UNEXPECTED RELATIVES

An octopus’s snail connection

A shared sea is less informative than a shared mollusc branch.

Sources & method ↓

A giant Pacific octopus shares a closer family-tree branch with a Roman snail than with an Atlantic salmon.

A snail rather than a salmon

A giant Pacific octopus shares a closer ancestral group with a Roman snail than with an Atlantic salmon. The salmon can feel like an easy match when you picture the octopus in water. But a setting is not a lineage. The comparison asks which animal stays with the octopus within the more specific shared group, not which animal is easiest to imagine in the same scene.

A shared phylum, then separate classes

The octopus and snail meet at Mollusca, the mollusc phylum. Below that group, the octopus follows Cephalopoda, the cephalopods, and the snail follows Gastropoda, the gastropods. The octopus and salmon meet at the wider Bilateria group instead. Mollusca sits within Bilateria, which is why the snail supplies the closer connection even though the two animals look very different.

The answer has a limited scope

This does not make a snail the octopus’s ancestor, nor does it identify the Roman snail as the octopus’s single closest living relative. It compares just two named candidates. It also does not say that every mollusc has the same body plan or lifestyle. The shared group tells you about ancestry; the distinct paths below it explain why belonging together does not mean being the same kind of animal.

Read the two pairs in order

Compare octopus–snail first, then octopus–salmon. Label their shared groups Mollusca and Bilateria. Because the first group is contained within the second, you can justify the snail choice without inventing a similarity percentage. When a family-tree clue seems surprising, this two-pair check is more useful than counting visible features or assuming that all water-associated animals should be close together.

Meet the animals

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Sources & method

Relationships follow NCBI Taxonomy. Branch order does not measure genetic similarity or time since divergence.

Keep exploring

A whale’s surprising land connectionBlue whale, hippo, or salmon: water is not a family tree.Read the story ↗An elephant’s unexpected water linkThe manatee shares a branch that the hippo does not.Read the story ↗