Genetic connectivity between and within coral reefs is an important component of resilience. Larval exchange between reefs promotes genetic diversity, which is critical in terms of resilience against any disturbance, particularly mass bleaching events. The spread of selectively advantageous genetic traits, such as bleaching resistance, is a potential consequence of larval coral exchange and migration1. Within species, susceptibility to bleaching and mortality can differ, even under the same environmental conditions. These differences between individuals suggest that genetic variation within coral populations can create resilience to increased thermal stress.
Several biological characteristics of corals may contribute to their resilience::
- 1. Fluorescent tissue proteins
Fluorescent proteins are common in many corals, providing a system for regulating light. These proteins protect the coral from broad-spectrum solar radiation by filtering out damaging UVA rays (blue light portion of spectrum), as well as by reflecting visible and infrared light, thereby reducing light stress on the corals. Concentrations of the pigments vary among species (pocilloporids and acroporids have relatively low densities of pigments, while poritids, faviids and other slow-growing massive corals have high densities). The protective capacity of these pigments provides a kind of internal defense mechanism that may have important implications for long-term survival of corals exposed to thermal stress. Corals containing fluorescent capacity have been found to bleach significantly less than non-fluorescent colonies of the same species.
Furthermore, a recent study2 identified an additional role of fluorescent pigments as supplemental antioxidants which may work to prevent oxidative stress in coral tissue and further supports the hypothesis that fluorescent pigments serve multiple functions. The diversity, temporal, and spatial variation in coral fluorescent pigments distribution, abundance, in combination with differential antioxidant potentials, suggest that fluorescent pigment roles may differ between coral species or with changing environmental conditions.
- 2. Mycosporine-like amino acids (MAAs)
MAAs absorb UV and dissipate UV energy as heat without forming toxic intermediates. While there is still a great deal of uncertainty in how MAAs are acquired, it is known that corals have a major influence on the complement and distribution of MAAs, thereby moderating the amount of UV that reaches the cells of the zooxanthellae and influencing the amount of damage sustained by the zooxanthellae3.
- 3. Heat-shock proteins
Many different heat-shock proteins are found in coral tissues and their activity influences the bleaching response. Heat-shock proteins help maintain protein structure and cell function, following stress3.
- 4. Colony integration
The extent of colony integration influences the degree to which the whole colony responds to thermal stress. Characteristics of colony integration include polyp dimorphism, intra-tentacular budding and complex colony morphology4. Species with a high colony integration (e.g., milleporids, pocilloporids and acroporids) are predicted to have a greater whole-colony response to increased temperatures than species with a low colony integration (e.g., poritids, faviids, and other massive corals). This pattern of mortality has been observed between Acropora and Porites. Acropora, with high colony integration,displayed high rates of whole-colony mortality and little partial mortality, while Porites, with low colony integration, had patches of bleached areas with little whole-colony mortality.
- 5. Change in diet in response to bleaching stress
Much of the energy needed for coral metabolism is derived from zooxanthellae, however many corals are also effective carnivores. Corals that increase carnivory survive experimental bleaching better than corals that cannot3. Changes in the transfer of photosynthetic products from the zooxanthellae to the coral in response to stress are currently being researched.
- 6. Tissue thickness
The thickness of coral tissues may contribute to the level of susceptibility to bleaching. Thin tissue is found in coral species that are more susceptible to bleaching. Thicker tissue may help shade zooxanthellae from intense light, thereby increasing the resilience of the coral. Corals from genera such as Porites that have thicker tissues and appear more robust to thermal stress than corals from genera such as Acropora which have thinner tissues5.
1 Van Oppen and Gates 2006
2 Palmer et al. 2009
3 Baird et al. 2009
4 Baird and Marshall 2002
5 Hoegh-Guldberg et al. 1999