ANATOMY OF AN ORPHANED MONOTHALAMID FORAMINIFERAN: FINE STRUCTURE AND PHYLOGENETIC POSITION

In comparison to their polythalamous, mineralized counterparts, modern single chambered monothalamid foraminifera are largely understudied. Despite their ubiquitous presence in diverse aquatic habitats, they often lack distinguishing test ornamentation rendering them difficult to identify and classify. Organic components of the test are vulnerable to taphonomic, particularly bacterial, degradation, they are fragile and commonly disintegrate rapidly following death or reproduction.

The “Fusiform” monothalamid occurs in varying salinities along many sites of the Sapelo Island, Georgia area: a low-salinity salt marsh (~10–22 ‰) at the Rod and Gun Marina near Darien, Georgia mudflats at the historical site on Sapelo locally known as “Chocolate,” mudflats near the Sapelo lighthouse and tidal creeks of Cabretta Island. Individuals exhibit a fusiform test shape, wide at the midriff and tapering on either side terminating with apertures. Test length always exceeds width and can range from 100-400µm. The test (10 µm thick) is flexible and composed of loosely agglutinated grains of predominantly quartz but also clay, indiscriminate fragments of diatom frustules and occasional dark, unidentified minerals. Agglutinated materials are loosely packed within an apparently featureless organic bioadhesive where bacterial colonies have also been observed. The inner organic lining (IOL) is roughly 4-5 µm thick, lies just beneath the agglutinated layer in direct contact with the cellular plasma membrane. Numerous vesicles of differing electron density, presumably with test construction materials, are just beneath and can be found merging with the plasma membrane. The IOL is electron transparent and contains electron-dense fibers and granules more dense and numerous toward the apertural ends. Cytoplasm may contain large vacuoles and most specimens were uninulceate. Apicomplexan parasites have also been found occupying vesicles within the cytoplasm.

Previous SSU rDNA phylogenetic analyses suggested that “Fusiform” was loosely related to Clade E, but it does not appear aligned with any of the currently defined genetic clades of monothalamids. Broad taxonomic sampling of monothalamids is necessary to increase phylogenetic resolution, particularly for orphaned taxa.