Intertidal Bioerosion by the Chiton Acanthopleura Granulata; San Salvador, Bahamas
Abstract:Rates of daily fecal pellet production and intertidal bioerosion by the chiton, Acanthopleura granulata Gmelin, 1791 were measured along the shore of Pigeon Creek, an interior marine tidal creek on San Salvador Island. Chitons grazed upon pitted, biokarstic Pleistocene limestones which form outcrops within the intertidal zone. The low-energy, tide-dominated character of the waterway fosters excellent preservation of delicate chiton fecal pellets, and allowed a more accurate, in situ measurement of bioerosion by directly counting their daily accumulation.
Chitons were abundant (5.5 chitons˙m−2), with adults and juveniles present in a 2.8:1 ratio along the exposed rocky shoreline. An average daily production rate of 67.2 pellets˙chiton−1 was calculated from 43 individuals monitored for 4 days. Pellet counts were highly variable among days and among individuals. Organically bound, 3–4 mm long fecal pellets were 94.3% CaCO3 by weight, and composed of a variety of constituent grain types and textures. Individual pellets from adults and juveniles contained about 2.1 and 0.7 mg CaCO3, respectively. Assuming that the mass of carbonate deposited as fecal pellets is equal to that eroded from the coast, an annual carbonate erosion rate of 41.5 g˙yr−1˙chiton−1 results. Combined with the 1.82 g˙cm−3 average density of the pelletal grainstone/packstone which they graze, a volumetric erosion rate of 22.8 cm−3˙yr−1˙chiton−1 results. Using local population density, an overall bioplanation rate of 0.12 mm˙yr−1 exists across the rocky intertidal zone due to the activities of A. granulata alone. Intertidal exposure hardens otherwise easily disaggregated chiton pellets, and may enhance their geologic preservation potential. If recognized in the fossil record, preserved chiton pellets may indicate proximity to low-energy, tide-dominated rocky shores, along which this process is most common.
Document Type: Research Article
Publication date: 1990-11-01
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