The Secret World Within

Phylosymbiosis: The Family Legacy

Just as you might inherit your grandmother's eyes or your father's smile, cephalopods inherit microbial communities from their evolutionary ancestors. This phenomenon, called phylosymbiosis, means closely related species share similar gut microbiomes. For example, octopuses (Octopus spp.) host distinct microbial profiles compared to squids (Loligo, Uroteuthis), reflecting millions of years of divergent evolution. A landmark 2022 study showed that host phylogeny explains 34% of microbiome variation—more than diet or habitat alone ^{2 3 7} .

Habitat: The Environment's Signature

Whether an animal dwells in the deep sea, a tidal pool, or freshwater drastically reshapes its gut microbes. Marine cephalopods like the Japanese flying squid (Todarodes pacificus) harbor salt-loving Photobacterium, while terrestrial snails host microbes adapted to soil and plants. Even within species, wild octopods carry more diverse—and potentially pathogenic—bacteria (like Vibrio) than their aquaculture-raised counterparts, which live in controlled, cleaner water ^{6 9} .

Wild Octopus

Higher microbial diversity including potential pathogens like Vibrio ⁶ .

Aquaculture Octopus

Reduced microbial diversity in controlled environments ⁶ .

Diet: The Microbial Menu

Carnivorous cephalopods feast on crabs and fish, selecting for protein-degrading bacteria like Mycoplasma. In contrast, herbivorous snails and slugs cultivate cellulose-breakers like Cloacibacterium. Diet's impact is so profound that even within the same class (e.g., Gastropoda), a snail eating algae vs. one scavenging carrion will develop wildly different gut communities ^{1 9} .

Cuttlefish

57.4% Mycoplasma (protein metabolism) ¹

Beka Squid

58.0% Photobacterium (digestion) ¹

Common Octopus

97.5% Mycoplasma (nutrient processing) ¹

The Breakthrough Experiment: A Deep Dive into Cephalopod Guts

Methodology: Decoding the Microbiome Blueprint

In 2022, researchers led by Kang et al. undertook the first comparative analysis of cephalopod and mollusk gut microbiomes:

1. Sample Collection: Gut tissues from 6 cephalopod species (e.g., cuttlefish, squid, octopus) and other mollusks (snails, oysters) were collected.
2. DNA Sequencing: Using 16S rRNA gene amplicon sequencing, they identified bacteria via the Illumina MiSeq platform.
3. Data Analysis: Microbial diversity was measured using alpha/beta diversity metrics. Host phylogeny was reconstructed using mitochondrial DNA (e.g., COI gene).
4. Comparison: Data were contrasted with marine fish and mollusks from varied habitats/diets ^{1 2 3} .

Results & Analysis: Core Taxa and Phylogenetic Signals

• Core Microbes: Photobacterium (23.8% abundance) and Mycoplasma (50.0%) dominated cephalopod guts.
• Host Specificity: Mycoplasma strains were host-specific (e.g., Octopus-associated types), while Photobacterium was broadly shared.
• Phylogeny Rules: Microbial communities clustered by host species/order (e.g., all octopods grouped together), mirroring their evolutionary tree (Fig 1).
• Habitat & Diet Matter: Mollusks split by environment (aquatic vs. terrestrial) and diet (herbivore vs. carnivore), but phylogeny was the strongest driver ^{1 2 5} .