Carbonate substrates prepared from the shells of living organisms and inorganic calcite were planted on the St. Croix shelf and periodically harvested to determine the nature and extent of microbial infestation by endolithic organisms. Substrates, 1 to 10 mm in diameter, were mounted on plexiglass sheets and anchored at five sites at depths ranging from the intertidal to 30 m. Samples were harvested after exposure times ranging from 9 days to 18 months. Scanning electron microscope (SEM) examination of plastic-impregnated endoliths and their resultant microborings, in association with specimens isolated through acid dissolution of host substrates, formed the basis for this investigation.

Blue-green algae were found to be the dominant microborers, and green algae were of secondary importance. Red algae and fungi were relatively scarce. In order of decreasing abundance, the following endolithic organisms were observed: Mastigocoleus testarum (blue-green alga), Plectonema terebrans (blue-green alga), Phaeophila engleri (green alga), Hyella sp. A (blue-green alga), Ostreobium brabantium (green alga), Calothrix sp. (blue-green alga), Ostreobium constrictum (green alga), moniliform fungus(?), Conchocelis stage of Porphyra sp. (red alga), Scytonema (blue-green alga), microboring sponge, reticulate fungus(?), and Hyella sp. B (blue-green alga).

Initial infestation of planted substrates was detectable after exposure periods of as little as 9 days and heavy infestation within 2 to 4 months. Less than 10 percent of the grains exposed for 9 days were colonized by endoliths; after 4 months approximately 95 percent of the grains were colonized. The distributional patterns of microborers that infested substrates planted on the St. Croix shelf are believed to be environmentally controlled and may be applicable in the paleoecological analyses of ancient carbonates.

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