Competitive effects of macroalgae on the fecundity of the reef-building coral Montastraea annularis

In recent decades, a rise in coral mortality, attributed to increased frequency of mass-bleaching events, increased prevalence of disease, and more frequent and severe hurricanes, has contributed to a rapid proliferation of macroalgae across many Caribbean reefs. As a consequence, the frequency of coral-algal interactions has risen. Here, we document the effects of 2 dominant Caribbean macroalgae, Dictyota spp. and Lobophora variegata, and a mixed algal community on the fecundity of a massive coral. Montastraea annularis is a dominant Caribbean reef-building coral characterised by a low recruitment rate. To investigate the effects of macroalgae on coral fecundity, algal contact was experimentally manipulated around the perimeter of M. annularis patches. Fecundity was measured as the diameter of eggs (ES), the number of eggs per gonad (E#) and the number of gonads per polyp (G#). Algal contact was shown to significantly reduce the diameter of eggs at both the coral-algal boundary and at the centre of coral patches. The presence of Dictyota spp. or a mixed algal community was shown to have more detrimental effects on ES than the presence of L. variegata. Removal of algal contact immediately prior to gametogenesis increased the reproductive output of polyps directly adjacent to the cleared areas, with an increase in ES, E# and G#. Our results imply that algal competitors can reduce the fecundity of M annularis through mechanical and/or allelochemical damage of polyps directly adjacent to the algae and by causing the reallocation of energy within the coral patch from reproduction to defence and repair.