Cyclammina cancellata Brady shows marked variations of diameter and thickness in Holocene sediments from depths of 500 m to more than 3,500 m in the Peru-Chile Trench area. The mean diameter increases consistently to a maximum of 5 mm at 2,000 m. Fluctuations between 4 and 5 mm follow from 2,000 to 3,500 m, and a decreasing trend characterizes deeper samples. The mean thickness increases steadily downward to about 3,500 m where a slight decrease sets in.
Using mean diameter, thickness, and a ratio between both, populations of this study can be characterized as to depth zones. Thus, small and relatively thick forms appear at about 500 m; larger and proportionally thinner forms live deeper than 1,000 m; large but relatively thick specimens characterize depths of about 2,000 to 3,500 m; and somewhat smaller and thicker ones are typical for depths below 3,500 m.
Temperature may be the principal factor affecting size, because it increases markedly to about 2,000 m—which coincides with the greatest size change in the populations. In deeper water other factors may play a role. Oxygen, salinity, and nitrate values do not show significant trends. Pressure alone is not directly involved, because off southern California similar size variations occur in different depth zones.