Because phoronid larvae swim and drift in seawater, they are much easier to sample than their adult forms, which live on the sea floor within sands, sediment or rubble. And the larvae do not look like the adults, making it difficult to tell which larva belongs to which adult without doing a sort of paternity testing: comparing larval DNA sequences to the DNA of their potential parents.

Named for the Egyptian goddess Phoronis, tubular adult horseshoe worms anchor their bodies to rocks or corals and wave a crown of ciliated tentacles to capture tiny food particles. To reproduce, their eggs and sperm unite to create embryos that hatch as swimming larvae, which then become members of the microscopic plankton.

The larvae have cylindrical bodies topped with a ring of tentacles and a large hood that serves to capture food. Many are decorated with yellow pigment spots and within some it is possible to see reddish or pinkish patches of blood cells through their translucent bodies. Eventually they descend to the seafloor, develop into adults and complete their life cycle, considered the most common cycle in the animal kingdom.

Scientists collected plankton from the Bay of Panama on the Pacific Coast and Bocas del Toro on the Caribbean coast. By examining plankton with a stereomicroscope, they found more than 50 phoronid larvae; 23 from the Pacific and 29 from the Atlantic. Using a genetic technique called barcoding based on DNA sequencing, they were able to distinguish three distinct phoronids in the plankton from the Bay of Panama and four others from the Caribbean.