The cuttlefish may be flashy, but its microbiome is super simple

In a collaboration led by Marine Biological Laboratory (MBL) scientist Jessica Mark Welch, scientists characterized the microbiome of the European common cuttlefish, Sepia officinalis, an animal whose impressive camouflage skills and behavior have long been studied. They published their findings in this month's issue of mSystems.

The project began when Roger Hanlon of the MBL and Jack Gilbert of University of California, San Diego, chose the cuttlefish to look for a connection between the gut microbiome and behavior. However, they first needed to identify the microbes present.

"Not much is known about the microbiomes of cephalopods (cuttlefish, squid and octopus), other than a famous symbiosis between the bobtail squid and Vibrio bacteria in its light organ," says Mark Welch.

Holly Lutz, a postdoctoral scientist from Gilbert's lab, examined microbial populations from the cuttlefish's digestive tract, gills, and skin. She found that the cuttlefish microbiome has just two families of bacteria: Vibrionaceae and Piscirickettsiaceae. By comparison, humans have hundreds of different kinds of bacteria in the gut microbiome alone.

The simplicity of the cuttlefish microbiome came with other surprises. Tabita Ramírez-Puebla, a postdoctoral scientist at the MBL, fluorescently probed the bacteria to reveal their organization under the microscope. "It turned out that most of the microbes were in the esophagus, an unusual place to find the most dense community of microbes," says Mark Welch.

Bacteria in the Vibrio family include some species that cause disease, but others form symbiotic relationships with their hosts, such as in the bobtail squid. "It's interesting that we find those Vibrios in association with the cuttlefish. It suggests that there is a long-term evolutionary relationship between Vibrios and marine invertebrates," says Mark Welch.

While Vibrio species are sometimes found in the gastrointestinal tract of fish, the findings hint at an expanded range of organisms and body sites associated with Vibrio. It's possible that Vibrio species in the esophagus serve as a reservoir of bacteria that seed the cuttlefish gut, bringing along enzymes that can aid in digestion. Or Vibrio colonization of the esophagus (and the rest of the animal) may be pathogenic or opportunistic.

Understanding the microbiome of cuttlefish can help aquariums care for these animals. "We need to know how to keep them healthy and knowing something about their microbiome is very useful," says Mark Welch. For example, antibiotic treatment can alter an animal's microbiome, causing unintended consequences elsewhere in the animal. In their study, the scientists treated cuttlefish with enrofloxacin, an antibiotic commonly used by aquarium veterinarians. They found enrofloxacin had no effect on the cuttlefish microbiome.