Larger agglutinated foraminifera of McMurdo Sound, Antarctica: Are Astrammina rara and Notodendrodes antarctikos allogromiids incognito?

Explorers Cove, an embayment on the oligotrophic western side of McMurdo Sound, Antarctica, is noted for physical and biological similarities to the deep sea. This locality is an important site for foraminiferal biologists because large, deep-sea-like agglutinated taxa are abundant at depths accessible to divers. Using microdissection methods, we found that many of the giant, single-chambered species from this locality display an allogromiid internal organization (i.e., a monothalamous cell body encased by an organic wall or “theca”). Another characteristic feature of these species is the presence of a distinct space separating the cell body and agglutinated test. We used high voltage transmission electron microscopy of thick sections to examine the theca of Astrammina rara, a dominant Explorers Cove species possessing a simple spherical test morphology, and Notodendrodes antarctikos, a distinctive arborescent species. We found that the simple gross morphology of A. rara's agglutinated test belies the complex ultrastructure of its allogromiid-like theca, and that the complex dendritic morphology of N. antarctikos' agglutinated test contradicts the simple ultrastructure of its theca. Additional field and laboratory observations on A. rara revealed that the agglutinated test can be a transient structure. Our observations raise questions regarding the taxonomy and phylogeny of unilocular agglutinated foraminifera and other primitive granuloreticuloseans. The results are also discussed in terms of test function in larger agglutinated foraminifera and the ecology of this important taxon.