They are a parasite common to crayfish. They are a type of worm, or Annelid, a commensal organism, meaning that they live on, but do not harm the crayfish. The specific scientific name for these are Branchiobdellida. They are members of the leech family. There is more specific data on the net, just type commensal worm in any search engine.
__________________ 10 Gal FW - 1 Male Cinnamon Dwarf Gourami, 3 Long Tail Zebra Danios, 2 Albino Corys, 2 Golden Chinese Algae Eaters, and 2 Ghost Shrimp.
½ Gal FW - 1 Male Betta (Jerry) |