Species of aplacophoran molluscs belonging to the burrowing family Prochaetodermatidae are often among the dominant species at particular localities in the deep-sea benthos, ranging up to 480 m−2 They outnumber most species of the three dominant groups: polychaetes,
peracarid crustaceans, and other molluscs. Their success is attributed to their utilization of a broad food source and rapid growth and maturation within 1 year after settlement. In three areas for which quantitative data are available—the northwest Atlantic, the northeast Pacific, and
the Aleutian Trench—the prochaetodennatid species at the midpoints of their vertical ranges were usually among the five top dominants and usually were outnumbered only by polychaetes. A Bray-Curtis similarity coefficient and group average sorting of the fifty most abundant species off
the Farallon Islands clustered six polychaete species and a prochaetodermatid (Blake et al., 1992). These results were tested statistically over the seven deepest box cores by median numbers and variances of the means among the seven species, and by correlations of the number of individuals
between paired species. The results showed that the single spionid species in the cluster apparently had little effect on the other six species. Correlation coefficients showed that every species except the spionid had a significant pairing effect with at least one other species in the group.
Of the five nonspionid polychaete species, all are motile burrowers; three belong to the Paraonidae and one each to the Cossuridae and Cirratulidae. Except for the cirratulid, all appear to be foragers like the prochaetodennatid. It is suggested that the same food source is shared by the six
species; that predator-prey interactions and competitive resource partitioning do not determine their relationship to each other; and that the group as a whole is probably a determinant of population structure. The presence of a mollusc amongst a nested group of polychaetes cautions against
using higher taxa as functional units.
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