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Analogy: Of Shrimp and Snails

Barnacles and limpets (shown below) have many superficial similarities: both are small creatures with conical shells and can be found in the ocean on rocky shores.

Barnacles Limpets

But the similarities end there. Inside their shells, they are very different:

Cross-section of a barnacle Cross-section of a limpet
Barnacles have the body layout of a shrimp. Limpets have the body layout of a snail.

What do these differences mean? They suggest that barnacles and limpets are not very closely related, and that their common ancestor (probably a worm-like animal) did not have a conical shell. Their similar shells evolved because both lineages faced a similar challenge: a rocky, wave-swept environment teeming with predators ready to pry a small creature off a rock for lunch — and conical, hard-to-get-a-grip-on armor offers a lot of protection in such an environment. These lineages evolved their shells independently — meaning that the shells of these two lineages are analogies.

Barnacle-limpet phylogeny

Barnacles photo by Charles Webber © California Academy of Sciences; Limpets photo by Sherry Ballard © California Academy of Sciences.

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