The growth of modular organisms is achieved by the asexual iteration of conserved units, and the biological implications of this type of growth are vast. One direct consequence of modularity is the potential for exponential growth through asexual reproduction and dispersal, thereby removing the genotype from the physiological constraints of senescence and permitting it to become virtually immortal. However, senescence at the level of individual modules may still exist. Scleractinian corals are an excellent model system to test for effects of age and size because colonies often experience fission, fusion, and fragmentation, thereby decoupling the relationship between age and size. Understanding how fission and fragmentation affect coral growth is timely because the likelihood of partial mortality and fission will increase due to global degradation of coral reefs, resulting in large numbers of small, yet old, colonies. In order to test the effects of age and size on growth in corals, two approaches were taken. First, age and size were manipulated experimentally by breaking branches of the coral, Madracis mirabilis, into young and old fragments, and growth subsequently was quantified as calcification rate. Growth scaled isometrically in both age groups, and although scaling exponents were statistically indistinguishable among ages, young fragments calcified faster than old fragments. In other words, the effect of age was absolute and independent of size. The second approach involved a mensurative analysis of the massive coral, Siderastrea siderea. This species often undergoes fission to produce small daughter colonies that are old. The growth of similarly sized sexual recruits (young) and daughter colonies (old) was monitored for a year, and these two colony types exhibited significant differences in lateral extension. Furthermore, age affected the scaling of calcification so that young corals grew disproportionately faster than old corals over the smallest size range. Together, the experiments with M. mirabilis and S. siderea demonstrate that age significantly affects coral growth, and suggest that the rapid growth of juvenile corals can be attributed to their young age, rather than their small size. Although most scleractinian corals are modular, certain species are not, and thus the relationship between age and size typically is predictable. In contrast to colonial corals that grow indeterminately and are typically only limited by space, solitary species are likely to exhibit an upper maximum size that may result from energetic constraints. An energetic model originally developed for anemones was modified in order to test the hypothesis that energetic constraints limit the maximum size of the solitary coral Fungia concinna. The model assumed that photosynthesis was the primary source of energetic intake and metabolic cost was quantified as aerobic respiration. The scaling exponent on mass was higher for energetic intake than metabolic cost, allowing large individuals to maintain an energetic surplus over the size range studied, even when the energy required for daily host tissue and symbiont growth was incorporated into the model. Therefore, it appears that growth in F. concinna is not limited energetically. Instead, mechanical constraints on locomotion may set the maximum size of this solitary coral. The results of these three studies demonstrate clearly that age and size separately affect the physiology of solitary and modular corals, but also highlight the potential for interactive effects of these two demographic parameters that likely are under strong selective pressure.
Thesis or Dissertation
Department of Biology, CSU Northridge