Environmental grain and polychaete species' diversity in a bathyal benthic community

Patterns of polychaete species' dispersion in the San Diego Trough, Southern California Continental Borderland, North Pacific Ocean, at a depth of 1230 m are analyzed. Samples consist of 4 replicate 0.25-m² box cores, each partitioned into twenty-five 0.01-m² subcores, and of 1 unpartitioned core. The sampler is biased but, when this bias is taken into account, few species show strong aggregation either between or within cores. If all species are considered together as replicates, intraspecific aggregation is detected between cores, but uniform dispersion dominates within cores. Patchiness or habitat partitioning on the assayed scales can thus account for little of the extremely high species' diversity observed. Apparently due to an environmental structure having a “grain” smaller than 0.01-m², the diversity of species assumed to be sedentary is higher than the diversity of species assumed to be mobile. One evidence of such structure is a negative covariance between paraonid abundance and density of “mudballs” constructed by a cirratulid. Consideration of these results suggests that either grain specialization or simultaneous, biogenic disequilibrium or both act on spatial scales smaller than 0.01-m² to maintain high polychaete species' diversity in the San Diego Trough.