When is a snail not a snail? What if it’s something totally, completely different? This mollusc-like shell was described as a new species of trochid in 1903, but in fact it comes from an entire different phylum (one of the highest levels of distinction between animals). Any ideas what it might be instead?

… It’s an anemone! … Obviously?? Bizarre as it may sound, this ‘shell’ is produced by a deep-sea anemone called Stylobates aeneus, which forms a symbiotic relationship with hermit crabs. The planula larva settles on a gastropod (snail) shell and grows a chitinous protein mat over its host, which then continues to expand and grow into a much larger shell form. Ideal, really, as it means the hermit crab rarely needs to exchange up for a large shell, as the Stylobates keeps growing along with it. The crab benefits from finding a home, with the added advantage of the anemone’s stinging cnidocyte cells, and the anemone is provided with transport by the crab. The original shell can usually be found at the apex of the shell structure, though its shape doesn’t seem to determine the anemone’s growth. It turns out there are actually four species in this genus capable of making these elaborate shells, and several others which can extend their host shell a little bit. But this is certainly a masterpiece of biological imitation.

Image courtesy of Chong Chen.

Glossary

Cnidocytes: Stinging cells which can inject a toxin to prey or potential predators.

Phylum: A high-level classification of organismal groups, e.g. arthropods (insects, crustaceans, spiders etc).

Planula: A swimming larval stage in the development of cnidarians (anemones, jellyfish and their relatives).

Symbiotic relationship: A ‘partnership’ between two organisms, where living together benefits both parties, e.g. providing defense, food or shelter.

Trochid: A snail belonging to the family Trochidae.