Sieve Plates and Habitat Adaptation in the Foraminifer Planulina ornata

Planulina ornata (d'Orbigny), a coarsely perforate species of foraminifera 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 structures with respiratory organelles (Berthold 1976, Leutenegger and Hansen 1979). An increasing number of perforate foraminifera representing various benthic and planktonic genera are shown to have their pores closed offfrom 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 foraminifera such as Ammonia (Banner and Williams 1973), but the majority of those studied are microporous and either calcified or not. 1 Partial support of this research was provided by a Nation-ai-Science Foundation grant (OCE-9201305) to c.R.G. This is University of Hawai'i School of Ocean and Earth Science and Technology Contribution No. 6005. Manuscript accepted 1 March 2002. 2 Department of Geology and Geophysics; University of Hawai'i at Manoa, Honolulu, Hawai'i 96822. Pacific Science (2003), vol. 57, no. 1:103-110 © 2003 by University of Hawai'i Press All rights reserved Jahn (1953) illustrated and described the microporous pore plates and termed them sieve plates. In some of the studied species, the pore plates occur singly at the base of each pore (Be et al. 1980), but other species have multiple plates, one with each growth lamina (LeCalvez 1947, Jahn 1953, Sliter 1974). Some pore-sieve plate and micropore diameters that may be typical of finely and coarsely perforate benthic species and planktonic species are as follows: I-j..lm sieve plate, O.I-j..lm micropores in a nonionid (Jahn 1953); 2to 6-j..lm sieve plate, 0.2to 0.3-j..lm micropores in Lamellodiscorbis aguayoi (Arnold 1954b); and 3.3-j..lm sieve plate, 0.1to 0.6-j..lm micropores in Globigerinoides sacculifer (Be et al. 1980). Planulina ornata, in contrast, has 6to lO-j..lm diameter sieve plates and 0.5to 1.0-j..lm diameter micropores. These well-defined structures are presumably functional within the species' habitat. HABITAT OF Planulina ornata Planulina ornata was originally described by d'Orbigny (1839) as Truncatulina ornata from off the Port of Valparaiso, Chile, where it was rare. Its geographic range continues northward to Alaska (Buzas and Culver 1990). Specimens occur free in the sediment and are reported to be most frequent on the outer continental shelf (Bandy and Arnal 1957 [Central America], Bandy 1961 [Gulf of California]) or on the upper continental slope between 160 and 260 m (Boltovskoy and Theyer 1970 [Chile]). Their bathymetric range generally extends deeper along the