Skeletal organization in the coral, Pocillopora damicornis

Abstract

ABSTRACT The skeleton of this Hawaiian reef coral was found to contain at least 99’9% by weight of the mineral aragonite, present as submicroscopic crystals in spheritic arrangements. The organic component of the skeleton comprises 0·01 to 0·1 % of the total weight and has 3 microscopic constituents: (1) filaments of lime-boring algae, (2) a dispersed network of fibres 1 μ in diameter, and (3) a transparent, milky, regionally biréfringent matrix of chitin. The chitin was observed to be a spongework of fibrils of average diameter 20 mμ. The chitin fibrils were inferred to be randomly oriented in the plane of the skeletogenic epithelium perpendicular to the direction of growth of the long axes of the aragonite crystals. The development of the skeleton is traced from the initial mineral deposit by the larva after its attachment, through the formation of the larval skeleton and growth into the fully formed, branching colony. The process of formation of chitin fibrils according to the contour of the skeletogenic epithelium and the later deposition of aragonite crystals as described accounts for the formation of all skeletal elements of Pocillopora. Evidence is presented for the hypotheses that (1) the amide group of the chitin molecule is responsible for the ability of certain organic substrates to be calcified (thus protein is not a necessary component of such substrates); (2) zooxanthellae in Pocillopora contribute a product of photosynthesis to the coral as the monomer of the chitin matrix; and (3) chitin synthesis thus depends on the activity of zooxanthellae and the rate of chitin synthesis controls the rate of skeletogenesis in Pocillopora.