Differences in coral-associated species on live and dead corals
Principal component analysis (PCA) of the entire community of fish and mobile macroinvertebrates associated with the finger corals indicated that the overall species composition of the communities differed significantly between live and dead corals (Figure 5A, see Figure S3 for explanation of PCA species scores). Species composition also differed among certain time points, especially between the first time point (July 2008) relative to other time points for live corals and dead corals, and between the last time point (February 2010) relative to other time points for live corals (Figure 5A). Live corals were characterized by high PC1, which increased across time points, and was strongly correlated with the presence of mutualistic xanthid crabs Trapezia ferruginea and Trapezia digitalis (Figure S3A). Dead corals were characterized by lower values of PC1, which was strongly correlated with the presence of the pencil urchin Eucidaris galapagensis (Figure S3A). Relative to the latter points, earlier time points were characterized by lower PC2 values, which were strongly correlated with the presence of the pencil urchin and juveniles of the ring-tailed damselfish Stegastes beebei , as well as a speciose assemblage of opportunistic snails (Figure S3B).
Live and dead corals exhibited strong overlap in fish assemblages (Figure 5B), but particular differences in the community of associated invertebrates (Figure 5C-D). With respect to coral-associated fishes, live and dead corals were dominated by two territorial and residential fish species: the coral hawkfish Cirrhitichthys oxycephalus and the juvenile ring-tailed damselfish Stegastes beebei ; though the abundance of these species varied seasonally, and ultimately declined by 65% and 90%, respectively (particularly on dead corals) between July 2008 and February 2010 (Figure 5B). Other common species at certain time points (e.g., Ophioblennius steindachneri in July 2008 andLythrypnus gilberti in January 2009 on dead corals andThalassoma lucasanum in July 2008 on live corals) also exhibited high seasonal variability in abundance (Figure 5B).
With regards to coral-associated invertebrates, dead corals hosted opportunistic and predatory macroinvertebrate species, dominated by the pencil urchin Eucidaris galapagensis , hermit crabs (Figure 5C), and a rich assemblage of 16 species of gastropods, especially two species of Engina as well as Muricopsis zeteki , which increased between July 2008 and January 2009 (Figure 5D). The abundance of pencil urchins on dead corals increased initially by 100% (21 to 42 individuals) between July 2008 and January 2009, and then declined by 62% between January 2009 and February 2010 (Figure 5C). Pencil urchins were observed feeding directly on the calcium carbonate structure of dead and occasionally bleached live coral tissue, while gastropods and hermit crabs were frequently observed feeding on the sessile invertebrates (barnacles, sponges, ascidians, bryozoans) that had colonized dead coral skeletons (O.K. Rhoades, pers. obs.). By contrast, the coral-associated invertebrates on live finger corals were dominated by hundreds of mutualistic xanthid crabs (Trapezia spp. includingTrapezia ferruginea and Trapezia digitalis ) (Figure 5C). The total number of mutualistic crabs increased by 300% (71 to 284 individuals) between July 2008 and February 2010 (Figure 5C). Gastropods were considerably (10 times) more abundant on dead corals relative to live corals (Figure 5D).