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Abstract

Abstract Observations off Bonaire, Netherlands Antilles, support the recent contention that Caribbean reefs are occasionally subjected to low-water emergence and are therefore more similar to reefs of the Indo-Pacific than previously believed.

Off Bonaire, some incipient reef flats and the shallow floor between the flats and shore are sporadically awash for extended periods of time. Most of the corals (representatives of over 20% of ail known Caribbean species) that inhabit the emergent biotopes appear to be able to endure subaerial exposure without severely deleterious effects. However, destruction of Montastrea annularis either results directly from exposure or, more probably, is initiated by it.

Bonaire’s shallow-water reefs are of three types. The type that occurs in turbulent water is composed almost entirely of Acropora palmata and is therefore similar to the typical, shallow-water reefs of the Caribbean. The two types that are found in calm water differ considerably in species composition from typical Caribbean reefs and differ from one another as a consequence of whether they undergo or escape emergence. Calm-water reefs that are constantly immersed consist almost exclusively of nearly continuous colonies of Montastrea annularis. Those that are intermittently emergent are the result of a succession of heterogeneous corals. Construction of the reefs of varied corals was begun by abundant large colonies of M. annularis, but most of these colonies were killed when they attained sizes at which their summits were subaerially exposed during exceptionally low stands of the sea. Following the decline of the M. annularis colonies and erosion of their coralla by boring organisms, construction of the reef framework was continued by an extraordinarily diverse coral assemblage. Framework building has been culminated upward by the establishment of exposure-tolerant corals and other cnidarians, but the diverse assemblage is maintained on the reef flanks and deeper portions of the flats.

The diversity of the corals (32 of the 48 species found off Bonaire) that are involved in the formation of the sometimes emergent reefs is unusual in shallow water in the Caribbean; such diversity is usually associated only with much deeper reef zones. Because Montastrea annularis has been shown to be dominant over most of the corals of the unusually diverse, shallow-water reef assemblage in the extracoelenteric-digestion hierarchy (i.e., a “pecking order” in which representatives of subordinate coral species are consumed and thereby eliminated from competition for reef space by those that are more aggressive), the diversity of the assemblage is attributed to the initial decimation and continued control by emergence of the once prolific colonies of this coral.

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