Colony integration during regeneration in the stony coral Favia favus

Modular organisms consist of repeated building blocks. An important consequence of modularity may be reflected in the ability of a colony to continually reallocate priority of resource transport among its units in response to stress. Hermatypic corals, the main organisms constructing tropical reefs, are prone to damage by a multitude of agents. Since colonization of lesions bY competitors is a potent threat to colonial organisms, fast recovery is an important component of colony survival. Previous regeneration studies have claimed that the energy requirements of this essential process are fueled only by the polyps directly bordering the injured area. This "localized regeneration hypothesis" rejects the necessity for wide colony integration during regeneration and sees no advantage to large colony size. The objective of the present study was to test an alternative regeneration hypothesis that argues, in contrast, that injury repair (i.e., closure of lesions by newly formed tissues) in corals may require extended colony integration (i.e., internal translocation of resources from sites of acquisition to sites of maximal demand). To test our hypothesis we examined: (1) the relationship between colony size and percentage recovery of lesions differing in size and shape; and (2) the effect of different sized lesions on the fecundity of polyps located at increasing distances from the lesion site. Both experiments were conducted on the common, spherically shaped coral Favia favus in the Red Sea near Eilat, Israel. The relatively small lesions (< 1 cm²) were the only ones to support the localized regeneration hypothesis, since their recovery was unaffected by colony size. However, the two larger lesion types (approximate sizes of 2 cm² and 3 cm²) confirmed the importance of large colony size for achieving fast recovery. In the second experiment we found that small lesions, repeated monthly, caused only a localized reduction in fecundity, while larger monthly repeated lesions caused significant reductions in fecundity up to a distance of 15 cm away from their site. Both experiments indicate that regeneration from injury may require an extended magnitude of energy integration throughout the colony, and that the extent of this integration is regulated by the colony in accordance with lesion characteristics. It is also concluded that in long-lived organisms such as corals, there is a priority of energy allocation to recovery rather than to reproduction. Our findings reveal the existence of injury thresholds within a colony that determine energy allocation and intra-colonial translocation of energy products toward regions of maximal demand. We suggest that such injury thresholds may characterize many other coral species and that colony integration during stress is a basic life-preserving ability and one of the most important advantages of clonal and colonial organisms.