial and temporal limits of coral-macroalgal competition: the negative impacts of macroalgal density, proximity, and history of contact

“…Macroalgae can harm corals via physical mechanisms such as shading, abrasion, and overgrowth (McCook et al 2001), chemical mechanisms such as allelopathy (Rasher et al 2011, Vieira et al 2016), suppression of coral settlement (Kuffner et al 2006, Paul et al 2011, Dixson et al 2014), or disruption of coral microbiomes that protect against coral pathogens (Nugues et al 2004, Smith et al 2006, Barott et al 2012, Zaneveld et al 2016). Macroalgae also alter coral interactions with corallivores (Wolf & Nugues 2013, Clements & Hay 2015, Brooker et al 2016). …”

Algae as Reservoirs for Coral Pathogens

“…Current evidence suggests that algae have the ability to directly harm corals in three principal ways; 1) shading, 2) abrasion resulting in a physical injury, and 3) the release of poisonous allelochemicals (reviewed by [23]). There are also numerous indirect ways the algae have been shown to have an effect, notably: 4) via the release of primary metabolites, which has been shown to stimulate microbial activity at the coral/algal interface [24]–[26], 5) via the release of secondary metabolites which alter the coral associated microbial community [27]–[30], 6) by attracting corallivores, which cause increased coral mortality [31], and lastly 7) by acting as a vector for coral disease pathogens [19]. In contrast to the above mentioned studies which illustrate numerous direct and indirect effects of algae on coral [32], showed that the presence of macro algae adjacent to corals had no observable effect on coral health and disease prevalence at least in relation to Yellow Band Disease within the Caribbean.…”

Harnessing ecological processes to facilitate coral restoration

“…). Competition between corals and the common green alga Halimeda opuntia can attract H carunculata , increasing the prevalence of coral disease and mortality (Wolf and Nugues ). Seeking ways to control both algal competitors, such as restoring areas with abundant fishes and/or urchins, and coral predators may also help to reduce the incidence and spread of coral diseases.…”