When describing coral shape and form the term phenotypic plasticity, i.e. environment-induced changes in morphology, is often used synonymously with intraspecific variation. Variation, however, may simply be due to genetic differentiation (polymorphism). Of the 1314 extant scleractinian coral species, less than 20 have been tested for plastic responses. Morphological plasticity has important implications for coral identification, as skeletal features used in coral systematics are directly affected by environment. Furthermore, plastic changes can indicate how corals acclimatise to environmental change. The studies that have examined phenotypic plasticity in corals experimentally can be divided into two groups, i.e. ‘non-clonal’—those that have transplanted whole colonies or fragments of colonies (but not treated the fragments as clones) to new environments, and ‘clonal’—those that have transplanted colony fragments and used them as clone-mates. The use of clone-mates is preferable as it facilitates the identification of among-genotype variation for plasticity. The heterogeneous nature of the reef environment makes identifying the parameters that affect coral morphology difficult in the field, but there are also many problems conducting suitable aquarium experiments. Nevertheless, evidence to date suggests light and water movement are the most important variables inducing change. As these factors are known to be axiomatic to coral growth, it is possible that associated plastic changes in corals are adaptive; however, this hypothesis is yet to be tested rigorously.

Morphological plasticity in scleractinian corals.

Morphological plasticity is common among clonal organisms, including scleractinian corals, yet the role of phenotypic plasticity in coral ecology and evolution is largely unexplored. Additionally, it is unclear how much variation in plastic responses exists among individuals, populations, and species, and thus how much potential there is for natural selection to act on coral reaction norms. In the branching coral Madracis mi- rabilis, corallite architecture and density, branch diameter and spacing, and overall aggregate morphology all vary among environments. To examine the role of phenotypic plasticity in generating these patterns, clonal replicates of five genotypes of M. mirabilis were trans- planted from each of two source populations into four treatment environments on the north coast of Jamaica. Flow rate, sedimentation, irradiance, water temperature, and salinity all varied among these environments. DNA fingerprinting was used to ensure that the 10 transplanted genotypes were genetically distinct. Six morphological traits (intersepta area, septa length, columella area, corallite area, corallite spacing, and branch tip diameter) were measured after transplantation to determine whether they were altered in response to en- vironmental conditions. Because these traits were correlated, principal components analysis was used to define new, uncorrelated traits for analysis. Four of the five corallite traits and branch diameter were significantly affected by the environment, demonstrating that mor- phological variation among environments in M. mirabilis is due in large part to phenotypic plasticity. No difference was detected between the two source populations in the magnitude or direction of their plastic responses, but there was substantial variation among genotypes (genotype 3 environment interaction). Many of the phenotypic changes of both populations resulted in the transplants becoming morphologically similar to resident conspecifics in each treatment environment. Genotypes from both populations were able to maintain similar growth rates under diverse environmental conditions. Such morphological convergence by phenotypic plasticity may expand the ecological range of this species by enabling genotypes to tolerate spatially and temporally variable environments.

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Clonal variation for phenotypic plasticity in the coral Madracis mirabilis

The nature and life-history consequences of modular construction in plants are discussed with particular reference to growth, reproduction and survival. Plants grow by the iteration of modular units and as a consequence growth can be described in terms of the population dynamics of these structural units. Changes in size, whether positive or negative, depend on the birth and death rates of modules; however, if the births continue to exceed the deaths, plants then have the capability of attaining enormous sizes, especially if they are clonal. The population nature of plant growth also means that plants of the same age may show large variation in individual size if individuals differ in their relative growth rates. Correlations between age and size are often, therefore, very weak. Constraints on the allocation of resources accumulated during growth have important implications for the reproductive schedules of plants, but the analysis of constraint functions has so far revealed little about the actual detail of these schedules. All the meristems of semelparous plants are involved in or die at reproduction and as a consequence death of the genet follows reproduction. For iteroparous plants, however, there are fundamental differences between the reproductive schedules of plants with a single shoot module and those with many shoot modules. The former demonstrate a relatively constant rate of reproduction from year to year following maturity whereas the latter show a continual increase in fecundity with size and age. The reproductive schedules of clonal plants are further discussed in relation to the allocation of meristems to either growth or reproduction. The pattern of mortality is examined at both the level of the module and the genet. Particular attention is focused on the survival and senescence of leaves and shoots; there is no equivalent regular shedding of organs in unitary organisms. Whereas genet senescence and death are coincident with shoot module death in semelparous plants, there is no evident relation between them in iteroparous plants. The life span of the genet reflects the birth and death rates of its modules and both aclonal and clonal plants that are iteroparous may achieve considerable longevity. The longevity of aclonal plants often seems to be restricted by the accumulation of dead material and the problems of being large. Clonal plants are, in contrast, potentially immortal. It is questionable whether the genets of iteroparous plants show senescence as defined for unitary organisms since there is no separation of germ plasm from soma and since apical meristems do not appear to senesce. Insofar as they retain the capacity for rejuvenescence from apical meristems, genets of modular organisms do not senesce; it is only the constituent organs that show senescence, death and decay.

