On 2 Florida reefs, submarine reef spurs 10-12 high and up to 50 ft wide were dissected with explosives so that internal structures could be examined. Millepora and alga-coated spurs were found to be composed mainly of in-situ coral (Acropora palmata). Comparison of living A. palmata with encrusted spurs suggests that a new interpretation of spur and groove formation is necessary. A. palmata growing in less than 20 ft of water on the seaward slope of reefs which face prevailing seas modifies its growth form so that the branches can accommodate the forward thrust of impinging waves. The branches become oriented in the direction of wave movement, and degree of modification is proportional to the wave strength. Continued unidirectional growth causes individual colonies to coalesce into fingerlike spurs that project as much as 200 ft into oncoming seas. These "living spurs" die from crowding when they reach the surface and subsequently become completely masked with calcareous algae and Millepora. Moving sand, in the grooves between spurs, prevents coral attachment, and periodic hurricane seas remove accumulating debris derived from the overhanging walls of adjoining spurs.

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