Geographic variation of shell bionts in the deep-sea snail Gaza

Recent studies of bathyal environments indicate that microhabitats contribute significantly to biodiversity. We suggest that microhabitats may also be useful in direct comparisons of biotic diversity patterns across large areas. Using a technique developed by palaeontologists, we document the diversity and abundance of taxa occurring on the hard substrate provided by gastropod shells of the bathyal (200 m) trochid gastropod Gaza. Because the shell surfaces are comparable, shell-associated biota and their diversity may be contrasted across basins with relatively few compounding variables. Animals found attached to the outside of the shells, epibionts, included folliculinids, encrusting bryozoans and foraminifera; within the shell, endobiontic spionid polychaetes had occupied the columella of the shell, apparently destroying both the protoconch and part of the shellʹs base. Both types of bionts were common on shells from continental margins of North and South America but were wholly absent on shells from the Lesser Antilles. Hypotheses that shell depth distributions (372–910 m) and shell age, biont dispersal potential and biotic interactions contribute to the large-scale differences in biont distribution are considered but rejected in favour of the hypothesis that terrigenous sediments and their associated nutrients are primary contributors to the observed geographic pattern.