Although further study is required in the deep Southern ocean to determine whether interregional variation is caused by a general poleward reduction of biodiversity, it has been found that in the North Atlantic and Norwegian Sea, the relationships between diversity and latitude are highly significant when depth is held constant. The study used deep sea bivalves, gastropods and isopods For bivalves, evenness is uncorrelated with latitude. Isopods and Gastropods, however, show significant decreases in evenness with latitude. It was stated in the paper in which the study was published that "sampling methods in the deep-sea benthos have been developed only during the past 25 years, and the geographic coverage of sampling programs is much more limited." (Nature, Vol 365)