Every evening, nocturnal Hawaiian bobtail squids (Euprymna scolopes) emerge from their burrows in shallow waters of the Pacific to hunt for shrimp. These soft-bodied, golf ball-size cephalopods don’t have much to protect them from predators like seals, eels and fish. So they rely on another organism to help out: the bacterium Vibrio fischeri. This microbe lives in an organ embedded in the squid’s ink sac and emits light throughout the night to match the illumination of the moon.

“It is basically acting like a little invisibility cloak for the squid,” said Jamie Foster, a microbiologist at the Space Life Sciences Lab at the University of Florida. In return for help with camouflage that protects against predators, the squid offers up sugars to feed the bacteria and lure them into the organ.

This mutually beneficial relationship has evolved over millions of years and is one of numerous examples of how multicellular animals and microbes work together to increase their chances of survival. But scientists still know little about how these relationships evolve or what spurs animals to grow specialized organs that encourage these symbioses.

Now, Foster and an international team of researchers have mapped the genome of a Hawaiian bobtail squid, creating a new tool to explore these questions. By parsing the squid’s genome, the team has already discovered that the evolution of its light organ followed a completely different pathway than that of a second symbiotic organ, which supports reproduction. Published in the Proceedings of the National Academy of Sciences, the findings lay the groundwork for future studies of animal-microbe interactions, including those in humans.

This work also marks the completion of the first genome for a squid — and only the second for a cephalopod of any kind, following the publication of a genome map for the octopus in 2015. “Having the genome available will be a tremendous resource for the field of studying symbiotic relations,” said Cliff Ragsdale, a researcher at the University of Chicago who helped map the octopus genome but wasn’t involved in this new study.

Given their tentacles, color-changing skin and other biological novelties, bobtail squids may not seem like the most obvious candidates for helping with the study of symbiosis in humans or other animals. But scientists have studied this species as a model of symbiosis for more than three decades. “We share a lot of genes and we share a lot of [genetic] pathways, so we can learn a lot from these model systems about our own health,” Foster said. For example, she noted, humans and squids share some of the same immune system components. In fact, our immune systems are so comparable that Foster sends squids into space to learn more about the human immune response to spaceflight.

In addition to these helpful commonalities, the bobtail squid has a unique quality that lends itself to symbiosis studies. Rather than going into partnership with a consortium of bacteria, as the human gut and most other symbiotic organs do throughout nature, the bobtail squid’s light organ cultivates a strictly monogamous relationship with V. fischeri. The squid’s immune system recognizes and nurtures only this one type of bacteria within the light organ, warding off all other suitors. “Because we just have one host and one symbiont, it is easier for us to tease apart what is going on,” said Spencer Nyholm, a symbiosis specialist at the University of Connecticut and a co-author on the study.