Some Life-History Consequences of Modular Construction in Plants

Existing life-history concepts deal inadequately with clonal organisms. To make them more generally applicable, several assumptions must be widened: (i) life histories may be expressed at different levels of individual organization, e.g. genets and modules; (ii) parts of a life cycle may be executed more than once by a single genetic individual; (iii) genetic individuals do not necessarily senesce; (iv) size and reproductive value of genetic individuals may increase indefinitely with age; and (v) dormancy, dispersal of progeny, and production of genetically different progeny are not necessarily linked to the same process, as they are in reproduction of unitary organisms. A model is presented of a size-structured population of genets, in which each genet comprises an age-structured metapopulation of modules. The model predicts that the extent to which clonal growth evolves depends on: constraints in genet architecture (high or low within-genet `dispersal'); cost of reproduction relative to that of clonal growth; the presence or absence of density-dependent regulation of population size; the presence of lethal events that kill entire genets, as opposed to genet mortality occurring only as a consequence of the mortality of all its modules; the ability of genets to predict such lethal events; and the temporal heterogeneity of the environment. The predicted responses to r- and K-selection differ markedly from those predicted for unitary organisms, especially for genets with high within-genet dispersal. For example, r-selection may favour clonal growth with no reproduction, whereas K-selection favours greater reproduction.

Life-history concepts and the population biology of clonal organisms

Coral mortality due to climate-associated stress is likely to increase as the oceans get warmer and more acidic. Coral bleaching and an increase in infectious disease are linked to above average sea surface temperatures. Despite the uncertain future for corals, recent studies have revealed physiological mechanisms that improve coral resilience to the effects of climate change. Some taxa of bleached corals can increase heterotrophic food intake and exchange symbionts for more thermally tolerant clades; this plasticity can increase the probability of surviving lethal thermal stress. Corals can fight invading pathogens with a suite of innate immune responses that slow and even arrest pathogen growth and reduce further tissue damage. Several of these responses, such as the melanin cascade, circulating amoebocytes and antioxidants, are induced in coral hosts during pathogen invasion or disease. Some components of immunity show thermal resilience and are enhanced during temperature stress and even in bleached corals. These examples suggest some plasticity and resilience to cope with environmental change and even the potential for evolution of resistance to disease. However, there is huge variability in responses among coral species, and the rate of climate change is projected to be so rapid that only extremely hardy taxa are likely to survive the projected changes in climate stressors.

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What are the physiological and immunological responses of coral to climate warming and disease

Sedentary reef-organisms such as sponges, colonial coelenterates, bryozoans and compound ascidians produce repeated modules (aquiferous systems, polyps, zooids) as they grow. Modular construction alleviates constraints on biomass imposed by mechanical and energetic factors that are functions of the surface area to volume ratio. Colonies thus may grow large whilst preserving optimal modular dimensions. Among corals, optimal polyp size is smaller in the more autotrophic than in the more heterotrophic species. Modular construction allows flexibility of growth form, which can adapt to factors such as water currents, silting, light intensity and proximity of competitors. Modular colonies have great regenerative capacities, even separated fragments may survive and grow into new colonies. All fragments from a parental colony are genetically identical and large branching corals frequently undergo clonal propagation through fragmentation during storms. Soft corals can also fragment endogenously. By spreading the risk of mortality among independent units, the generation and dispersal of fragments lessens the likelihood of clonal extinction.



In spite of their ability to propagate asexually, most benthic colonial animals also reproduce asexually. The selective advantages of the genetic diversity among sexually produced offspring seem not to be linked with dispersal, but probably lie in the biological interactions with competitors, predators and pathogens in the parental habitat. Age at first sexual maturity and the proportional investment of resources in sexual reproduction are related to colonial survivorship. Small branching corals on reef flats grow quickly, attain sexual maturity within 1–4 years, planulate extensively, but reach only small sizes before dying. Massive corals are longer lived and have the opposite characteristics of growth and reproduction.



Most sessile reef organisms compete for space, food or light. Faster growers can potentially outcompete slower growers, but are often prevented from doing so by several forms of aggression from competitors and by the damage inflicted by storms. Competitive interactions among sedentary organisms on coral reefs are unlikely to be linear or deterministic, and so the co-existence of diverse species is possible.

R. N. Hughes

Papers

Evolutionary ecology of colonial reef‐organisms, with particular reference to corals