GEOGRAPHIC-VARIATION OF SHELL BIONTS IN THE DEEP-SEA SNAIL GAZA

Recent studies of bathyal environments indicate that microhabitats con tribute significantly to biodiversity. We suggest that microhabitats m ay also be useful in direct comparisons of biotic diversity patterns a cross large areas. Using a technique developed by palaeontologists, we document the diversity and abundance of taxa occurring on the hard su bstrate provided by gastropod shells of the bathyal (>200 m) trochid g astropod Gaza. Because the shell surfaces are comparable, shell-associ ated biota and their diversity may be contrasted across. basins with r elatively few compounding variables. Animals found attached to the out side of the shells, epibionts, included folliculinids, encrusting bryo zoans and foraminifera; within the shell, endobiontic spionid polychae tes had occupied the columella of the shell, apparently destroying bot h the protoconch and part of the shell's base. Both types of bionts we re common on shells from continental margins of North and South Americ a but were wholly absent on shells from the Lesser Antilles. Hypothese s that shell depth distributions (372-910 m) and shell age, biont disp ersal potential and biotic interactions contribute to the large-scale differences in biont distribution are considered but rejected in favou r of the hypothesis that terrigenous sediments and their associated nu trients are primary contributors to the observed geographic pattern.