The sequences of foraminiferal SSU ribosomal genes contain highly variable regions--so variable that closely related species will have sequence differences. This is extremely useful for detecting individual species' DNA. We have designed species-specific PCR products for several of the most common foraminifera in the Antarctic benthos, which are capable of picking out a single species' DNA from a mixed pool of DNA extracts. 
Diagram showing one of the two variable inserts in the foram SSU. Note the immediate sequence divergence within the insert region. Positions of primers indicated by black arrows.
PCR reactions, using the same template, using primers specific for all forams (3A) or for specific forams (blue box). Sizes are in bp. Species-specific primers are for: Crithionina delacai, Gloiogullmia sp., Rhabdammina cornuta, Notodendroides hyalinosphairos, Psammosphaera fusca, Pyrgo peruviana, and Astrammina rara.
About the image: The cells above are foraminifera from a sample of Explorers' Cove sediment. Notice the considerable morphological variability--yet completely different molecular types can be housed in very similar tests.