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Molluscan grazing of sublittoral algal-bored shells and the production of carbonate mud in the Firth of Clyde, Scotland

Published online by Cambridge University Press:  06 July 2012

George E. Farrow  and
Julian Clokie
George E. Farrow
Affiliation:
Department of Geology, University of Glasgow
Julian Clokie
Affiliation:
Department of Botany, University of Glasgow, Marine Station, Millport, Isle of Cumbrae
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Synopsis

The limpet Acmaea virginea is an important agent of erosion of bivalve shells within the photic zone. This lower limit is governed by the depth range of shell-boring algae on which it feeds, the dominant alga being conchocelis. Any authigenic ferromanganese coatings on shells are removed by the grazing and the shells may become wafer thin and highly fragmented. The chiton Lepidopleurus also grazes shell surfaces but feeds more on superficial detritus than on the boring algae, and so its depth range is not so restricted. Both molluscs leave highly distinctive radula marks on shells which are potentially preservable in the fossil record. Acmaea virginea produces broad scoops, six-pronged, with very sharp narrow interstitial ridges. Lepidopleurus produces short, sharp scratches in sets of two, three or four, with wide interstitial ungrazed plateaux.

The faecal pellets of both species contain 5–10 μm sized carbonate particles. Whereas there is little else in Acmaea pellets, which are white, cylindrical and only loosely held together, chiton pellets include much fine manganiferous material, detrital quartz and broken diatom frustules, and are well bound with mucilage which makes them dirty brown-grey in colour and ovoid. The data indicate that physical processes need no longer be assumed to be the major factor in the recycling of carbonate from shells, and in their fragmentation.

Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 70 , Issue 5-9 , 1979 , pp. 139 - 148
Copyright
Copyright © Royal Society of Edinburgh 1979

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