Habitat-related patterns of injury and mortality among Jamaican gorgonians

Patterns of injury and mortality were surveyed in four gorgonian assemblages on the north coast of Jamaica during the summer of 1977. Both processes varied significantly across the reef in every aspect examined. Death by detachment was rare, restricted to the most exposed reef zone, and due mainly to the structural failure of the underlying substratum. Frequencies of death by overgrowth differed among zones and were complicated by variable rates of injury, regeneration and fouling as well as by the persistence of overgrown skeletons.

Injuries differed among the assemblages in frequency, type, cause, location within colonies, and relationship to colony size. Frequencies of total injury increased from the exposed Fore Reef Zones to the more protected Rear Zones. Patterns of recent injury (those still capable of regeneration) differed from those of permanent injury (encrustation or branch fracture). The primary inferred causes of injury were abrasion on the Fore Reef and predation in the Rear Zones. Small colonies were relatively immune to injuries by abrasion on the Fore Reef, but were susceptible to injuries by predation in the Rear Zones. Within colonies, injuries were concentrated on peripheral branches by both abrasion and predation. Loose fragments were rare and heavily injured. The aggregation of injuries within and among gorgonian colonies was due to small-scale patchiness in the causes of initial losses, and to mechanisms by which they are multiplied and expanded. The energetic and reproductive consequences of frequent and aggregated injury may be as important in structuring these assemblages as are the complex and unpredictable sources of mortality.