Coral defence against macroalgae: differential effects of mesenterial filaments on the green alga Halimeda opuntia

Abstract

Several studies have shown that the growth of macroalgae is reduced when in contact with corals, but very few have addressed the mechanisms involved. This study provides for the first time an explicative mechanism for the inhibition of algal growth by scleractinian corals. In experimental field contacts between 8 species of scleractinian corals and the green alga Halimeda opuntia, corals extruded their mesenterial filaments (MFs) onto the plant. In some coral species, this extrusion was followed by persistent discolouration of the contacting algal segments. Microscopic examination of these segments revealed migration of chloroplasts away from the surface of segments, as well as the presence of nematocysts from the MFs fired into the algal epidermis. There was a significant relationship between the extent of algal discolouration and both the occurrence and length of the MFs, potentially explaining differences among coral species in their ability to damage algal segments. MF extrusion also occurred in experimental contacts with 2 other species of macroalgae (Lobophora variegata and Dictyota sp.). The use of extruded MFs by scleractinian corals against macroalgae is similar to their aggressive behaviour against other scleractinian coral species. However, the ranking of competitive ability against H. opuntia (measured by the extent of algal discolouration) differed from that established in interspecific aggressions among corals. The great abundance and wide distribution of the species of corals and macroalgae used in this study, together with observations of MFs in natural interactions, suggest that the use of MFs is a common and widespread mechanism of interaction between corals and macroalgae. We conclude that coral-algal competition involves complex mechanisms and is therefore likely to represent a complex hierarchical structure, in which macroalgae are not always able to overgrow and kill corals.

Differential competitive abilities between Caribbean coral species and a brown alga: a year of experiments and a long-term perspective

“…Few of these mechanisms are known to occur against algae. However, corals have recently been shown to extrude mesenterial filaments over algae (Nugues et al 2004). Interestingly, the effect of these filaments on the green alga Halimeda opuntia differed depending on coral species and was closely related to the physical properties of the filaments (i.e.…”

Inhibition of coral recruitment by macroalgae and cyanobacteria

“…Thus, it appears that these species can kill coral recruits using mechanisms other than simple shading or abrasion. As the juvenile coral reaches a certain threshold size, defense mechanisms on the part of the coral may come into play, such as the use of mesenterial filaments in reducing the growth and damaging the thalli of neighboring algae (Nugues et al 2004a). It remains unknown how old corals have to be to employ these tactics, but apparently adult P. astreoides do not use this defense mechanism against algae (Nugues et al 2004a).…”

“…As the juvenile coral reaches a certain threshold size, defense mechanisms on the part of the coral may come into play, such as the use of mesenterial filaments in reducing the growth and damaging the thalli of neighboring algae (Nugues et al 2004a). It remains unknown how old corals have to be to employ these tactics, but apparently adult P. astreoides do not use this defense mechanism against algae (Nugues et al 2004a). Lirman (2001) found that the growth of adult P. astreoides and Montastrea annularis was reduced when caged with Dictyota spp.…”

The effect of the SW Atlantic burrowing crab Chasmagnathus granulatus on the intertidal razor clam Tagelus plebeius

“…Non-trophic interactions appear to be comparatively less significant (but see Jones et al, 1994), albeit both positive and negative non-trophic interactions can be a major structuring force under certain conditions (e.g. Dunstan and Johnson, 2003;Nugues et al, 2004).…”