The medicinal leech, Hirudo medicinalis , carries in its digestive tract a pure culture of Aeromonas veronii biovar sobria. This simplicity provides the opportunity to investigate the interaction of the partners.
Some animals have special organs where they house the symbionts.
A well-studied example is the Hawaiian squid, Euprymna scolopes . The symbiotic bacteria, Vibrio fischeri, produce light and are housed in a light organ. The light produced by the bacteria is used by the squid to camouflage themselves. Other symbioses are so intimate that the symbionts live inside the host cells. An example of this is the symbiosis of Aphids, Aphidoidea .