design: Hosts were collected from Oshoro, Hokkaido, Japan, dissected, and had their intestines removed, sliced into smaller pieces in sterile 0.2 m-filtered seawater and agitated by pipetting. The liquid was examined for archigregarines using an inverted microscope. Archigregarines were manually isolated with a glass micropipette and washed in 0.2 m-filtered seawater. A single cell was then deposited in SmartSeq2 lysis buffer for single-cell transcriptomics (Picelli et al 2014). Cells were subjected to two to three freeze/thaw cycles, after which cDNA was generated and amplified following the SmartSeq2 protocol, using 22 PCR cycles. The resulting cDNA was made into libraries with Illumina Nextera XT or Illumina DNA prep, and sequenced on a Illumina MiSeq run (2x300 bp paired end).