Constructional Azooxanthellate Coral Communities: An Overview with Implications for the Fossil Record

Abstract

As evidenced by the success of zooxanthellate coral species today, symbiosis with algae confers great advantages to organisms inhabiting the reef environment. Although not as well known as their zooxanthellate counterparts, azooxanthellate coral communities, dominated by so-called corals, can and do build constructionalframework, harboring a variety of associated vertebrate and invertebrate fauna. As has long been noted, when it comes to reefs and constructional roles, zooxanthellate and azooxanthellate corals exhibit mutually exclusive relationships. This is traditionally explained in terms of competition. When zooxanthellate species are absent due to restrictive environmental conditions, azooxanthellate species may inhabit relatively shallow water, even building limited constructional framework. In some non-reef settings where zooxanthellate corals are not living under optimal conditions, azooxanthellate species can be found living alongside them. On the shallow-water reef as a rule, azooxanthellate corals are relegated to cryptic habitats not colonizable by the overwhelmingly dominant zooxanthellate species. A review of 16 well-documented examples of modern constructional azooxanthellate coral communities and their physical and biological characteristics reveals a wide variety of latitudinal and depth ranges, temperatures, species composition, thicknesses, and species diversity. In all major oceans, they are best developed in cold and relatively deep water, as deep as 1500 m, and from the equator to latitudes of over 70?. Besides having a predominantly coral composition, some deep-water, azooxanthellate coral communities with extensive constructional framework have unexpected faunal diversity and presumably also complex interactions. The paleobiogeography and evolution of some of these deeper-water, coral-dominated azooxanthellate communities, known to have inhabited atolls and oceanic seamounts, may have been influenced by plate tectonic seafloor movements. While we are becoming increasingly cognizant of Holocene examples, counterparts in the geologic record are much more poorly known. The apparent paucity of ancient analogs may stem from difficulties in their interpretation and recognition in the geologic record. Judging from the size, facies relationships, faunal content, and diversity of Holocene examples, ancient deep-water coral structures, when preserved in the rock record, could be easily misconstrued as features of shallow-water origin. More careful analysis of deep-water Holocene examples can greatly expand our awareness of ancient analogs. In some cases, depending on the amount and thickness of construction, they could hold potential as hydrocarbon reservoirs. Azooxanthellate scleractinian communities are common in today's oceans and examples of them extend back to the Middle Triassic period. Fossil examples, unfortunately, are rare and sporadic in stratigraphic distribution, with the most notable reports from the Tertiary. Our view of the relative abundance of ancient counterparts may be hampered by poor preservation and problems of recognition.