Reproductive Strategies of Modular Organisms: Comparative Studies of Reef‐ Building Corals

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The fecundity of clonal animals and plants is determined by the number of gravid modules (polyps, zooids, shoots, ramets) and their reproductive output. We examined the fecundity of modules (polyps) and colonies of six reef—building corals (Acropora hyacinthus, A. nana, A. gemmifera, A. millepora, Goniastrea retiformis, and Stylophora postillata) to explore how modularity influences reproductive strategies and the evolution of life histories in clonal animals. In unitary, aclonal organisms, fecundity often reaches a plateau or declines in old age. In contrast, we found that fecundity of colonies was indeterminate because vegetative growth can lead to an indefinite number of repeated, reproductive modules. However, colony fecundity was not a simple linear function of the number of modules. The species ranking of polyp fecundities (G. retiformis > the Acropora species > S. pistillata) was very different from the ranks of size—specific colony fecundities (A. hyacinthus, A. millepora, A. gemmifera > A. nana and G. retiformis). Colony fecundity for small S. pistillata was the lowest of all, but large colonies were more fecund than any of the other species at a comparable colony size (despite Stylophora having the lowest polyp fecundity). This contrast highlights clearly the effects of modularity on the reproductive biology and life histories of clonal organisms. The volume of eggs and testes, and the number of eggs and testes per polyp varied greatly among species (20—, 13—, 17—, and 3—fold, respectively). Egg volume was smallest for S. pistillata (the only brooder in the study) and for G. retiformis (which also had the most eggs per polyp, indicating a trade—off between egg size and number). The total reproductive volume per polyp (egg and testes combined) varied three—fold among the six species, and was correlated with a similar range of polyp diameters. In some cases, components of polyp fecundity depended on colony size; egg size in A. hyacinthus and A. nana and the number of planulae per polyp in S. pistillata increased by two— to fourfold with a one to two order of magnitude change in colony size. However, egg size was constant for the other species. Similarly, colony size had no effect on egg or testes number per polyp or testes volume per polyp for any species. We also found striking differences in sex allocation, which may relate to the fertilization biology of these hermaphroditic species. The ratio of total egg volume to testes volume per polyp varied fivefold, and was lowest for S. pistillata (a brooder) and G. retiformis (which produced the most and smallest eggs). The ratio increased with colony size for all species, especially A. hyacinthus. An early investment predominantly in testes allows sex to commence without the initial expense of egg production, perhaps so that colony growth can continue to a larger, safer size. All species had a characteristic colony size at maturation, which varied ninefold among taxa, corresponding to a minimum puberty age of 1—3 yr. This is the first comparative study to simultaneously measure male and female function in a range of clonal animals.