Biodiversity of Modern Temperate to Subtropical Monothalamid Foraminifera

Among the Foraminifera, the “monothalamids” include a series of basal clades that encompass members of the orders Allogromiina and Astrorhizida. Representatives have few defining morphological characters and typically possess just a single-chambered organic or agglutinated test which may be absent in freshwater forms. Modern monothalamids often are not recognized in distributional studies, particularly those in which sediments are dried prior to examination. As a result, little is known about monothalamid occurrences, distributions, adaptations, and modes of life. This study examines modern “monothalamid” biodiversity using morphological and molecular surveys conducted at three previously well-studied sites: Long Key (Florida Keys), Sapelo Island (Georgia), and the Sippewissett marshes of Cape Cod (MA). Overall, our morphological and molecular surveys revealed a remarkably high monothalamid biodiversity at these sites, much higher than that reported in previous studies, and most representatives are undescribed. High monothalamid biodiversity can no longer be viewed as typical only of cold and/or deep marine settings. Our sites in the Keys, for example, hosted a variety of both large and small organic-walled and agglutinated taxa, and phytal substrates hosted more diverse assemblages than sediments. Several organic-walled taxa were particularly abundant as squatters, occupying empty spirorbid shells or constructing protective domes of carbonate grains. By contrast, the mudflats sampled in the Sapelo marsh systems were dominated by diverse assemblages of agglutinated monothalamids, and their distributions varied with salinity. The molecular environmental screens consistently revealed a higher biodiversity than the morphological surveys alone, indicating that some species have very cryptic occurrences and are not readily observed morphologically. Furthermore, although the monothalamids have few gross morphological characters, the fine structure of their tests (examined via TEM and SEM) is remarkably varied. Indeed, the monothalamids are represented by a wide variety of shell constructions that belies their otherwise generalized morphology.