Why are coral reefs considered one of the most biologically productive environments?
Coral reefs are ecosystems with well-defined structures that involve both photosynthetic algae and consumers (see Ecology). The outer layer of a reef consists of living polyps of coral. Within the coral animals live single-celled, round algae called zooxanthellae. Below and surrounding the polyps is a calcareous skeleton, both living and dead, that contains filamentous green algae. Other species of algae, both fleshy and calcareous, grow in the surface of old skeletal deposits. These algae make up most of the primary producers. The photosynthetic zooxanthellae and filamentous green algae transfer some food energy directly to the coral polyps. Coral animals also feed at night on zooplankton, which they capture with their tentacles. Coral animals prey on zooplankton not so much for the calories but for scarce nutrients, especially phosphorus. Through digestion, coral animals release these nutrients to the algae. Coral and algae then apparently cycle these nutrients between them, reducing
