A little research for the hobby's addicted
from Gerin F. Rivera and Peter J. Edmunds in Journal of Experimental Marine Biology and Ecology.
After several decades of disturbance, many coral reefs in the Caribbean are dominated by macroalgae. One process affecting this transition is coral¯macroalgal competition, yet few studies have addressed the mechanisms involved. In this study, Rivera and Edmunds investigated competition between the tall and bushy macroalga Sargassum hystrix (J. Agardh) and the branching coral Porites porites (Pallas) on a shallow reef in Jamaica. Experiments were designed to expose coral branches to different treatments to test the role of shading and abrasion by Sargassum on coral growth and polyp expansion. Corals exposed to Sargassum grew significantly more slowly (80% reduction) than controls, but this effect was absent when corals were caged to prevent physical contact with macroalgae. Light levels were reduced in both the algal and cage treatments, but shading apparently had little effect on the growth of corals in cages. Short-term measurements of integrated net water flow did not detect variation among
treatments. In algal-mimic treatments, where clear plastic strips could touch but not shade the corals, growth rates were 25%
lower than controls, but this effect was not statistically significant. Thus, the growth of corals in contact with Sargassum was
reduced by abrasion and, to a lesser extent, by factors unique to living macroalgae. Analysis of polyp expansion showed that
polyps were more frequently retracted when corals were in contact with macroalgae or algal-mimics compared to controls or
cage treatment; the frequency of polyp contraction was correlated positively with growth. Together, these results suggest that
abrasion-mediated polyp retraction is one of the primary mechanisms of competition utilized by tall (ca. 17 cm) macroalgae
against scleractinian corals.
_________________
1960 recession
from Gerin F. Rivera and Peter J. Edmunds in Journal of Experimental Marine Biology and Ecology.
After several decades of disturbance, many coral reefs in the Caribbean are dominated by macroalgae. One process affecting this transition is coral¯macroalgal competition, yet few studies have addressed the mechanisms involved. In this study, Rivera and Edmunds investigated competition between the tall and bushy macroalga Sargassum hystrix (J. Agardh) and the branching coral Porites porites (Pallas) on a shallow reef in Jamaica. Experiments were designed to expose coral branches to different treatments to test the role of shading and abrasion by Sargassum on coral growth and polyp expansion. Corals exposed to Sargassum grew significantly more slowly (80% reduction) than controls, but this effect was absent when corals were caged to prevent physical contact with macroalgae. Light levels were reduced in both the algal and cage treatments, but shading apparently had little effect on the growth of corals in cages. Short-term measurements of integrated net water flow did not detect variation among
treatments. In algal-mimic treatments, where clear plastic strips could touch but not shade the corals, growth rates were 25%
lower than controls, but this effect was not statistically significant. Thus, the growth of corals in contact with Sargassum was
reduced by abrasion and, to a lesser extent, by factors unique to living macroalgae. Analysis of polyp expansion showed that
polyps were more frequently retracted when corals were in contact with macroalgae or algal-mimics compared to controls or
cage treatment; the frequency of polyp contraction was correlated positively with growth. Together, these results suggest that
abrasion-mediated polyp retraction is one of the primary mechanisms of competition utilized by tall (ca. 17 cm) macroalgae
against scleractinian corals.
_________________
1960 recession
