By day, the immobile, stony corals that give form to coral reefs bask in watery sunlight. They are the very picture of harmony, living in concert with their symbiotic algae. Their diverse colors and shapes coming together to create a wonderful mosaic of life. But by night, that sense of accord dissolves, literally -- neighbor corals vomit their digestive filaments onto one another, trying to disintegrate their opponents.
The competition for space and light on coral reefs is intense, and the stakes are high -- the loser may die, while the winner gains precious resources that will enable it to thrive. But the way in which some of these conflicts are played out -- with the extrusion through the corals' mouths of long filaments attached to internal structures called mesenteries, seems, to me, rather unusual, if not a bit extreme.
The tendency for stony corals to act aggressively toward one another was described as early as 1910
, in experiments conducted on corals of the genus Isophyllia
. Marine biologists later discovered that extracoelenteric digestion is in fact a naturally occurring form of aggression, offering at least one explanation for how some species of immobile corals compete for limited resources on reefs. (A second, and more common, form of aggression that also was discovered among stony corals, and that is deployed by some soft corals, involves the use of sweeping tentacles and stinging nematocysts.)
I find it particularly interesting that coral communities actually have hierarchies of aggression. The endangered coral Ctenella chagius
, for example, holds an intermediate position in a hierarchy of aggression that exists among corals
in the reefs of the Chagos Archipelago
, which is located in the central Indian Ocean. Specifically, Ctenella chagius
was found to kill subordinate coral species, such as Gardineroseris ponderosa
and Porites lutea
, but was attacked by more aggressive corals, such as those of the genus Acropora
. A species known as Galaxea clavus
is one of the most dominant species found in the reefs of the Chagos.
Coral aggression hierarchies are reminiscent of the types of dominance hierarchies that have been described in groups of social mammals, such as baboons, and among birds. Dominance hierarchies, which essentially are pecking orders (a term derived from the hierarchies of chickens), form when individuals are forced to compete for resources. Within many mammalian and avian hierarchies, the most aggressive individuals generally are the most successful in claiming resources, but actual fighting is relatively limited. Among corals, however, conflict seems to be the norm, with wars waged over long periods of time.
Furthermore, with corals, aggression can occur between colonies (inter-colony), and it may be interspecific, involving different species, or intraspecific, involving individuals of the same species. Colonies of large, slow-growing corals frequently contain the most aggressive species, presumably to counter the advantage given to faster-growing corals, which are able to quickly populate gaps on the reef. The massive corals, however, seem to be the ultimate victors, for despite their very gradual rate of growth, their large size plays a central role in
giving coral reefs their foundational structure.