Large-scale biodiversity pattern of Cumacea (Peracarida : Crustacea) in the deep Atlantic

Large-scale biodiversity pattern is best known as latitudinal species diversity gradients (LSDGs) on land and may be causally linked to ecological and evolutionary processes influenced by solar insolation. Although similar LSDGs may exist in the sea, patterns are contradictory and not well understood. LSDGs among small benthic taxa in the deep sea are particularly problematic because of sampling and taxonomic limitations, and remoteness from solar-driven processes at the surface. Such explanatory processes are, in any case, likely to be very different from those on land. We investigate large-scale biodiversity pattern in deep-sea benthos by analysis of a data set of 225 species among 55 933 individuals of Cumacea identified by the late N. S. Jones from 122 epibenthic sled samples (depth range greater than or equal to500 m to less than or equal to4000 m) mostly taken near the continental margin throughout the deep Atlantic Ocean. Although the plotted data showed considerable scatter, and samples from the South Atlantic were relatively few and extended only to mid-latitudes, they showed a parabolic relationship to latitude, peaking at the equator. Sample diversity from the eastern North Atlantic (62 samples) tended to be higher than that from the western North Atlantic (19 samples), with linear regression relationships of diversity to latitude indicating a poleward decline differing only in elevation of the regression line, although those for the western North Atlantic, like those for the South Atlantic, were not significant. Adding 4 samples from the Nordic Seas as high-latitude end member markedly increased the slope of the regression for the deep eastern North Atlantic. This is consistent with impoverished cumacean diversity in the deep Nordic Seas basins caused by Quaternary extinctions and isolation from the deep Atlantic. Other regional differences in diversity, particularly between the western and eastern North Atlantic, also suggested a strong basin-scale imprint. This underlies the, probably productivity-related, negative relationships between diversity and latitude found, which were similar to those found previously in other deep-sea macrofaunal taxa and Foraminifera. Analysis of the whole Atlantic data set did not support a molluscan-type zoogeographic dispersion from the south into the South and then North Atlantic. Hence, although bathyal LSDG may exist in deep-sea Cumacea, evidence suggests that large-scale pattern may reflect regional history as much as modern ecology.