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Planulina ornata (d'Orbigny), a coarsely perforate species of forami nifera having a low trochospiral test, was recovered attached to phosphatic hardgrounds from the lower oxygen-minimum zone off Peru. Above the base of individual pores are calcified, perforate sieve plates, the largest so far described. Structure of the pores suggests a possible association with mitochondria and respiratory function. These large pores may facilitate extraction of the severely limited amount of oxygen from the ambient bottom waters at that locale. IN THE COURSE of investigating foraminifera encrusting phosphoritic hardgrounds from the lower part of the oxygen-minimum zone of the Peru continental slope (Resig and Glenn 1997), Planulina ornata (d'Orbigny), a species in which the calcified pore plates and their perforations are unusually large, was recovered. Documentation of this occurrence is presented here to supplement the small body of literature on pore microstructure and function in foraminifera and to lend support to the proposed association of these struc tures with respiratory organelles (Berthold 1976, Leutenegger and Hansen 1979). An increasing number of perforate fora minifera representing various benthic and planktonic genera are shown to have their pores closed off from the chambers by organic membranes or plates (Table 1), a condition that may exist for most perforate foraminifera (Boltovskoy and Wright 1976). The plates are reported to be nonporous in some fora minifera such as Ammonia (Banner and Wil liams 1973), but the majority of those studied are microporous and either calcified or not.

Sieve Plates and Habitat Adaptation in the Foraminifer Planulina ornata