The biodiversity of modern monothalamid Foraminifera (predominantly agglutinated taxa as well as naked and organic-walled forms) is considerably higher than that recognized in most previous studies. The numerous mudflats associated with salt marshes of Sapelo Island, Georgia, host an extensive array of mainly undescribed monothalamids (allogromiids and astrorhizids), in addition to the better-known, multi-chambered forms. Among these monothalamids are two new cryptic species, Xiphophaga minuta and X. allominuta, nov. gen., nov. spp., which may be distinguished by SSU rDNA sequences, but are closely similar in morphology. These new species are small (typically ~200 μm), translucent, and comparable in shape and shell construction. Both agglutinate clay platelets and have a single aperture located on a distinct neck. Both species belong to the monothalamid clade E of Pawlowski and others (2002). Xiphophaga minuta occurred abundantly on Sapelo’s back-barrier mudflats during the summer of 2003, and X. allominuta was recovered from the same mudflats during the summers of 2005 and 2007. Xiphophaga minuta is the primary focus of this report.

Gametogenesis, common in populations of both species during the summer, occurs in uninucleate individuals and follows a comparable pattern. Some foreign material is egested at the onset, but most is sequestered within a large, highly vacuolated central region. Gametogenesis occurs in the peripheral cytoplasm around this central region, producing numerous biflagellated gametes that are released directly into the surrounding seawater via the aperture. Xiphophaga minuta and X. allominuta therefore are gametogamous, as are most species of Foraminifera.

Non-reproductive individuals of both species appear golden-brown in color, reflecting the ingestion of numerous pennate diatoms within the cytoplasm. Such individuals are packed with diatoms, which, although mainly digested, retain intact chloroplasts. We suggest that these foraminiferans, which lack hard mineralized structures, are unable to fracture diatoms and remove their chloroplasts. Rather, they ingest entire diatoms as a form of chloroplast sequestration.

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